Kumpulan skematik elektronika 2

circuitideas

w w w . e f y m a g . c o m104 • mar ch 2008 • electronics for you

Fig. 1: Glow plug controller

Fig. 2: Pin configurations of bs170 and bc548

T.A. BABu

GLOW PLuG CONTROLLER s.c. dwivedi

In diesel engines, the air in the cylinders is not hot enough to ignite the fuel under cold condi-

tions. Therefore each cylinder of these

engines is fitted with an electric heater known as ‘glow plug.’ A control circuit is necessary to optimise the functioning of glow plugs. It raises the air tem-perature inside the engine cylinder for quick and reliable starting, extended battery life and reduced diesel con-sumption.

The glow plug controller (Fig. 1) uses a simple timer circuit built around MOSFET T1 for reliability and simplic-ity. Momentary pushing of switch S2

charges capacitor C1 rapidly via resis-tor R1. When the voltage on capacitor C1 exceeds the threshold voltage of the gate (G) of MOSFET T1, it starts charging reservoir capacitor C2 and simultaneously energises relay RL1.

MOSFET T1 remains conducting as long as the voltage on C1 is greater than the threshold voltage of the MOS-FET gate.

The ‘on’ time period depends on the value of capacitor C1 and resistor R2, which govern the discharge current of capacitor C2. The component values given here will produce ‘on’ time of around 25 seconds. In effect, when you press switch S2 momentarily, the relay energises for about 25 seconds and

the glow plug gets the power supply through its contact.

The red LED (LED1) indicates that the heating process of glow plugs is

‘on.’ When the ‘on’ time is over, the green LED (LED2) turns on for a while, followed by a short beep from the buzzer, which in-dicates that the engine is ready for starting. Glow plugs draw a heavy current, hence high-cur-

rent-rating contacts of an automotive relay are required.

Assemble the circuit on any gen-eral-purpose PCB and house in a suitable case. Connect the glow plug wire to the relay contact. 12V battery already available with the vehicle is used to power the circuit. Connect the piezobuzzer and LED1 and LED2 through an external connection and place it at a convenient location for the driver to operate.

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108 • Dec em b er 2010 • electronics for you w w w . e f y m a g . c o m

Raj K. GoRKhali

GuitaR EffEct PEdal PowERs.c. dwivedi

connecting points as shown in Fig. 2. The circuit (Fig. 2) can be divided

into two sections: power supply and signal handling. The power supply section is built around transformer X1, regulators 7805 and 7905, bridge recti-fier comprising diodes D1 through D4, and a few discrete components. The signal-handling circuit is built around two OP27 op-amps (IC3 and IC4).

The power supply of about 9V for the effect pedals is derived from step-down transformer X1. MOV1 is a metal-oxide varistor that absorbs any large spike in mains power.

IC 7905 (IC1) is a -5V low-power regulator. By using a 3.9V zener diode (ZD1) at its ground terminal, you get -8.9V output. The same technique is also applied to IC 7805 (IC2)—a +5V regulator to get 8.9V. Use good-qual-ity components and heat-sinks for the

regulators. This supply is more than enough for the five effect pedals.

The greater the voltage drop across the regulator, the lower the output current potential. Resistors R1 and R2 provide a constant load to ensure that the regulators keep regulating. Capacitors C3 through C8 ensure that the supplies are as clean as possible. It is very important to use proper heat-sinks for IC1 and IC2. Otherwise, these could heat up.

Working of the circuit is simple. The input signal stage uses a basic differentiation amplifier to accept the incoming signal and a voltage fol-lower to buffer the output to the power amplifier. The differential amplifier is built around IC3. It works by effective-

ly looking at the signals presented to its inputs. If the input signals are of different amplitudes, IC3 amplifies the difference by a factor determined by R4/R3 (where R4=R6 and R3=R5). If the input signals have same ampli-tudes, these are attenuat-ed by the common-mode rejection ratio (CMRR) of the circuit. The value of CMRR is determined by the choice of the op-amp the auxiliary components used and circuit topolo-gy. You can use standard resistors. With the values shown, you get an overall gain of unity.

The combinat ion of resistor R7 and C13 serves as a passive low-pass filter, progressively attenuating unwanted high-frequency signals. The second op-amp (IC4)

Fig. 1: A typical guitar pedal switch

Fig. 2: Pedal power circuit

A friend of mine plays guitar with several guitar effect pedals. He had a problem with battery

eliminators and cables of the pedals cluttering the stage and so he asked for help. The solution is simple as de-scribed here.

A small box is fitted to the rear of the amplifier providing a 9V output for the effect pedal. The amplifier section

gets 9V through a pedal switch (refer Fig. 1). This power out-put and guitar s igna l input lines are com-bined into a single unit with multi-way cable

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electronics for you • December 2010 • 109w w w . e f y m a g . c o m

forms a simple voltage follower (its output follows its input), providing a low output impedance to drive into the standard power amplifier.

Assemble the circuit on a general-purpose PCB and fit it to the rear of an

amplifier. The unit must be compact, yet robust. So use a very sturdy alu-minium extrusion for the cabinet in or-der to neatly house the assembled PCB.

To ensure simple operation, there are only three connections to the unit.

First, mains power is tapped from the transformer. The second lead carries the 9V output to the amplifier. The third is the guitar signal input at the five-way socket for connection to the effect pedal.

electronics for you • January 2011 • 59w w w . e f y m a g . c o m

Solar TipS

P.S. DeoDhar

all about home Solar SyStemSKnow the right current controller, inverter and battery combination to get the most out of your PV solar panel

Going for home solar power is a wise decision as solar energy is not only natural but is free

to us unlike any other source of elec-trical power. So you will be helping to reduce greenhouse gas emissions. However, home solar power systems need significant initial investment. So you have to be careful.

Unfortunately, for unscrupulous companies, consumer ignorance about photovoltaic (PV) solar power is often an opportunity to cheat. In fact, many vendors are in the solar business to chase your money with little interest in the technology, environmental benefits or your needs. Here is some informa-tion that will help you fortify yourself against being shortchanged.

PV solar modules convert energy from the sun into free and non-pol-luting DC power. Unfortunately, this power cannot be directly used due to varying light intensity of the sun over the day. You need to use a current controller to convert it into a constant voltage under all conditions of light.

The only controller type that you have to buy is the one that harvests the maximum solar energy from the sun. This is called maximum peak power tracking (MPPT) controller. MPPT helps you to harvest about 30 per cent more energy from the solar module than any other type. Solar panels are expensive and the MPPT controller costs maximum 10 per cent of the cost of the panel. Since only an MPPT con-troller gives 30 per cent more energy from the same panel, this investment

is fully justified. The sun shines bright during the

day. In India, often there is no electrici-ty even during the day for many hours. During daytime, the MPPT controller can directly feed DC into your inverter and you can use that AC power for feeding electric fans, television or com-puters. For this, you need only a small-capacity battery. The inverter directly uses the solar panel output to give you AC supply. If you are using it for of-fice, you need power mostly during the day, except for some time in the early morning and late evening. A small-size battery is good enough for this.

However, home users need solar power at night after the sun has set. For this, one needs to accumulate all the available solar energy during the day and then use it at night. In such a case, you need a battery with big enough capacity to accumulate all the available solar energy from the sun and deliver it to the inverter at night feeding your 230V AC loads like lights, fans, PCs and TVs.

Depending on your energy need at night, your supplier will recommend certain battery capacity; its voltage and ampere-hour (Ah) capacity. Small home inverters of 300-1500W rating usually work off one or two 12V lead-acid batteries depending on the invert-er power and its input voltage rating.

Electrical power and energy often confuse people. But it is easy to under-stand the difference between the two. Your CFL light, normal bulb, fan and TV are rated by their power consump-tion and the unit used is watt (W) or volt-ampere (VA). You can add all these to find out how much power you

60 • Janua ry 2011 • electronics for you w w w . e f y m a g . c o m

Solar TipS

need to operate them. For example, your three CFL lights (16W×3), a fan (40W) and a TV (60W) will add up to 150W. This is the minimum power rat-ing that you need for your inverter. It is necessary to buy an inverter with at least 50 per cent or even 100 per cent more capacity in watts.

However, for finding your energy needs, you need to multiply the watt rating of the appliance by the time in hours you need to use it. For instance, in the above example, if you need to run these appliances for five hours, your energy need is 150W×5h = 750 Wh. So your inverter could be of 225-300W (continuous) rating but your energy need is higher.

It is important to remember that your battery capacity depends on your energy need. The battery has to be large enough to accumulate from the sun and store 750 Wh of energy and not 150W power as some people often misunderstand.

Your home inverter generally needs 12V or 24V DC input depending on what you buy. This is because lead-acid batteries are often rated at 12V. For 24V, you need to connect two of them in series.

Another rating of the battery is its capacity specified in Ah. A 40Ah battery means that it can deliver 8A for five hours, 2A for 20 hours, etc. In other words, its energy storage capac-ity is 12V×40 Ah=480 Wh.

So if you need 750 Wh as in the example given above, you will need a 12V, 80Ah battery that can store 12x 80=960Wh energy for night use. Even here, it is essential to buy a battery with 50 to 60 per cent more capacity in order to ensure a longer battery life. Selection criteria is similar to that for a water tank capacity; buy always more than what you need.

So for the most economic size of the solar power system you need, calculate how many hours you may want to

use each of your appliances and find energy need of each by multiplying by its wattage rating. For example, using a 15W CFL light for four hours will require 60 Wh of energy. A 40W fan that you may use all night long—say, for ten hours—will require 400 Wh of energy. Add up energy needs of all the appliances to calculate your energy need in Wh.

Your PV solar modules are rated in peak watts. But, in practice, these, on an average, deliver only 70 to 75 per cent power (over about five hours) of the solar energy that you get to ac-cumulate from the sun. Therefore a panel rated at, say, 100Wp, will give you 70W for five hours—that is, energy of 350 Wh (5×0.7×100). So if your en-ergy need is 700 Wh, your PV module should be rated at 200Wp.

The author is ex-chairman, Electronics Commis-sion, government of India, and a former advisor to late Rajiv Gandhi

CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • A U G U S T 2 0 0 7 • 9 7W W W . E F Y M A G . C O M

� PRADEEP VASUDEVA

S.C. DWIVE

DIINFRARED FIRECRACKER IGNITER

F irecrackers are normally ignitedby using a matchstick or acandle. You have to run away

quickly after igniting the fuse of thefirecracker. This method of igniting

firecracker is unsafe, because the dan-ger of the firecracker bursting beforeyou reach a safe distance is alwaysthere.

The device described here uses re-mote control, usually used with TV re-ceivers or CD players, to burst the fire-cracker. Thus the firecracker can be ig-nited from a safe distance using thecircuit described below in conjunction

with the remote control.In the figure shown here, normally

the output of IC1 is low and greenLED2 is ‘on’ and the red LED3 ‘off.’This indicates that the circuit isready for use. When any key on theremote control is pressed, output pin

3 of IRX1 (IR receiver moduleTSOP1738) goes low. This output isconnected to pin 2 of IC1 via LED1and resistor R4 to trigger themonostable operation of IC1.

The output of IC1 remains high fora period equal to 1.1×R2×C2. With thevalues of the components given in thecircuit diagram here, the period worksout to 3.5 seconds approximately. This

activates relay RL1 and red LED3glows and green LED2 turns off. ‘On’state of red LED3 indicates that thefirecracker is about to burst.

R7 is a small part of the element ofan electric heater (220V,1000W), which is kept awayfrom the electronic circuitand connected to the relaycontacts through a thickelectric cable. The resistancevalue of short length of theheater element (R7) is 3 to3.5 ohms. A current ofaround 4 amperes flowsthrough it when connectedto a 12V battery. Flow of 4Acurrent through R7 for 3.5seconds makes it red hot,which ignites the fire-cracker.

The circuit is poweredby a 12V, 7AH battery. IC2provides about 9V for theoperation of the circuit. The

circuit should be housed in a metalliccabinet to prevent it from being dam-aged by bursting of the firecracker. TheIR receiver and the two LEDs shouldbe fixed on the front panel of the cabi-net.

Wiring and relay used in the cir-cuit should be chosen such that theyare able to carry more than 5 amperesof current. �

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112 • Dec em b er 2010 • electronics for you w w w . e f y m a g . c o m

dark gray, blue or even black. They come in various configurations and radiation patterns, but 5mm types with 15- to 40-degree patterns are the most popular.

Typically, IR LEDs run at around 1.3 to 1.7 volts, depending on the

LED current (typically 10 to 30 mA). However, this may vary with the type and manufacturer. Practically, IR il-luminators may have 6 or 60 to 100 or more LEDs, depending on the output needed.

The circuit (refer Fig. 1) can be divided into three parts: ambient light sensor, relay driver and IR LEDs. The ambient light sensor is built around multiturn linear potmeter VR1 and light-dependent resistor LDR1. The relay driver section is built around transistors T1 through T3. The IR LEDs section is built around LED1 through LED40.

The light sensor circuit is a simple transistor switch with the base of the Darlington pair (formed by T1 and T2) connected to a voltage divider. Variable resistor VR1 and the 10mm encapsulated LDR are used to sense the ambient light. As light falls on the surface of LDR1, its resistance changes. The amount of minimum light needed

to actuate the relay through driver transistor T3 can be varied by adjust-ing VR1. Diode 1N4001 eliminates any back voltage when the relay de-energises. Switch S1 is the mains

power on/off switch and switch S2 is added to bypass the ambient light de-tection function.

Relay RL1 energises only when the ambient light level falls below a threshold value set by VR1, i.e., when it’s dark. Normally-opened (N/O) con-tacts of the relay ground path to the IR LEDs (LED1 through LED40) to make them glow. The blue LED (LED41) in-dicates the circuit activity. When there is ambient light and you want to use the illuminator, switch S2 ‘on.’ All the LEDs (LED1 through LED40) glow to fulfil your requirement.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The IR LEDs assembly is very important. A set of 40 (5×8) 5mm infra-red LEDs (IR LED1 through IR LED40) with independent current-limiting resistors (R3 through R10) per string is used. This section is powered by the input DC supply through the relay contacts.

T.K. Hareendran

Infrared IllumInaTors.c. dwivedi

Infrared (IR) illuminators are widely used to improve the image-capturing quality of security cam-

eras fitted in dark zones. Just like our eyes, cameras also can’t record move-

ments in dark. However, un-like our eyes, most of the lat-est cameras can capture infra-red light.

In an IR I l l u m i n a t o r , many infrared

IR LEDs are grouped together to throw good amount of IR light. Typically, LEDs output at 470 nm (blue region), 525 nm (green region) and 625 nm (red region). IR LEDs produce longer wavelengths, 880 nm and 940 nm being the common ones. Most CCD cameras are a little more sensitive to 880 nm, although when these LEDs are used for security applications, some indi-viduals can detect a very dim red glow from them. The 940nm LED radiations are completely invisible to the eye. Some of these LEDs are clear, while others are tinted with pale shades of

Fig. 1: Circuit for infrared illuminator

Fig. 2: Infrared illuminator

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electronics for you • December 2010 • 113w w w . e f y m a g . c o m

Mount IR LEDs on the general-pur-pose PCB board such that these make three circles. After soldering, carefully cut the outside of the circuit board in a round shape and fit it in a suitable metal/plastic cabinet. If available, add

a suitable reflector sheet for the IR LED bank. Finally, fit the LDR bank on top of the enclosure with switches, indicator-sensitivity-control pot and power input socket. Fig. 2 shows the infrared illuminator unit.

To make the circuit actuate the re-lay when the intensity of ambient light is less than the preset light level, throw light on LDR1 and then slowly adjust the potentiometer until LED1 lights up and the relay energises.

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116 • September 2009 • electronics for you w w w . e f y m a g . c o m

Here is a simple but inexpen-sive inverter for using a small soldering iron (25W, 35W,

etc) in the absence of mains supply. It uses eight transistors and a few resis-tors and capacitors.

Transistors T1 and T2 (each BC547) form an astable multivibrator that pro-duces 50Hz signal. The complementary outputs from the collectors of transis-tors T1 and T2 are fed to pnp Darling-ton driver stages formed by transistor

LoveLy T.P.

InverTer for SoLderIng Iron

s.c. dwivedi

pairs T3-T5 and T4-T6 (utilising BC558 and BD140). The outputs from the drivers are fed to transistors T7 and T8 (each 2N3055) connected for push-pull operation. Use suitable heat-sinks for transistors T5 through T8.

A 230V AC primary to 12V-0-12V, 4.5A secondary transformer (X1) is used. The centre-tapped terminal of the secondary of the transformer is connected to the battery (12V, 7Ah), while the other two terminals of the secondary are connected to the collec-tors of power transistors T7 and T8,

respectively.When you power the circuit using

switch S1, transformer X1 produces 230V AC at its primary terminal. This voltage can be used to heat your sol-dering iron.

Assemble the circuit on a general-purpose PCB and house in a suitable

cabinet. Connect the battery and trans-former with suita-ble current-carrying wires. On the front panel of the box, fit power switch S1 and a 3-pin socket for con-necting the soldering iron.

Note that the rat-ings of the battery, transistors T7 and T8, and transformer may vary as these all depend on the load (soldering iron).

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1 0 8 • J A N U A R Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

Using this circuit, audio musi-cal notes can be generatedand heard up to a distance of

Fig. 1: Transmitter circuit

Fig. 2: IR audio receiver circuit

� PRADEEP G.

IR MUSIC TRANSMITTER ANDRECEIVER

S.C. DWIVE

DI10 metres.The circuit can be divided into

two parts: IR music transmitter andreceiver. The IR music transmitterworks off a 9V battery, while the

IR music re-ceiver works offregulated 9V to12V.

Fig. 1 showsthe circuit ofthe IR musictransmitter. Ituses popularmelody genera-tor IC UM66(IC1) that cancont inuous lygenerate musi-cal tones. Theoutput of IC1 isfed to the IR

driver stage (built across the transis-tors T1 and T2) to get the maximumrange.

Here the red LED (LED1) flickersaccording to the musical tones gener-ated by UM66 IC, indicating modula-tion. IR LED2 and LED3 are infraredtransmitting LEDs. For maximumsound transmission these should beoriented towards IR phototransistorL14F1 (T3).

The IR music receiver uses popu-lar op-amp IC µA741 and audio-fre-quency amplifier IC LM386 along withphototransistor L14F1 and some dis-crete components (Fig. 2).

The melody generated by IC UM66is transmitted through IR LEDs, re-

ceived by phototransistorT3 and fed to pin 2 ofIC µA741 (IC2). Its gaincan be varied usingpotmeter VR1. The outputof IC µA741 is fed to ICLM386 (IC3) via capaci-tor C5 and potmeter VR2.The melody producedis heard through thereceiver’s loudspeaker.Potmeter VR2 is used tocontrol the volume ofloudspeaker LS1 (8-ohm,1W).

Switching off thepower supply stopsmelody generation. �

CIRCUIT

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1 2 2 • O C T O B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

age through a water-activated tiltswitch only when the probes in thetilt switch make contact with water.When the tilt switch is kept in the hori-zontal position, the inverting input ofIC1 gets a higher voltage than its non-inverting input and the output remainslow.

IC CD4538 (IC2) is used as amonostable with timing elements R5and C1. With the shown values, theoutput of IC2 remains low for a pe-riod of three minutes. CD4538 is a pre-cision monostable multivibrator freefrom false triggering and is more reli-able than the popular timer IC 555. Itsoutput becomes high when power isswitched on and it becomes low whenthe trigger input (pin 5) gets a low-to-high transition pulse.

The unit is fixed inside the laptopcase in horizontal position. In this po-sition, water inside the tilt switch ef-

fectively shorts the contacts, so the out-put of IC1 remains low. The alarm gen-erator remains silent in the standbymode as trigger pin 5 of IC2 is low.

When someone tries to take thelaptop case, the unit takes the verticalposition and the tilt switch breaks theelectrical contact between the probes.Immediately the output of IC1 be-comes high and monostable IC2 is trig-gered. The low output from IC2 trig-gers the pnp transistor (T1) and thebuzzer starts beeping.

Assemble the circuit as compactlyas possible so as to make the unitmatchbox size. Make the tilt switch us-ing a small (2.5cm long and 1cm wide)plastic bottle with two stainless pinsas contacts. Fill two-third of the bottlewith water such that the contacts nevermake electrical path when the tiltswitch is in vertical position. Make thebottle leakproof with adhesive or wax.

Fix the tilt switchinside the enclo-sure of the circuitin horizontal posi-tion. Fit the unit in-side the laptopcase in horizontalposition using ad-hesive.

Use a minia-ture buzzer and amicro switch (S1)to make the gadgetcompact. Keep thelaptop case in hori-zontal position andswitch on the unit.Your laptop is nowprotected. �

P rotect your valuable laptopagainst theft using this minia-ture alarm generator. Fixed in-

side the laptop case, it will sound aloud alarm when someone tries to takethe laptop. This highly sensitive cir-cuit uses a homemade tilt switch toactivate the alarm through tilting ofthe laptop case.

The circuit uses readily availablecomponents and can be assembled ona small piece of Vero board or a gen-eral-purpose PCB. It is powered by a12V miniature battery used in remotecontrol devices.

IC TLO71 (IC1) is used as a volt-age comparator with a potential di-vider comprising R2 and R3 provid-ing half supply voltage at the non-in-verting input (pin 3) of IC1. The in-verting input receives a higher volt-

� D. MOHAN KUMAR

LAPTOP PROTECTOR S.C. DWIVE

DI

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electronics for you • january 2009 • 97w w w . e f y m a g . c o m

This automatic door opener can be made using readily available components. The electromag-

netic relay at the output of this gadget can be used to control the DC/AC door-opener motor/solenoid of an electromechanical door opener as-sembly, with slight intervention in its electrical wiring.

A laser diode (LED1) is used here as the light transmitter. Alternatively, you can use any available laser pointer. The combination of resistor R1 and diode D1 protects the laser diode from over-current flow. By varying muliturn trimpot VR1, you can adjust the sensitivity. (Note that ambient light reflections may slightly degrade the performance of this unit.)

Initially, when the laser beam is falling on photo-transistor T1, it con-

ducts to reverse-bias transistor T3 and the input to the first gate (N1) of IC1 (CD4001) is low. The high output at pin 3 of gate N1 forward biases the LED-driver transistor (T4) and the green standby LED (LED2) lights up continuously. The rest of the circuit remains in standby state.

When someone interrupts the laser beam, photo-transistor T1 stops conducting and transistor T3 becomes forward-biased. This makes the output of gate N1 go low. Thus LED-driver transistor T4 becomes reverse-biased and LED2 stops glowing. At the same time, the low output of gate N1 makes the output of N2 high. Instantly, this high level at pin 4 of gate N2 triggers the monostable multivibrator built around the remaining two gates of IC1 (N3 and N4). Values of resistor R8 and capacitor C1 determine the time period of the monostable.

T.K. Hareendran

Laser-guided door opener s.c. dwivedi

The second monostable built around IC2 (CD4538) is enabled by the high-going pulse at its input pin 12 through the output of gate N4 of the first monostable when the laser beam is interrupted. As a result, relay RL1 energises and the door-opener motor starts operating. LED3 glows to indicate that the door-opener motor is getting the supply. At the same time, piezobuzzer PZ1 sounds an alert. Transistor T5, whose base is connected to Q output (pin 10) of IC2, is used for driving the relay. Transistor T6, whose base is connected to Q output of IC2, is used for driving the intermittent pi-ezobuzzer. ‘On’ time of relay RL1 can be adjusted by varying trimpot VR2. Resistor R9, variable resistor VR2 and capacitor C3 decide the time period of the second monostable and through it

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98 • january 2009 • electronics for you w w w . e f y m a g . c o m

on time of RL1.The circuit works off 12V DC

power supply. Assemble it on a gen-eral-purpose PCB. After construction, mount the laser diode and the pho-

totransistor on opposite sides of the doorframe and align them such that the light beam from the laser diode falls on the phototransistor directly. The motor connected to the pole of

relay contacts is the one used in elec-tromechanical door-opener assembly. If you want to use a DC motor, replace mains AC connection with a DC power supply.

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9 2 • D E C E M B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� RAJ K. GORKHALI

LED LIGHTING FOR CHRISTMAS S.C. DWIVE

DI

U sing light effects for decora-tion on festive occasions is anormal practice. Designers are

coming up with varieties of electroniccircuits to fill the imagination of us-ers.

Here is an easy-to-assemble circuitfor christmas decoration as shown inFig.1. It comprises four transistors,eighteen LEDs, a few resistors and twocapacitors. Transistors T1 and T2 areconfigured as an astable multivibrator,which means one of the two transis-tors is always conducting. Thus thecombination produces clock pulses.

The values of time-constantsformed with R6-C2 and R8-C1 pairshave been selected to produce a low-frequency clock that is visible to hu-man eye. The collectors of transistors

T1 and T2 are connected to driver tran-sistors T3 and T4. These are used tolight up two rows of LEDs connectedin parallel with alternate clock pulses.The frequency at which LED1 throughLED9, and LED10 through LED18, al-ternately light up is about 2 Hz. Youcan easily change this frequency bychanging the values of capacitors C2and C1.

Resistors R2 and R4 are used to setthe current through the LEDs. Red(LED1 through LED9) and green LEDs(LED10 through LED18) are used forsimulating christmas decoration effects.For the brightness variation, you canchange the values of resitors R2 and R4.

Take any general-purpose PCB andcut it into a star shape. Thereafter, as-semble the circuit and solder the colourLEDs onto it such that it looks like achristmas star.

Alternatively, you can design thePCB in circular shape with a festivewhite lacquer finish on component sideand conductor tracks on the other. Placethe control circuit at the centre of thePCB board, with LEDs mounted alongthe outer edge as shown in Fig. 2. Alongthis edge, there are three circular tracks:The middle one is the positive supply,which goes to the anodes of all LEDs.The outer track is connected to the cath-odes of the red LEDs and the innertracks are connected to the cathodes ofthe green LEDs.

To obtain the best effect with thecombination of red and green LEDs,mount them alternately on the PCBboard. Exercise care so that you donot accidentally connect the red andgreen LEDs in parallel. The forwardvoltage drops of red and green LEDsare different.

The circuit works off a 3V-9V bat-tery. It consumes little current, so two/four AA cells or a 9V battery can easilypower the electronic star. You can alsouse a stabilised 3V-9V DC mains adap-tor in place of the battery. �Fig.1: LED lighting circuit for Christmas

Fig.2: Christmas Star

C I R C U I T I D E A S

ELECTRONICS FOR YOUJANUARY 2004 119

LED-BASEDMESSAGE DISPLAY

SANI THEO

S.C. DWIVEDI

This LED-based message display isbuilt around readily availble, low-cost components. It is easy to fabri-

cate and makes use of 3mm red LEDs. Atotal of 172 LEDs have been arranged todisplay the message “HAPPY NEW YEAR2004.”

The arrangement of LED1 throughLED11 is used to display ‘H’ as shown inFig. 1. The anodes of LED1 through LED11are connected to point A and the cath-odes of these LEDs are connected to pointB. Similarly, letter ‘A’ is built using LED12through LED21. All the anodes of LED12through LED21 are connected to point A,while the cathodes of these LEDs are con-nected to resistor R8 (not shown in thecircuit diagram). Other letters/words canalso be easily arranged to make the re-quired sentence.

The power supply for the message dis-play circuit (Fig. 2) comprises a 0-9V, 2Astep-down transformer (X1), bridge recti-fier comprising diodes D1 through D4,and a filter capacitor (C1). IC 7806 (IC1)

Fig. 1: LED arrangement for word ‘H’

Fig. 2: Circuit diagram

of LED-based m

essage display

C I R C U I T I D E A S

ELECTRONICS FOR YOU JANUARY 2004120

provides regulated 6V DC to the displaycircuit comprising timer 555 (IC2) anddecade counter CD4017 (IC3). The astablemultivibrator built around IC2 produces1Hz clock at its output pin 3. This outputis connected to clock pin (pin 14) of thedecade counter.

The decade counter can count up to10. The output of IC3 advances by onecount every second (depending on thetime period of astable multivibrator IC2).

When Q1 output of IC3 goes high, tran-sistor T1 conducts and the current flowsthrough LED1 through LED48 via resistorsR7 through R11. Now the word ‘HAPPY’built around LED1 through LED48 is dis-played on the LED arrangement board.

Next, when Q2 output of IC3 goes

high, transistor T2 conducts and the cur-rent flows through LED49 through LED87via resistors R12 through R14. Now theword ‘NEW’ is displayed on the LED ar-rangement board.

Again, when Q3 output goes high, tran-sistor T3 conducts and the current flowsthrough LED88 through LED128 via resis-tors R15 through R18. Now the word‘YEAR’ is displayed on the LED arrange-ment board.

Similarly, when Q4 output goes high,transistor T4 conducts and the currentflows through LED129 through LED172via resistors R19 through R22. Now dig-its ‘2004’ are displayed on the LED ar-rangement board.

During the entire period when Q5,

Q6, Q7, or Q8 output go high, transistorT5 conducts and the current flows throughall the LEDs via diodes D9 through D12and resistors R7 through R22. Now thecomplete message “HAPPY NEW YEAR2004” is displayed on the LED arrange-ment for four seconds.

Thus, the display board displays‘HAPPY,’ ‘NEW,’ YEAR’ and ‘2004’ oneafter another for one second each. Afterthat, the message “HAPPY NEW YEAR2004” is displayed for 4 seconds (becauseQ5 through Q8 are connected to resistorR6 via diodes D5 through D8).

At the next clock input output Q9 goeshigh, and IC3 is reset and the display isturned off for one second. Thereafter thecycle repeats.

CIRCUIT

IDEAS

8 4 • J U N E 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� D. MOHAN KUMAR

LIGHT FENCE S.C. DWIVE

DI

T he basic problem with most ofstandard light sensors is thatthey require precise alignment

of light beam to mute the circuit dur-

ing standby mode. The circuit de-scribed here is so sensitive that it willdetect a moving person at a distanceof few metres in daylight or underelectric lighting without cumbersomealignment of light beam. It requires vir-tually no set up, and may be simplyplaced within the line-of-sight of al-most any light source including ambi-

ent day light or fluorescent electriclight. The beep generated from the cir-cuit will be loud enough to detect theentry of a person in the room or theprotected area being guarded.

The circuit uses a voltage compara-tor and a monostable timer to give thewarning alarm on detecting a movingperson. IC µA741 (IC1) is used as avoltage comparator with two poten-tial dividers in its inverting and non-inverting inputs. Resistors R1 and R2provide half-supply voltage of 4.5volts to its inverting input (pin 2).

LDR1 and preset VR1 form anotherpotential divider to provide a variablevoltage input to the non-inverting in-put (pin 3).

If VR1 is properly adjusted for therequired light level, the output of IC1will be high, which drives pnp tran-sistor T1 out of conduction. This is dueto the high potential at the base of T1.The emitter voltage of T1 will be high

in this condition, which inhibitsIC2 from oscillation and LED1from lighting. IC2 is wired as amonostable timer. R6 and C2 pro-vide a preset time delay.

As a person crosses the pro-tected area, his shadow will besensed by LDR1 due to changein the light intensity leveland the voltage at the non-invert-ing input of IC1 will drop mo-mentarily. The output of IC1 sud-denly becomes low, allowing T1to conduct. This triggers themonostable (IC2) and the alarmsounds.

Assemble the circuit on acommon PCB and house in aplastic case. Keep LDR1 inside ablack tube to increase its sensi-tivity. Adjust preset VR1

until LED1 turns off at the particularlight level. Keep LDR1 facingthe entrance of the room or the areato be protected. Sensitivity of the cir-cuit depends on the proper adjust-ment of VR1. If VR1 is correctly ad-justed, the circuit can detect a movingperson from a distance of about threemetres. �

circuitideas

electronics for you • July 2009 • 85w w w . e f y m a g . c o m

LoveLy T.P.

Liquid LeveL ALArm s.c. dwivedi

Here is a simple circuit for liquid level alarm. It is built around two BC547 transistors

(T1 and T2) and two timer 555 ICs (IC1 and IC2). Both IC1 and IC2 are wired in astable multivibrator mode. Timer IC1 produces low frequency, while timer IC2 produces high frequency. As a result, a beeping tone is generated when the liquid tank is full.

Initially, when the tank is empty, transistor T1 does not conduct. Con-sequently, transistor T2 conducts and

pin 4 of IC1 is low. This low voltage disables IC1 and it does not oscillate. The low output of IC1 disables IC2 and it does not oscillate. As a result, no sound is heard from the speaker.

But when the tank gets filled up, transistor T1 conducts. Consequently, transistor T2 is cut off and pin 4 of IC1 becomes high. This high voltage enables IC1 and it oscillates to produce low frequencies at pin 3. This low-fre-quency output enables IC2 and it also oscillates to produce high frequencies. As a result, sound is produced from the speaker. Using preset VR1 you can

control the volume of the sound from the speaker.

The circuit can be powered from a 9V battery or from mains by using a 9V power adaptor.

Assemble the circuit on a general-purpose PCB and enclose in a suit-able cabinet. Install two water-level probes using metal strips such that one touches the bottom of the tank and the other touches the maximum level of the water in the tank. Interconnect the sensor and the circuit using a flexible wire.

CIRCUIT

IDEAS

9 2 • M A Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� D. MOHAN KUMAR

MAT SWITCH S.C. DWIVE

DI

This simple circuit produces awarning beep when somebodycrosses a protected area in your

home or office. The switch, hidden be-low the floor mat, triggers the alarmwhen the person walks over it.

The circuit uses a conductive foam

as the switch. It can be two smallpieces of conductive pads usually usedto pack sensitive ICs as antistatic cover.Alternatively, you can make the switchby coating conducting carbon ink ontwo small pieces of a copper-cladboard.

When the circuit is in standbymode, transistor T1 does not conduct,

since its base isfloating. Whenthe personwalks, the switchis pressed andcurrent flowsthrough R1 andthe switch toprovide positivebias to transistorT1. Transistor T1conducts and itscollector voltagedrops, whichacts as a negativetrigger input forthe monostable

wired around IC NE555 (IC1).IC1 outputs a pulse of fifty-seconds

duration with preset values of R4 andC3. This pulse is applied to the buzzerthrough transistor T2. The buzzersounds a warning beep onunauthorised entry. The pulse dura-tion can be changed to the desiredvalue by changing the values of R4 andC 3 .Resistor R2 in the circuit makes thetrigger pin of IC1 high to prevent falsetriggering.

Assemble the circuit on a general-purpose PCB and enclose in a plasticcase. Use a 9V battery to power thecircuit. Connect the touchpad switchwith the PCB and hide under the matat the entrance. The PCB can bemounted on the nearby wall.

Make the switch carefully usingconducting foam or copper clad coatedwith conducting ink. Place the twopieces with their conducting surfacefacing each other. Solder carefully athin copper electric wire and ensurethat it makes contact when the twoplates touch together on pressing. Pro-vide two 1cm rubber tabs between theplates to avoid touch in the standbymode. �

circuitideas

electronics for you • February 2009 • 95w w w . e F y m a g . c o m

Most thefts happen after midnight hours when peo-ple enter the second phase

of sleep called ‘paradoxical’ sleep. Here is an energy-saving circuit that causes the thieves to abort the theft attempt by lighting up the possible sites of intrusion (such as kitchen or backyard of your house) at around 1:00 am. It automatically resets in the morning.

The circuit is fully automatic and uses a CMOS IC CD 4060 to get the desired time delay. Light-dependent resistor LDR1 controls reset pin 12 of IC1 for its automatic action.

During day time, the low resist-ance of LDR1 makes pin 12 of IC1 ‘high,’ so it doesn’t oscillate. After

sunset, the high resistance of LDR1 makes pin 12 of IC1 ‘low’ and it starts oscillating, which is indicated by the flashing of LED2 connected to pin 7 of IC1. The values of oscillator compo-nents (resistors R1 and R2 and capaci-tor C4) are chosen such that output pin 3 of IC1 goes ‘high’ after seven hours, i.e., around 1 am. This high output drives triac 1 (BT136) through LED1 and R3.

Bulb L1 connected between the phase line and M2 terminal of triac 1 turns on when the gate of triac 1 gets the trigger voltage from pin 3 of IC1. It remains ‘on’ until pin 12 of IC1 be-comes high again in the morning.

Capacitors C1 and C3 act as power reserves, so IC1 keeps oscillating even if there is power interruption for a few seconds. Capacitor C2 keeps trigger

pin 12 of IC1 high during day time, so slight changes in light intensity don’t affect the circuit. Using preset VR1 you can adjust the sensitivity of LDR1.

Power supply to the circuit is de-rived from a step-down transformer X1 (230V AC primary to 0-9V, 300mA secondary), rectified by a full-wave rectifier comprising diodes D1 through D4 and filtered by capacitor C1.

Assemble the circuit on a general-purpose PCB with adequate spacing between the components. Sleeve the exposed leads of the components. Using switch S1 you can turn on the lamp manually. Enclose the unit in a plastic case and mount at a location

that allows adequate daylight.

Caution. Since the circuit uses 230V AC, many of its points are at AC mains voltage. It could give you lethal shock if you are not careful. So if you don’t know much about work-ing with line voltages, do not attempt to con-struct this circuit. EFY will not be responsible for any kind of resulting loss or damage.

D. MOHAN KUMAR

MIDNIGHT secURITy lIGHTs.c. dwivedi

circuitideas

94 • November 2009 • electronics for you w w w . e f y m a g . c o m

D. Mohan KuMar

Mini uPS SySteM s.c. dwivedi

This circuit provides an uninter-rupted power supply (UPS) to operate 12V, 9V and 5V

DC-powered instruments at up to 1A current. The backup battery takes up the load without spikes or delay when the mains power gets interrupted. It can also be used as a workbench power supply that provides 12V, 9V and 5V operating voltages. The circuit im-

mediately disconnects the load when the battery voltage reduces to 10.5V to prevent deep discharge of the battery. LED1 indication is provided to show the full charge voltage level of the bat-tery. miniature white LEDs (LED2 and LED3) are used as emergency lamps during power failure at night.

A standard step-down trans-former provides 12V of AC, which is rectified by diodes D1 and D2. Ca-pacitor C1 provides ripple-free DC to charge the battery and to the remain-ing circuit. When the mains power is on, diode D3 gets forward biased to charge the battery. Resistor R1 limits the charging current. Potentiometer VR1 (10k) with transistor T1 acts as the voltage comparator to indicate the voltage level. VR1 is so adjusted that LED1 is in the ‘off’ mode. When the battery is fully charged, LED1 glows indicating a full voltage level of 12V.

When the mains power fails, diode D3 gets reverse biased and D4 gets forward biased so that the battery can automatically take up the load without any delay. When the battery voltage or input voltage falls below 10.5V, a cut-off circuit is used to prevent deep discharging of the battery. Resistor R3, zener diode ZD1 (10.5V) and transistor T2 form the cut-off circuit. When the volt-age level is above 10.5V, transistor

T2 conducts and its base becomes negative (as set by R3, VR2 and ZD1). But when the voltage reduces below 10.5V, the zener diode stops conduc-tion and the base voltage of transis-tor T2 becomes positive. It goes into the ‘cut-off’ mode and prevents the current in the output stage. Preset VR2 (22k) adjusts the voltage below 0.6V to make T2 work if the voltage is above 10.5V.

When power from the mains is available, all output voltages—12V, 9V and 5V—are ready to run the load. On the other hand, when the mains power is down, output volt-ages can run the load only when the battery is fully charged (as indicated by LED1). For the partially charged battery, only 9V and 5V are available. Also, no output is available when the voltage goes below 10.5V. If battery voltage varies between 10.5V and 13V, output at terminal A may also

vary between 10.5V and 12V, when the UPS system is in battery mode.

Outputs at points B and C provide 9V and 5V, respectively, through regu-lator ICs (IC1 and IC2), while output A provides 12V through the zener diode. The emergency lamp uses two ultra-bright white LEDs (LED2 and LED3) with current limiting resistors R5 and R6. The lamp can be manually

switched ‘on’ and ‘off’ by S1.The circuit is assembled on a gen-

eral-purpose PCB. There is adequate space between the components to avoid overlapping. heat sinks for tran-sistor T2 and regulator ICs (7809 and 7805) to dissipate heat are used.

The positive and negative rails should be strong enough to handle high current. Before connecting the circuit to the battery and transformer, connect it to a variable power supply. Provide 12V DC and adjust VR1 till LED1 glows. After setting the high voltage level, reduce the voltage to 10.5V and adjust VR2 till the output trips off. After the settings are com-plete, remove the variable power sup-ply and connect a fully-charged battery to the terminals and see that LED1 is on. After making all the adjustments connect the circuit to the battery and transformer. The battery used in the circuit is a 12V, 4.5Ah UPS battery.

CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • J A N U A R Y 2 0 0 8 • 1 3 5W W W . E F Y M A G . C O M

� D. MOHAN KUMAR

MOBILE BUGS.C.

DWIVEDI

T his handy, pocket-size mobiletransmission detector can sensethe presence of an activated

mobile phone from a distance of one-and-a-half metres. So it can be used toprevent use of mobile phones in ex-amination halls, confidential rooms,etc. It is also useful for detecting theuse of mobile phone for spying andunauthorised video transmission.

The circuit can detect both the in-coming and outgoing calls, SMS andvideo transmission even if the mobilephone is kept in the silent mode. Themoment the bug detects RF transmis-sion signal from an activated mobilephone, it starts sounding a beep alarmand the LED blinks. The alarm contin-ues until the signal transmission ceases.

An ordinary RF detector usingtuned LC circuits is not suitable fordetecting signals in the GHz frequencyband used in mobile phones. Thetransmission frequency of mobilephones ranges from 0.9 to 3 GHz witha wavelength of 3.3 to 10 cm. So a cir-cuit detecting gigahertz signals is re-

quired for a mobile bug.Here the circuit uses a 0.22µF disk

capacitor (C3) to capture the RF signalsfrom the mobile phone. The lead lengthof the capacitor is fixed as 18 mm witha spacing of 8 mm between the leads toget the desired frequency. The disk ca-pacitor along with the leads acts as asmall gigahertz loop antenna to collectthe RF signals from the mobile phone.

Op-amp IC CA3130 (IC1) is usedin the circuit as a current-to-voltageconverter with capacitor C3 connectedbetween its inverting and non-invert-ing inputs. It is a CMOS version usinggate-protected p-channel MOSFETtransistors in the input to provide veryhigh input impedance, very low inputcurrent and very high speed of perfor-mance. The output CMOS transistoris capable of swinging the output volt-age to within 10 mV of either supplyvoltage terminal.

Capacitor C3 in conjunction withthe lead inductance acts as a transmis-sion line that intercepts the signalsfrom the mobile phone. This capacitor

creates a field, stores energy and trans-fers the stored energy in the form ofminute current to the inputs of IC1.This will upset the balanced input ofIC1 and convert the current into thecorresponding output voltage.

Capacitor C4 along with high-valueresistor R1 keeps the non-inverting in-put stable for easy swing of the out-put to high state. Resistor R2 providesthe discharge path for capacitor C4.Feedback resistor R3 makes the invert-ing input high when the output be-comes high. Capacitor C5 (47pF) isconnected across ‘strobe’ (pin 8) and

‘null’ inputs (pin 1) of IC1 for phasecompensation and gain control tooptimise the frequency response.

When the mobile phone signal isdetected by C3, the output of IC1 be-comes high and low alternately ac-cording to the frequency of the signalas indicated by LED1. This triggersmonostable timer IC2 through capaci-tor C7. Capacitor C6 maintains thebase bias of transistor T1 for fastswitching action. The low-value tim-ing components R6 and C9 producevery short time delay to avoid audionuisance.

Assemble the circuit on a general-purpose PCB as compact as possibleand enclose in a small box like junkmobile case. As mentioned earlier, ca-pacitor C3 should have a lead lengthof 18 mm with lead spacing of 8 mm.Carefully solder the capacitor in stand-ing position with equal spacing of theleads. The response can be optimisedby trimming the lead length of C3 forthe desired frequency. You may use ashort telescopic type antenna.

Use the miniature 12V battery of aremote control and a small buzzer tomake the gadget pocket-size. The unitwill give the warning indication ifsomeone uses mobile phone within aradius of 1.5 metres. �

circuitideas

118 • OctO b er 2010 • electronics for you w w w . e f y m a g . c O m

T.K. Hareendran

Mobile Car STereo Player

Using a mobile phone while driving is dangerous. It is also against the law. However,

you can use your mobile phone as a

powerful music player with the help of a stereo power amplifier. This does away with the need of a sophisticated in-dash car music system.

Most mobile phones have a music player that offers a number of features

including pre-s e t / m a n u a l sound equal-i sers . They have standard 3.5mm stereo sockets that allow music to be played through stand-a r d s t e r e o headphones/sound ampli-fiers. Nokia 2700 classic is an example.

A car au-dio amplifier with 3.5mm socket can be designed and simply connected to the mobile phone output via a shield-ed cable with suitable connectors/jacks (readymade 3.5mm male-to- male connector cable is a good alter-native).

Fig. 1 shows the circuit of car ste-reo player. It is built around popular single-chip audio power amplifier TDA1554Q (IC1). The TDA1554Q is an integrated class-B power amplifier in a 17-lead single-in-line (SIL) plastic power package.

IC TDA1554Q contains four 11W identical amplifiers with differential input stages (two inverting and two

s.c. dwivedi

Fig. 1: Circuit of mobile car stereo player

Fig. 3: Proposed enclosure

Fig. 2: (a) 3.5mm stereo socket and (b) 3.5mm stereo jack

non-inverting) and can be used for single-ended or bridge applications. The gain of each amplifier is fixed at 20 dB. Here it is configured as two 22W stereo bridge amplifiers.

The amplifier is powered from the 12V car battery through RCA socket J2. Diode D1 protects against wrong-polarity connection. LED1 indicates the power status.

Connect stereo sound signal from the 3.5mm headset socket of the mo-bile phone to audio input socket J1. When you play the music from your mobile, IC1 amplifies the input. The output of IC1 is fed to speakers LS1 and LS2 fitted at a suitable place in your car. Electrolytic capacitor C5 connected between pin 4 of IC1 and GND improves the supply-voltage ripple rejection. Components R2 and C4 connected at mute/standby pin (pin 14) of IC1 eliminate the switch on/off plop.

The circuit is quite compact. A good-quality heat-sink assembly is crucial for IC1. Fig. 2 shows the stereo socket and stereo jack.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Small dimensions of the power amplifier make it suitable for being en-closed in a plastic (ABS) case with vent holes. Signal input socket, speaker out-put terminals, on/off switch, indicator, fuse holder and power supply socket are best located on the front panel of the enclosure as shown in Fig. 3.

CIRCUIT

IDEAS

9 2 • A U G U S T 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� D. MOHAN KUMAR

MOBILE SHIELD MALAYAPPAS

AMY

P rotect your mobile phone fromunauthorised use or theft usingthis simple circuit. It can gener-

ate a loud chirping sound when some-body attempts to take away the mo-bile handset. The added feature is thatthe circuit also works as a mobilecharger.

The circuit is powered by astep-down transformer X1 with recti-fier diodes D1 and D2 and filter ca-

pacitor C1. Regulator IC 7812 (IC1)along with noise filter capacitors C2and C3 provides regulated power sup-ply.

The circuit utilises two NE555timer ICs: One as a simple astablemultivibrator (IC2) and the second asa monostable (IC3). The astablemultivibrator has timing resistors R1and R2 but no timing capacitor as itworks with stray capacitance. Its pins6 and 2 are directly connected to a pro-tecting shield made up of 10cm×10cmcopper-clad board.

The inherent stray capacitance ofthe circuit is sufficient to given an out-put frequency of about 25 kHz withR1 and R2. This arrangement provides

greater sensitivity and enables the cir-cuit with hand capacitance effect. Out-put pulses from the oscillator are di-rectly given to trigger pin 2 of themonostable. The monostable uses alow-value capacitor C6, resistors R3and preset VR1 for timing.

The output frequency of themonostable is adjusted using presetVR1 such that it is slightly less thanthat of the astable. This makes thecircuit standby, when there is nohand capacitance present. So in the

standby mode, the astable’s outputwill be low. This makes the triggerinput of monostable low and outputhigh.

The warning LED1 and buzzer areconnected such that they become ac-tive only when the output of themonostable sinks current. In thestandby state, the LED1 remains ‘off’and the buzzer is silent. As somebodytries to take the mobile phone fromthe protecting shield, his hand comesnear the shield or makes contact withthe shield, which introduces hand ca-pacitance in the circuit. As a result, theastable’s frequency changes, whichmakes the trigger pin of themonostable low and its output oscil-

lates. This produces chirping soundfrom the buzzer and also makes theLED1 blink.

The circuit can also be used as amobile charger. It provides output of6V at 180 mA through regulator IC7806 (IC4) and resistor R5 for charg-ing the mobile phone. Diode D3 pro-tects the output from polarity rever-sal.

The circuit can be wired on acommon PCB. Enclose it in a suitablecase with provision for charger out-

put leads. Make the protective shieldusing 10cm×10cm copper-clad boardor aluminium sheet. Connect it tothe circuit using a 15cm plastic wire.Leads of all capacitors should beshort.

Adjust VR1 slowly using a plasticscrewdriver until the buzzer stopssounding. Bring the hand close to theshield and adjust VR1 until the buzzersounds. With trial-and-error proce-dure, set it for the maximum sensitiv-ity such that as soon the hand comesnear the shield, the buzzer startschirpring and the LED blinks. Insteadof using the copper cladding for shield,a metallic mobile phone holder can beused as the shield. �

CIRCUIT

IDEAS

1 1 6 • O C T O B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

T his simple-to-build alarm canbe fitted in bikes to protectthem from being stolen. The

tiny circuit can be hidden anywhere,without any complicated wiring. Vir-tually, it suits all bikes as long as theyhave a battery. It doesn’t drain out thebattery though as the standby currentis zero.

The hidden switch S1 can be asmall push-to-on switch, or a reedswitch with magnet, or any other simi-lar simple arrangement. The circuitis designed around a couple of low-voltage MOSFETs configured asmonostable timers. Motorbike key S2is an ignition switch, while switch S3is a tilt switch.

Motorbike key S2 provides powersupply to the gate of MOSFET T2, when

turned on. When you turn ignition offusing key S2, you have approximately15 seconds to get off the bike; this func-tion is performed by resistor R6 to dis-charge capacitor C3. Thereafter, if any-one attempts to get on the bike or moveit, the alarm sounds for approximately15 seconds and also disconnects theignition circuit.

During parking, hidden switch S1is normally open and does not allowtriggering of MOSFET T1. But whensomeone starts the motorbike throughignition switch S2, MOSFET T2 trig-gers through diode D1 andresistor R5. Relay RL1 (12V,2C/O) energises to activatethe alarm (built aroundIC1) as well as to discon-nect the ignition coil fromthe circuit. Disconnection ofthe ignition coil prevents

� T.A. BABU

MOTORBIKE ALARM S.C. DWIVE

DI

generation of spark from the spark plug.Usually, there is a wire running fromthe alternator to the ignition coil, whichhas to be routed through one of the N/C1 contacts of relay RL1 as shown inFig. 1. Fig. 2 shows the pin configura-tions of SCR BT169, MOSFET BS170and transistor BC548.

Also, on disconnection of the coil,sound generator IC UM3561 (IC1) getspower supply through N/O2 contactof relay RL1. This drives the darlingtonpair built around T3 and T4 to pro-duce the siren sound through loud-speaker LS1.

To start the vehicle, both hiddenswitch S1 and ignition key S2 shouldbe switched on. Otherwise, the alarmwill start sounding. Switching on S1

triggers SCR1, which, inturn, triggers MOSFET T1.MOSFET T1 is configuredto disable MOSFET T2from functioning. As a re-sult, MOSFET T2 does nottrigger and relay RL1 re-mains de-energised, alarm

Fig. 1: Cheap motorbike alarm

Fig. 2: Pin configurations ofBT169, BS170 and BC548

CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • O C T O B E R 2 0 0 7 • 1 1 7W W W . E F Y M A G . C O M

deactivated and ignition coil connectedto the circuit. Connection to the igni-tion coil helps in generation of sparkfrom the spark plug. Keeping hiddenswitch S1 accessible only to the ownerprevents the bike from pillaging.

Tilt switch S3 prevents attempt tomove the vehicle without starting it.Glass- and metal-bodied versions ofthe switch offer bounce-free switchingand quick break action even whentilted slowly. Unless otherwise stated,

the angle by which the switch must betilted to ensure the contact operation(operating angle), must be approxi-mately 1.5 to 2 times the stated differ-ential angle. The differential angle isthe measure of the ‘just closed’ posi-tion to the ‘just open’ position.

The tilt switch has characteristicslike contacts make and break with vi-bration, return to the open state at rest,non-position sensitivity, inert gas andhermetic sealing for protection of con-

tacts and tin-plated steel housing. Ifyou find difficulty in getting the tiltswitch, you may replace it with a reedswitch (N/O) and a piece of magnet.The magnet and the reed switchshould be mounted such that the con-tacts of the switch close when the bikestand is lifted up from rest.

EFY note. Make sure that whiledriving, the two internal contacts ofthe Tilt switch don’t touch eachother. �

circuitideas

118 • August 2009 • electronics for you w w w . e f y m A g . c o m

PradeeP G.

Multitone Siren s.c. dwivedi

This multitone siren is useful for burglar alarms, reverse horns, etc. It produces five different

audio tones and is much more ear-catching than a single-tone siren.

The circuit is built around popular CMOS oscillator-cum-divider IC 4060

and small audio amplifier LM386. IC 4060 is used as the multitone genera-tor. A 100µH inductor is used at the input of IC 4060. So it oscillates within the range of about 5MHz RF. IC 4060 itself divides RF signals into AF and ultrasonic ranges. Audio signals of different frequencies are available at pins 1, 2, 3, 13 and 15 of IC 4060 (IC1).

These multifrequency signals are mixed and fed to the audio amplifier built around IC LM386.

The output of IC2 is fed to the speaker through capacitor C9. If you want louder sound, use power ampli-fier TBA810 or TDA1010.

Only five out-puts of IC1 are used here as the other five outputs (pins 4 through 7 and 14) produce ultrasonic signals, which are not audible.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a suitable cabinet. Regulated 6V-12V (or a battery) can be used to power the circuit.

circuitideas

96 • october 2008 • electronics for you w w w . e f y m a g . c o m

Ashok k. Doctor

MusculAr stiMulAtor s.c. dwivedi

Here is a circuit that stimulates nerves of that part of your body where electrodes are

aid in removing cellulitis. The system comprises two units:

muscular stimulator and timer. Fig. 1 shows the circuit of the

muscular stimulator. IC 7555 is wired as an astable multivibrator to generate about 80Hz pulses. The output of IC1 is fed to tran-sistor T1, whose emitter is further connected to the base of transistor T2 through R3 and VR1. The col-lector of transistor T2 is connected to one end of the secondary wind-ing of transformer X1. The other end of the secondary winding of the transformer is connected to ground.

When IC1 oscillates, trans-former X1 is driven by the pulse frequencies generated to produce high voltage at its primary termi-nals. Separate electrodes are con-nected to each end of the primary winding of transformer X1. Diode 1N4007 (D1) protects transistor T2

against high-voltage pulses generated by the transformer.

Using potmeter VR1 you can con-trol the intensity of current sensing at the electrodes. The brightness level of LED1 indicates the amplitude of the pulses. If you want to increase the in-tensity level, replace the 1.8-kilo-ohm resistor with 5.6 kilo-ohms or higher value up to 10 kilo-ohms.

X1 is a small mains transformer with 220V primary to 12V, 100/150mA secondary. It must be reverse connect-ed, i.e., connect the secondary winding to the collector of T2 and ground, and primary winding to the output elec-trodes. The output voltage is about 60V but the output current is so small that there is no threat of electric shock.

Electrodes are made of small, thin-guage metallic plates measuring about 2.5×2.5 cm2 in size. Use flexible wires to solder electrodes and connect to the Fig. 2: Timer circuit

Fig. 1: Muscular stimulator circuit

attached. It is useful to relieve head-ache and muscular pain and revive frozen muscles that impair movement. Though it provides muscles stimula-tion and invigoration, it’s mainly an

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electronics for you • october 2008 • 97w w w . e f y m a g . c o m

output of the device. Before attaching metal electrodes to the body, wipe them with a damp cloth. After attach-ing the electrodes to the body (with the help of elastic bands on velcro straps), flip switch S1 to activate the circuit and rotate the knob of intensity-control preset VR1 very slowly until you feel a slight tingling sensation.

Fig. 2 shows the timer circuit. It uses IC NE555 wired in monostable mode. Initially, when you press switch

S2, the monostable triggers and its out-put goes high for 10 minutes. Thereaf-ter, its output goes low to give a beep sound from the piezobuzzer and lights up the red LED (LED2) indicating that stimulation time is over.

Assemble the timer with a separate switch and a 9V DC battery in the same cabinet as the stimulator. Tape the elec-trodes to the skin at opposite ends of the chosen muscle and rotate VR1 knob slowly until you sense light itching

when the muscular stimulation circuit is powered on. At the same time, flip switch S2 to start the timer for counting the time. At the end of the timing cycle, the piezobuzzer beeps. Each session should last about 10 minutes.

Caution: Heart patients and pregnant women should not use this device. Also, do not attach elec-trodes to burns, cuts, wounds or any injury. Consult your physician before using this circuit.

circuitideas

electronics for you • November 2008 • 85w w w . e f y m a g . c o m

D. Mohan KuMar

night alert s.c. dwivedi

Idea of this circuit came to me at midnight when my pet dog started barking continuously on sensing

a moving shadow, perhaps that of an intruder. Dogs have a night adaptation capability to maximise the sensitivity of vision in low light. They are well adapted to see moving objects rather

than stationary ones in darkness. This circuit turns a lamp ‘on’ for

a short duration when the dog barks, giving an impression that the occu-pants have been alerted.

The condenser microphone fitted in the dog’s cage senses barking sound and generates AC signals, which pass through DC blocking capacitor C1 to the base of transistor BC549 (T1).

Transistor T1 along with transistor T2 amplifies the sound signals and provides current pulses from the col-lector of T2.

The input trigger pulse is applied to the collector of transistor T3 and coupled by capacitor C3 to the base of transistor T4 causing T4 to cut off. The

collector voltage of transistor T4 for-ward biases transistor T3 via resistor R8. Transistor T1 conducts and capaci-tor C3 discharges to keep transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3 charges enough to enable it to conduct.

When transistor T4 conducts, its col-lector voltage goes low to drive transis-tor T3 into cut-off state. Resistor R9 and

capacitor C3 are timing components. When fully charged, capacitor C3 takes about two minutes to discharge.

So when sound is produced in front of the condenser mic, TRiAC1 (BT136) fires and the bulb (B1) glows for about two minutes.

Assemble the circuit on a general-purpose PCB and enclose in a plastic cabinet. Power to the circuit can be derived from a 12V, 500mA step-down transformer with rectifier and smooth-ing capacitor. Solder the triac ensuring

sufficient spacing between the pins to avoid short circuit. Fix the unit in the dog’s cage, with the lamp inside or outside as de-sired. Connect the microphone to the circuit us-ing a short length of shielded wire. Enclose the mi-crophone in a tube to increase

its sensitivity.Caution. Since the circuit uses

230V AC, many of its points are at AC mains voltage. it could give you le-thal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. EFY will not be responsible for any kind of resulting loss or damage.

circuitideas

104 • February 2010 • electronics for you w w w . e F y m a g . c o m

Most water-level indicators for water tanks are based upon the number of LEDs

that glow to indicate the correspond-ing level of water in the container. Here we present a digital version of the water-level indicator. It uses a 7-seg-

ment display to show the water level in numeric form from ‘0’ to ‘9.’

The circuit works off 5V regulated power supply. It is built around prior-ity encoder IC 74HC147 (IC1), BCD-to-7-segment decoder IC CD4511 (IC2), 7-segment display LTS543 (DIS1) and a few discrete components. Due to high input impedance, IC1 senses water in the container from its nine input termi-nals. The inputs are connected to +5V via 560-kilo-ohm resistors. The ground

Daniyal SyeD

nUMeRiC WaTeR-leVel inDiCaTOR

s.c. dwivediterminal of the sensor must be kept at the bottom of the container (tank). IC 74HC147 has nine active-low inputs and converts the active input into ac-tive-low BCD output. The input L-9 has the highest priority.

The outputs of IC1 (A, B, C and D) are fed to IC2 via transistors T1 through T4. This logic inverter is used

to convert the active-low output of IC1 into active-high for IC2. The BCD code received by IC2 is shown on 7-seg-ment display LTS543. Resistors R18 through R24 limit the current through the display.

When the tank is empty, all the inputs of IC1 remain high. As a result, its output also remains high, making all the inputs of IC2 low. Display LTS543 at this stage shows ‘0,’ which means the tank is empty. Similarly,

when the water level reaches L-1 posi-tion, the display shows ‘1,’ and when the water level reaches L-8 position, the display shows ‘8.’ Finally, when

the tank is full, all the inputs of IC1 become low and its output goes low to make all the inputs of IC2 high. Display LTS543 now shows ‘9,’ which means the tank is full.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a box. Mount 7-segment LTS543 on the front panel of the box. For sensors L-1 though L-9 and ground, use corrosion-free conductive-metal (stainless-steel) strips.

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90 • august 2008 • electronics for you w w w . e f y m a g . c o m

limiter and capacitors C1 and C4 act as buffers.

Working of the cir-cuit is simple. Audio signals from the PC au-dio socket/headphone socket are fed to the am-

plifier circuit through components R2 and C2 (left channel), and R3 and C3

(right channel). Pot-meter VR1 works as the volume control-ler for left (L) chan-nel and potmeter VR2 works for right (R) channel. Pin 7 of TDA2822M receives t h e l e f t - c h a n n e l sound signals and pin 6 receives the right-channel signals through VR1 and VR2, respectively. Amplified signals for driving the left and right loudspeak-ers are available at pins 1 and 3 of IC1, respectively. Com-ponents R5 and C8, and R6 and C10 form the traditional zobel network.

Assemble the cir-cuit on a medium-size, general-purpose PCB and enclose in a suitable cabinet. It is advisable to use a socket for IC TDA2822M. The external connections should be made using suitably screened wires for better result.

T.K. Hareendran

PC MulTiMedia SPeaKerS s.c. dwivedi

This circuit of multimedia speak-ers for PCs has single-chip-based design, low-voltage pow-

er supply, compatibility with USB power, easy heat-sinking, low cost, high flexibility and wide temperature tolerance.

At the heart of the circuit is IC TDA2822M. This IC is, in fact, mono-

lithic type in 8-lead mini DIP pack-age. It is intended for use as a dual audio power amplifier in battery-pow-ered sound players. Specifications of TDA2822M are low quiescent current, low crossover distortion, supply volt-

age down to 1.8 volts and minimum output power of around 450 mW/channel with 4-ohm loudspeaker at 5V DC supply input.

An ideal power amplifier can be simply defined as a circuit that can deliver audio power into external loads without generating significant signal distortion and without consuming excessive quiescent current.

This circuit is powered by 5V DC

supply available from the USB port of the PC. When power switch S1 is flipped to ‘on’ position, 5V power supply is extended to the circuit and power-indicator red LED1 lights up instantly. Resistor R1 is a current surge

RIGHT

Fig. 1: Circuit for PC multimedia speaker

Fig. 2: Pin configuration of TDA2822M

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84 • June 2009 • electronics for you w w w . e f y m a g . c o m

T.K. Hareendran

PC POWer ManaGer s.c. dwivedi

fier comprising diodes D1 through D4, smoothed by capacitors C1 and C2, and regulated by IC LM7812 (IC1). The regulated 12V DC is used to energise relay RL1. LED1 works as a power- ‘active’ indicator.

To set up the circuit, first connect the input socket (SOC1) of the circuit to a proper AC mains wall outlet us-ing a three-core power cable. Now connect one end of a standard USB cable to the B-type USB input socket and the other end of the cable to any vacant USB port (A-type) of the PC. Fi-nally, plug one standard four-way switchboard (extension cord) into the supply output socket (SOC2) of the circuit and take power from this switchboard to acti-vate all loads like moni-tor, scanner, printer and even your PC.

To activate the PC manager circuit, proceed as follows: Press ‘start’ switch S1 and hold it in this position for a few minutes. When power-‘active’ indicator LED1 lights up, relay RL1 energises and the 230V mains power supply from SOC1 is fed to output socket SOC2 through the contacts of relay RL1.

Very often we forget to switch off the connected peripher-als like monitor, scanner and

printer while switching off our PC. This leads to needless energy con-sumption and possible shortening of the life of the peripheral. PCs with an ATX switch-mode power supply (SMPS) unit are not provided with a mains switch outlet. It is therefore not possible to achieve automatic switch-ing (on/off) of peripheral units with the computer power switch.

Here is a simple circuit that turns the connected peripherals on/off along with your PC. It consists of a regulated power supply, a simple USB interface and two electromagnetic relays used as power switches.

The power supply for the circuit is derived from the AC mains via trans-former X1. The 15V AC available at the secondary winding of transformer X1 is first rectified by a bridge recti- Fig. 2: Wiring diagram for PC power manager

Fig. 1: Circuit of PC power manager

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electronics for you • June 2009 • 85w w w . e f y m a g . c o m

PC manager is ready to use.When you switch off your PC, relay

RL2 de-energises. As a result, electric power from the switchboard (to which all peripherals are connected) is cut off. Switch S2 works here as an emergency bypass switch.

Assemble the circuit on a general-purpose PCB and enclose in a suitable

Now start your computer as usual, by pressing the power button on the front panel. When the PC runs, there will be 5V DC at the USB interface socket. As a result, relay RL2 energises via diode D6. The contacts of relay RL2 close switch S1 permanently, and LED2 glows continuously.

Release ‘start’ switch S1. Now your

cabinet. Connect SOC1, SOC2 and USB socket along with switches S1 and S2 and LEDs (LED1 and LED2) on the front panel of the cabinet. Refer Fig. 2 for connections.

EFY note. Take care during fab-rication and testing, as the circuit is at mains potential and may give you lethal shock.

CIRCUIT

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9 2 • S E P T E M B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� RAJ K. GORKHALI

PC TEMPERATURE ALARM S.C. DWIVED

I

I f your PC overheats, it coulddamage its expensive components.Here’s a circuit that warns you of

your PC getting heated.Today’s computers contain most

of the circuitry on just a few chipsand reduced power consumption isa byproduct of this LSI and VSLIapproach. Some PCs still havepower supplies that are capable ofsupplying around 200W, but fewPCs actually consume power to thisextent.

On the other hand, apart fromsome portable and small desktop com-puters that use the latest micro-powercomponents, most PCs still consumesignificant amount of power and gen-erate certain amount of heat.

The temperature inside the aver-age PC starts to rise well above theambient temperature soon after it isswitched on. Some of the larger inte-grated circuits become quite hot andif the temperature inside the PC rises

too high, these devices may not beable to dissipate heat fast enough.This, in turn, could lead to failure ofdevices and eventually of the PC.

Various means to combat overheat-ing are available, ranging from simpletemperature alarms to devices liketemperature-activated fans to keep themicroprocessor cool.

Here is a temperature alarm thatactivates an audio ‘beeper’ if the tem-perature inside the PC exceeds a pre-set threshold. This temperature is user-adjustable and can be anywhere be-tween 0°C and 100°C.

The unit is in the form of asmall PC expansion card, which yousimply need to plug into any avail-able slot of the host PC. It is poweredfrom the PC and consumes only about12 mA.

The sensor (LM35) used here pro-vides a substantial amount of on-chipsignal conditioning, including ampli-fication, level shifting and phase in-version. As a result, it provides an out-put of 10 mV per degree centigrade

rise in temperature. It caters to a tem-perature measurement range of 0°C to100°C, which corresponds to 0V to 1Vof voltage.

The voltage-detector stage com-pares the output voltage of the tem-perature sensor with the preset refer-ence voltage. The output of the com-parator goes high if the output po-tential from the sensor exceeds thereference voltage. When this happens,the voltage comparator enables a low-frequency oscillator, which, in turn,activates the audio oscillator. The out-put of the audio oscillator is connectedto a loudspeaker (LS1), which soundsa simple ‘beep-beep’ alarm. The ref-erence voltage determines the tem-perature at which the alarm is acti-vated.

Fig. 1 shows the circuit of the PCtemperature alarm and Fig. 2 showsthe pin configuration of sensor LM35.IC LM35 (IC1) is an easy-to-usetemperature sensor. It is basically athree-terminal device (two supplyleads plus the output) that operates

Fig. 1: Circuit for PC temperature alarm

CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • S E P T E M B E R 2 0 0 7 • 9 3W W W . E F Y M A G . C O M

over a wide supply range of 4 to 20V.It consumes only 56 µA at 5V andgenerates insignificant heat.

IC2 is an operational amplifier usedhere as a voltage comparator. VR1 pro-vides a reference voltage that can beset anywhere from 0V to approxi-mately 1V, which matches the outputvoltage range of IC1. This referencevoltage is applied to the inverting in-put of IC2 and the output of IC1 iscoupled to the non-inverting input.Consequently, the output of IC2 is lowif the output of IC1 is below the refer-ence voltage, or high if the output ofIC1 exceeds the reference voltage.

The low-frequency oscillator IC3 isa standard 555 astable multivibratorcircuit. It is gated via the reset inputat pin 4, which holds output pin 3 lowwhen IC3 is gated ‘off’ (when the out-put of IC2 is low). This prevents IC4from oscillating. IC4 is another 555

Fig. 2: Pindetails of LM35

astable multivibratorcircuit, gated via itsreset input. It has anoperating frequencyof approximately 2.5kHz.

When IC3 is acti-vated, its output pro-vides a square waveof 1 Hz. This is used

to trigger IC4, which gives an audiooutput of 2.5 kHz in bursts. It is con-nected to loudspeaker LS1 to generatealarm.

The alarm circuit can be fitted intoany spare expansion slot of the PC, butbe careful to fit it the right way round.Before setting VR1 to a suitable thresh-old temperature, decide what that tem-perature should be. The technical speci-fication in your computer’s manualmight be of help here.

If we assume that the room tem-

perature will not normally exceed25oC, the temperature of the interiorof the computer would be up to 35oC.Unless you have good reason to use adifferent threshold temperature, VR1should be set for a wiper potential of350 mV.

Trial-and-error method can beused in the absence of test equip-ment to enable VR1, but it would bea bit time-consuming. There is aslight complication in that thecomputer’s outer casing must be atleast partially removed to provideaccess to VR1. Once VR1 has beenadjusted, the outer casing must be putback into place so that the interior ofthe computer can warm up in thenormal way. You must therefore al-low time for the temperature insidethe computer to rise back to its nor-mal operating level each time VR1 isreadjusted. �

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w w w . e f y m a g . c o m88 • june 2008 • electronics for you

EFY LAB

Poor MAn’s HEAring Aid s.c. dwivedi

This miniature stereo preampli-fier-cum-headphone amplifier circuit works off a 3V battery

(lithium non-rechargeable coin cell). Although its performance is not comparable to that of commercially available sophisticated hearing aids, still it can serve the purpose well for persons with a low degree of hearing impairment. Its maximum power output at 1 kHz is around 8 mW, which is adequate for driving the headphones.

The circuit, as shown in Fig. 1, is wired around Sanyo’s MSI (me-dium-scale-integrated) surface-mount 10-pin DIL IC LA4537M, which measures just 8×6.4×1.5 mm3. A functional block diagram of LA4537M IC is shown in Fig. 2. Since the MSI’s pin-to-pin (centre-to-centre) distance is only 1 mm, the circuit has to be assembled on a properly designed PCB using soldering iron with a pointed bit. Two ICs (LA4537M) have been cascaded to increase the overall sensitivity and thereby the reception range. You can adjust the volume of the stereo channels indi-

house it in a thin metallic case, which can then be mounted in the middle of a metallic/plastic headband (gener-ally used by telephone operators), while the two microphones with their associated earpieces are to be extended using screened wires so that these (microphone-earpiece sets) can be kept closest to the respective earlobes.

Caution. Ensure that shielded microphone wires do not touch ( shor t ) the sh ie lded earpiece

w i r e s , a s these are con-nected to dif-ferent p ins (reference in-put pin 5 and ground pins 3 and 8, respec-tively) of ICs L A 4 5 3 7 M . You may use an insula t -i n g s l e e v e over each of the shielded wires.

Fig. 1: Circuit for the hearing aid

Fig. 2: Functional diagram of iC la4537m

vidually, as per your requirement, using presets VR1 and VR2, respec-tively.

With 3V supply voltage, you can afford to use 1/8-watt resistors, while the electrolytic capacitors’ voltage rating can be as low as 5V. This will allow the assembled circuit to occupy very little space. Apart from the usual battery ‘on’/‘off’ switch S1, muting ‘on’/‘off’ switch S2 has also been provided. Both S1 and S2 could be PCB-mount slide switches.

After assembling the main circuit,

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w w w . e f y m a g . c o m92 • june 2008 • electronics for you

T.K. Hareendran

PorTable lamP FlasHer s.c. dwivedi

Here is a portable, high-power incandescent electric lamp flasher. It is basically a dual

flasher (alternating blinker) that can handle two separate 230V AC loads (bulbs L1 and L2).

The circuit is fully transistorised and battery-powered. The free-run-ning oscillator circuit is realised using two low-power, low-noise transistors

T1 and T2. One of the two transistors is always conducting, while the other is blocking. Due to regular charging and discharging of capacitors C1 and C2, the two transistors alternate be-tween conduction and non-conduction states.

The collector of transistor T1 is con-nected to the base of driver transistor T4 through current-limiting resistor R5. Similarly, the collector of transistor T2 is connected to the base of driver

transistor T3 through limiting resistor R6. These transistors are used to trigger Triac1 and Triac2 (each BT136) through optotriacs IC1 and IC2, re-spectively, and switch on the power supply to external loads L1 and L2. IC1 and IC2 operate alternatively at a low frequency determined by the values of

capacitors C1 and C2. The oscillator circuit built around

transistors T1 and T2 generates low fre-quencies. When transistor T3 conducts, IC1 is enabled to fire Triac1 and bulb L1 glows. Similarly, when transistor T4 conducts, IC2 is enabled to fire Triac2 and bulb L2 glows.

Connect the power supply line (L) of mains to bulbs L1 and L2, and neutral (N) to T1 terminals of Triac1 and Triac2. You can also con-nect neutral (N) line of the external 230V mains supply to both loads (bulbs L1 and L2) as a common line and then route supply line (L) to respective loads (bulbs L1 and L2).

The circuit works off only 3 volts. Since current consumption is fairly low, two AA-type cells are sufficient to power the cir-cuit. Assemble the circuit

on a general-purpose PCB and enclose in a suitable plastic cabinet with integrated AA-size pen-light cell holder. Drill holes for mounting the ‘on’/‘off’ switch and power switching terminals. Also connect two bulb holders for bulbs L1 and L2. Refer Fig. 2 for pin configurations.

EFY note. While assembling, test-ing or repairing, take care to avoid the lethal electric shock.

Fig. 2: Pin configurations of MOC3021, BT136 and BC550/547

Fig. 1: Circuit for portable lamp flasher

ELECTRONICS PROJECTS Vol. 22

PRECISION AMPLIFIER

WITH DIGITAL CONTROL

This circuit is similar to the pre-ceding circuit of the attenuator.Gain of up to 100 can be achieved

in this configuration, which is useful forsignal conditioning of low output of trans-ducers in millivolt range.

The gain selection resistors R3 to R6can be selected by the user andcan be anywhere from 1 kilo-ohm to 1meg-ohm. Trimpots can be used for ob-taining any value of gain required by theuser. The resistor values shown in thecircuit are for decade gains suitable foran autoranging DPM.

Resistor R1 and capacitor C1 reduceripple in the input and also snub tran-sients. Zeners Z1 and Z2 limit the inputto ±4.7V, while the input current is lim-ited by resistor R1. Capacitors C2 andC3 are the power supply decoupling ca-pacitors.

Op-amp IC1 is used to increase theinput impedance so that very low inputsare not loaded on measurement. The usercan terminate the inputs with resistance

of his choice (such as 10 meg-ohm or 1meg-ohm) to avoid floating of the inputswhen no measurement is being made.

IC5 is used as an inverting buffer torestore polarity of the input while IC4 isused as buffer at the output of CD4052,because loading it by resistance of valueless than 1 meg-ohm will cause an error.An alternative is to make R9=R10=1 meg-ohm and do away with IC4, though thismay not be an ideal method.

Gains greater than 100 may not bepractical because even at gain value of100 itself, a 100μV offset will work out tobe around 10 mV at the output (100μV x100). This can be trimmed using the offsetnull option in the OP07, connecting a

trimpot between pins 1 and 8, and con-necting wiper to +5V supply rails. Forbetter performance, use ICL7650 (not pin-

compatible) in place of OP07 and use ±7.5Vinstead of ±5V supply.

Eight steps for gain or attenuationcan be added by using two CD4051 andpin 6 inhibit on CD4051/52. More stepscan be added by cascading many CD4051,or CD4052, or CD4053 ICs, as pin 6 workslike a chip select.

Some extended applications of this cir-cuit are given below.

1. Error correction in transducer am-plifiers by correcting gain.

2. Autoranging in DMM.3. Sensor selection or input type se-

lection in process control.4. Digitally preset power supplies or

electronic loads.5. Programmable precision mV or mA

sources.6. PC or microcontroller or micropro-

cessor based instruments.7. Data loggers and scanners.

Truth Table (Control Input vs Gain)X,Y (On-switch (2) (1) GainPair) B A (Av.)X0,Y0 0 0 1/10X1,Y1 0 1 1X2,Y2 1 0 10X3,Y3 1 1 100

REGULADOR DE CORRIENTE

- Esquema eléctrico + Topográfico.

- Pistas

- Componentes

- Soldaduras.

- Terminado componentes.

CARLOS DAMIAN LOPEZ HERRERO

circuitideas

88 • april 2009 • electronics for you w w w . e f y m a g . c o m

D. Mohan KuMar

reMote-operateD Master switch

s.c. dwivedi

tial divider comprising resistors R4 and R5 maintains half of 5.1V at pin 2 of IC1. In brief, the voltage at pin 2 of IC1 is higher than at pin 3 and its output remains low. LED2 remains ‘off’ and transistor T2 does not conduct. Relay RL1 remains de-energised and, as a re-sult, security lamps (both indoors and outdoors) remain switched off.

When you press any key of the remote TV handset, IR rays fall on the

receiver (IRX1) and its output goes low. LED1 flashes in sync with pulsation of the IR rays. At the same time, transis-tor T1 (BC558) conducts to take pin 3 of IC1 high. IC1 is used as a comparator with timer action.

When transistor T1 conducts, pin 3 of IC1 gets a higher voltage than pin 2 making the output of IC1 high. Mean-while, capacitor C4 charges to full voltage and keeps pin 3 high for a few minutes even after T1 is non-conduct-ing. Resistor R3 provides discharge path for capacitor C4, which decides the time period for which the output of comparator IC1 should remain high.

The high output of IC1 energises re-lay RL1 through relay-driver transistor T2. Thus the load, i.e., security lamps, turn on for three to four minutes. LED2

glows to indicate activation of the relay as well as switching ‘on’ of the security lights. Connect a single-pole, single-throw ‘on’/‘off’ switch (MS) to activate the security lamps manually

when required.Zener diode ZD1 provides 5.1V DC

for safe operation of the IR receiver and associated circuit. Power for the circuit is derived from a step-down transformer (X1) and a bridge recti-fier comprising diodes D1 through D4. Smoothing capacitor C1 removes rip-ples, if any, from the power supply.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Drill holes on the front panel for mounting the IR sensor and LEDs. Connect the master switch between the normally-open (N/O) contact and pole of relay RL1 so that the master switch can be used when needed. The relay contacts rating should be more than 4A. Mount the unit near the master switch using minimal wiring.

Generally, a bedside master switch is used to switch on lamps both indoors and

outdoors when there is a threat of intruder. This circuit can be used to activate the master switch from the bed without searching for the switch in darkness. It can be activated by the TV remote handset. The security lamps

glow for three minutes and then turn off. The circuit is sensitive and can be activated from a distance of up to 25 metres.

IR receiver module TSOP 1738 (IRX1) is used to sense the pulsed 38kHz IR rays from the TV remote handset. The IR receiver module has a PIN photodiode and a preamplifier enclosed in an IR filter epoxy case. Its open-collector output is 5 volts at 5mA current in the standby mode.

In the standby mode, no IR rays from the remote handset fall on the IR receiver, so its output pin 3 remains high and LED1 doesn’t glow. Through resistor R2, the base of transistor T1 remains high and it does not conduct. As a result, the voltage at pin 3 of IC CA3130 (IC1) remains low. The poten-

circuitideas

140 • January 2010 • electronics for you w w w . e f y m a g . c o m

An audio signal can be used as a form of input to control any security system. For example,

an automatic security camera can be configured to respond to a knock on the door. The circuit described here allows the security system to automati-

cally switch on when a master switch is in on state. It uses a transducer to detect intruders and a 5V regulated DC power supply provides power to the circuit.

As shown in Fig. 1, a condenser microphone is connected to the input

of small signal preamplifier built around transistor T1. Biasing resistor R1 determines to a large extent the microphone sensitivity. A microphone usually has an internal FET which requires a bias voltage to operate. The sound picked up by the microphone is amplified and fed to input pin 2 of IC1 (LMC555) wired in monostable

configuration.IC2 (CD4538B) is a dual,

precision monostable mul-tivibrator with independent trigger and reset controls. The output of IC1 is connected to the first trigger input pin 4 of IC2(A) through switch S1. If an intruder opens or breaks the door, IC1 is triggered by sound signals; the timer out-put pin 3 of IC1 goes high and enables first monostable multi vibrator IC2(A). IC2(A) pro-vides a time period of around

5 to 125 seconds, which is adjusted with preset VR1.

Another monostable multivibrator IC2(B) also provides a time period of around 25 to 600 seconds, which is adjusted with preset VR2. The output of IC2(B) is used to energise relay RL1.

T.K. Hareendran

SecuriTy SySTem SwiTcHer s.c. dwivedi

Fig. 1: Security system switcher

Fig. 2: Proposed cabinet

+5V ADAPTORFOR POWER

SUPPLY

CONNECTORFOR

SECURITYGADGET

Indicator LED1 is provided to display the relay activity. Any AC/DC oper-ated security gadget is activated or deactivated through a security switch. Thus, the security switch of the gadget

is connected in the n/o contacts of the relay. you can also operate high-power beacons, sirens or hooters in place of the security switch for any AC/DC operated security gadget.

Assemble the circuit on a gen-eral-purpose PCB and enclose it in a cabinet as shown in Fig. 2 along with 5V adaptor for powering the circuit. Connect the security switch according to the circuit diagram and use appropriate AC/DC power sup-ply required to operate the security gadget.

Warning! All relevant electrical safety precautions should be taken when connecting mains power supply to the relay contacts. With the help of single pole double throw (SPDT) switch S1, internal or external trig-ger input (active high signal) can be selected.

CIRCUIT

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E L E C T R O N I C S F O R Y O U • N O V E M B E R 2 0 0 7 • 9 5W W W . E F Y M A G . C O M

This circuit simulates a seismicsensor to detect vibrations/sounds. It is very sensitive and

can detect vibrations caused by themovement of animals or human be-ings. So it can be used to monitor pro-tected areas to restrict entry of un-wanted persons or animals.

The circuit uses readily availablecomponents and the design is straight-forward. A standard piezo sensor isused to detect vibrations/sounds dueto pressure changes. The piezo elementacts as a small capacitor having a ca-pacitance of a few nanofarads. Like acapacitor, it can store charge when apotential is applied to its terminals. Itdischarges through VR1, when it is dis-turbed.

In the circuit, IC TLO71 (IC1) iswired as a differential amplifier with

� D. MOHAN KUMAR

SEISMIC SENSOR

both its inverting and non-invertinginputs tied to the negative rail througha resistive network comprising R1, R2and R3. Under idle conditions (as ad-justed by VR1), both the inputs receive

almost equal voltages, which keeps theoutput low.

TLO71 is a low-noise JFET inputop-amp with low input bias and off-set current. The BIFET technology pro-vides fast slew rates. Capacitor C1 isprovided in the circuit to keep thedifferential input of IC1 for better per-formance.

When the piezo element is dis-turbed (by even a slight movement), itdischarges the stored charge. This al-ters the voltage level at the inputs ofIC1 and the output momentarilyswings high as indicated by greenLED1. This high output is used to trig-ger switching transistor T1, which trig-gers monostable IC2. The timing pe-

riod of IC2 is determined by R7 andC5. With the shown values, it will bearound two minutes. The high outputfrom IC2 activates T2 and the buzzer

starts beeping along with red light in-dication from LED2.

Assemble the circuit on a commonPCB and enclose in a suitable cabinet.Connect the piezo element to the PCBusing single-core shielded wire. En-close the piezo element inside a rust-proof, small aluminium box. The piezoelement should be firmly glued to theenclosure facing the fine side towardsthe case. Fix the sensor assembly onthe back side of a ceramic tile or gran-ite tile with good adhesive. Fix the tile(or bury it in the earth) near the en-trance with the sensor assembly fac-ing downwards. Whenever a pressurechange develops near the sensor, thecircuit will be activated. �

S.C. DWIVE

DI

circuitideas

100 • OctO ber 2009 • electronics for you w w w . e f y m a g . c O m

Many a times you need to power an adjoining acces-sory circuit from the power

supply used in the main module cir-

Pratik Panchal

Short-circuit Protection in Dc low-Voltage SyStemS

cuit. Here is a circuit to derive the addi-tional power supply from the main cir-cuit. The main circuit is protected from any damage due to short-circuit in the

additional power supply circuit by cutting off the derived supply voltage. The derived supply volt-age restores automatically when shorting is removed. An LED is used to indicate whether short-circuit exists or not. Author’s prototype of short-circuit protection module is shown in Fig. 1.

In the main power sup-ply circuit, 230V AC is stepped down by trans-

former X1 (230V AC primary to 0-9V, 300mA secondary), rec-tified by a full-wave rectifier comprising di-odes D1 through D4, filtered by capaci tor C1 and regulated by IC 7805 to give regulated 5V (O/P1).

T r a n s i s -tors SK100 and BC547 are used to derive the secondary out-put of around 5V (O/P2) from the main 5V supply (O/P1).

Working of the circuit is simple. When the 5V DC output from regulator IC 7805 is available, transistor BC547 conducts through resistors R1 and R3 and LED1. As a result, transistor SK100 conducts and short-circuit protected 5V DC output appears across O/P2 termi-nals. The green LED (LED2) glows to indicate the same, while the red LED (LED1) remains off due to the presence of the same voltage at both of its ends.

When O/P2 terminals short, BC547 cuts off due to grounding of its base. As a result, SK100 is also cut-off. Thus during short-circuit, the green LED (LED2) turns off and the red LED (LED1) glows. Capacitors C2 and C3 across the main 5V output (O/P1) ab-sorb the voltage fluctuations occurring due to short-circuit in O/P2, ensuring disturbance-free O/P1. The design of the circuit is based on the relationship given below:

RB = (HFE X Vs)/(1.3 X IL)where,RB = Base resistances of transistors

of SK100 and BC547HFE = 200 for SK100 and 350 for

BC547Switching Voltage Vs = 5V1.3 = Safety factorIL = Collector-emitter current of

transistorsAssemble the circuit on a gen-

eral-purpose PCB and enclose in a suitable cabinet. Connect O/P1 and O/P2 terminals on the front panel of the cabinet. Also connect the mains power cord to feed 230V AC to the transformer. Connect LED1 and LED2 for visual indication.

s.c. dwivedi

Fig. 1: Prototype of short-circuit protection in DC low-voltage systems

Fig. 2: Circuit diagram of short-circuit protection

CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • S E P T E M B E R 2 0 0 7 • 9 7W W W . E F Y M A G . C O M

� D. MOHAN KUMAR

SHUTTER GUARDS.C.

DWIVEDI

This sensitive vibration sensor isexclusively made for shops toprotect against burglary. It will

detect any mechanical or acoustic vi-bration in its vicinity when somebodytries to break the shutter and immedi-

ately switch on a lamp and sound awarning alarm. A 15-minute time de-lay after switch-on allows sufficienttime for the shop owner to close theshutter.

The front end of the circuit has atimer built around the popular binarycounter IC CD4060 (IC1) to provide15-minute time delay for the remain-ing circuitry to turn on. Resistors R3and R4 and capacitor C2 will makeQ9 output high after 15 minutes. Di-ode D1 inhibits the clock input (pin11) to keep the output high till thepower is switched off. Blinking LED1indicates the oscillation of IC1.

The high output from IC1 is used

to enable reset pin 4 of IC2 so that itcan function freely. Transistor T1 am-plifies the piezo-sensor signal and trig-gers monostable IC2. The base of tran-sistor T1 is biased using a standardpiezo element that acts as a small ca-pacitor and flexes freely in responseto mechanical vibrations so that the

output of IC2 is high till the prefixedtime period.

In the standby mode, the alarm cir-cuit built around IC3 remains dormantas it does not get current. Timing com-ponents R8 and C6 make the outputof IC2 high for a period of three min-utes.

When any mechanical vibration(caused by even a slight movement)disturbs the piezo element, trigger pin2 of IC2 momentarily changes its stateand the output of IC2 goes high. Thistriggers triac 1 and the alarm circuitactivates. Triac BT136 completes thelamp circuit by activating its gatethrough resistor R9. IC UM3561 (IC4)

generates a tone simulating the policesiren with R11 as its oscillation-controlling resistor. Zener diode ZD1provides stable 3.1V DC for the tone-generating IC.

Assemble the circuit on a general-

purpose PCB and enclose in a suit-able, shockproof case. Connect thepiezo element to the circuit by usinga single-core shielded wire. Glue acircular rubber washer on the fineside of the piezo element and fixit on the shutter frame with thewasher facing the frame so that thepiezo element is flexible to sensethe vibrations. Fix the lamp andthe speaker on the outer side andthe remaining parts inside the case.Since triac is used in the circuit, mostpoints in the PCB will be at mainslethal potential. So it is advised notto touch any part of the circuit whiletesting. �

ELECTRONICS PROJECTS Vol. 22�

SIMPLE INTERCOM CIRCUITThe circuit of a two-position in-

tercom is presented here. This circuit is very simple yet it functions

quite satisfactorily. The circuit does not involve any complicated switching. The switches S1/S2 must be fixed in such a way

that when the handset is resting on the cra-dle, the switch is OFF and when it is taken off the cradle, the switch turns on.

Both the sets used are identical in construction. When one set (say, party 1) is switched on, the other set’s (party 2’s) bell

energises. When party 2 turns on his own set, his bell automati-cally stops and he can talk to party 1 via his m i c r o p h o n e . One can sub-stitute the BEL 1895 IC based amplifier and bell circuit with any other low power amplifier and bell circuit. The block diagram clari-fies the connection of

the two sets. Only three wires are required to connect the two sets if separate battery is used in each set. However, if the battery is common for the two sets, it requires four wires for interconnections.

The circuit can be easily assembled on

a general-purpose PCB. Intercom cases are also available in the market which may be used for giving it a professional outlook.

circuitideas

88 • June 2009 • electronics for you w w w . e f y m a g . c o m

D. Mohan KuMar

SKin reSponSe Meter s.c. dwivedi

Human skin offers some resistance to current and voltage.

This resistance changes with the emotional state of the body. The circuit proposed here measures changes in your skin resistance following changes in your mental state.

In the relaxed state, the resistance offered by the skin is as high as 2 mega-ohms or more, which reduces to 500 kilo-ohms or less when the emotional stress is too high. The reduction in skin resistance is related to increased blood flow and permeability followed by the physiological changes during high stress. This increases the electrical conductivity of the skin.

This circuit is useful to monitor the skin’s response to relaxation techniques. It is very sensitive and shows response during a sudden moment of stress. Even a deep sigh will give response in the circuit.

The circuit uses a sensitive amplifier to sense variations in the skin resistance. IC CA3140 (IC1) is designed as a resist-ance-to-voltage converter that outputs varying voltage based on the skin’s con-ductivity. It is wired as an inverting am-plifier to generate constant current to skin in order to measure the skin resistance.

IC CA3140 is a 4.5MHz BiMOS op-erational amplifier with MOSFET inputs and bipolar output. The gate-protected inputs have high impedance and can sense current as low as 10 pA. This de-vice is ideal to sense small currents in low-input-current applications.

The inverting input (pin 2) of IC1 is connected to ground (through preset VR1) and one of the touch plates, while the non-inverting input (pin 3) is ground-ed directly. The output from IC1 passes through current-limiting resistor R1 to the second touch plate. R1 act as a feedback

resistor along with the skin when the touch plates make contact with the skin. So the gain of IC1 depends on the feed-back provided by R1 and the skin.

In the inverting mode of IC1, a posi-tive input voltage to its pin 2 through the feedback network makes its output low. If the skin offers very high resistance in the relaxed state, input voltage to pin 2 reduces and the output remains high. Thus the gain of IC1 varies depending on the current passing through the skin, which, in turn, depends on the skin re-sponse and emotional state.

In the standby state, touch plates are free. As there is no feedback to IC1, it gives a high output (around 6 volts), which is indicated by shifting of the meter to right side.

When the touch plates are shorted by the skin, the feedback circuit completes and the output voltage reduces to 4 volts or less depending on the resistance of the skin. Since the feedback network has a fixed resistor (R1) and VR1 is set to a fixed resistance value, the current flowing through it depends only on the resistance of the skin. The output from IC1 is dis-played on a sensitive moving coil meter (VU meter). By varying preset VR2, you can adjust the sensitivity of the meter.

For easy visual observation, an LED display is also included. IC LM3915 (IC2) is used to give a logarithmic display through LED indications. It can sink

current from pin 18 to pin 10 with each increment of 125 millivolts at its input pin 5. Using VR3 you can adjust the input voltage of IC2, while using VR4 you can control the brightness of the LEDs.

In practice, the circuit pro-vides both meter reading and LED indications. If the LED display is not needed, IC2 can be omitted.

Assemble the circuit on a general-purpose PCB and

enclose in a suitable cabinet with touch pads glued on the top, 5-10 mm apart. Touch pads can be any type of conduct-ing plates, such as aluminium or copper plates, having dimensions of 1×1 cm2. The moving coil meter can be a small VU meter with 1-kilo-ohm coil resistance and 0-10 digit reading.

After assembling the circuit, adjust the presets such that IC1 outputs around 6 volts. None of the LEDs (LED1 through LED3) glows in this position with the touch plates open.

Now gently touch the touch plates with your middle finger. Maintain the finger still allowing one minute to bond with the pads and keep your body relaxed. Adjust VR3 until the green LED (LED1) lights up and the meter shows full deflection. Adjust VR2 to get maximum deflection of the meter. This indicates normal resistance of the skin, provided the body is fully relaxed.

If you are stressed or have ill feel-ing, skin resistance decreases and the blue LED lights up followed by the red LED along with a deflection of the meter towards the lower side. In short, the red LED and zero meter reading indicate you are stressed, and the green LED and high meter reading indicate you are re-laxed. Practise some relaxation technique and observe how much your body is relaxed.

circuitideas

88 • Mar ch 2009 • electronics for you w w w . e f y M a g . c o M

which is used as a comparator. The reference voltage (Vref) at the non-inverting terminal (pin 3) of IC1(A) is set using preset VR1. The preset is also used to control the sensitivity of the sound signals received by the cir-cuit. The output from pin 1 of IC1(A) is fed to the trigger input (pin 2) of timer NE555, which is configured in

monostable mode.When sufficient sound signal

strength is detected at the base of tran-sistor T1, the pulsating voltage at its collector exceeds the reference voltage at pin 3. As a result, output pin 1 of IC1(A) goes low. The low output from IC1(A) triggers the NE555 timer and its output goes high for a preset duration. R4 and C2 are the timing components for setting the time duration. The high output of the timer is directly used as the power source for the sound ampli-fier section.

The sound amplifier section is built around transistors T2 through T5. The last amplifier stage T5 (pnp transistor BC558) drives the earphone. The sound

signal received from the mic is fed to the non-inverting pin of the second op-amp of IC1(B) which is wired in unity follower configuration. The unity follower mode resolves the problem of impedance mismatch which would have occured if the output of the mic

is fed directly to amplifier stage. The output from pin 7 of IC1(B) is fed to the base of transistor T2. The weak signal received at transistor stage T2 is further amplified by transistors T3, T4 and T5. An earphone to listen to the sound is connected between the collector of T5 and ground. It is recommended to use a mono earphone with volume control attached.

With 9V DC supply, when sound is detected through the mic, the amplifier section is automatically triggered and the current consumption of the circuit is about 96 mA. When the amplifier cir-cuit is ‘off,’ the circuit draws a current of about 6 mA only, thus saving con-siderable amount of battery power.

Devrishi Khanna anD rohit MoDi

sMart hearing aiD sani theo

Normally, hearing aid circuits consume battery power continuously once they are

switched on. The circuit given here saves battery power by switching on the sound amplifier section only

when sound is detected. The sensi-tivity of the detection section and the ‘on’ time duration of the sound am-plifier circuit can be set by the user. Also the circuit uses only a single condenser mic for sound detection and amplification.

As is clear from the above, this hearing aid consists of a condenser microphone, earphone, and sound detection and amplification sections. The sound detection section employs a quad op-amp IC LM324 (IC1(A)) and a timer NE555 (IC2). The sound signal received at the mic is pre-amplified by transistor BC549 (T1). The voltage at its collector is fed to the inverting terminal (pin 2) of op-amp IC1(A),

circuitideas

w w w . e f y m a g . c o m88 • april 2008 • electronics for you

T.K. Hareendran

SmarT VibraTion SenSor s.c. dwivedi

going pulse output from the vibra-tion sensing mechanism built around piezo-ceramic wafer and associated components. As a result, control input pins 2 and 6 of IC1 latch are grounded. Output pin 3 of IC1 now goes high. The positive supply from output pin 3 of IC1 is extended to three-tone siren generator UM3561 (IC2) through R5, D1 and R6. Components R6 and ZD1 stabilise the input power supply of IC2 to around 3.3V. Output signals from IC2 are amplified by Darlington-pair transistors T2 and T3 to produce alert tone (police siren sound) via loud-

In this vibration sensor alarm cir-cuit, initially, when power switch S1 is flipped to ‘on’ position, power

indicator LED1 lights up immediately. IC LM555 (IC1), wired as a simple latch circuit with control input, is powered and R-C components R4 and C5 con-nected at its reset pin 4 force the latch to standby mode (with inactive low output). The circuit is driven into sleep mode.

As soon as vibration is detected, MOSFET T1 is fired by the positive-

speaker LS1.Reset switch S1 can be used to

switch off the alarm sound by resetting the latch circuit. For safety, use key-lock type switches for S1 and S2. A relay can also be connected at the output socket (SOC1) of the circuit to energise high-power beacons, emergency sirens and fence electrification units.

The circuit works off 9V DC. A compact PP3-/6F22-type alkaline battery can be used to power the circuit.

circuitideas

80 • May 2009 • electronics for you w w w . e f y M a g . c o M

P.V. Vinod Kumar TheKKumuri

Solar Panel baSed Charger and Small led lamP

s.c. dwivedi

You can save on your electric-ity bills by switching to alter-native sources of power. The

photovoltaic module or solar panel

described here is capable of deliver-ing a power of 5 watts. At full sun-light, the solar panel outputs 16.5V. It can deliver a current of 300-350 mA. Using it you can charge three

types of batteries: lead acid, Ni-Cd and Li-ion. The lead-acid batteries are commonly used in emergency lamps and UPS.

The working of the circuit is sim-ple. The output of the solar panel is

fed via diode 1N5402 (D1), which acts as a polarity guard and pro-tects the solar panel. An ammeter is connected in series between diode D1 and fuse to measure the current flowing during charging of the batteries. As shown in Fig. 1, we have used an analogue mul-timeter in 500mA range. Diode D2 is used for protection against reverse polarity in case of wrong connection of the lead-acid battery. When you connect wrong polarity, the fuse will blow up.

For charging a lead-acid bat-tery, shift switch S1 to ‘on’ posi-tion and use connector ‘A.’ After

you connect the battery, charging starts from the solar panel via diode D1, multimeter and fuse. Note that pulsating DC is the best for charging lead-acid batteries. If you use this cir-

Fig. 1: Circuit of solar panel based charger

Fig. 2: LED lamp circuit

cuit for charging a lead-acid battery, replace it with a normal pulsating DC charger once a week. Keep checking

the water level of the lead-acid battery. Pure DC voltage normally leads to deposition of sulphur on the plates of lead-acid batteries.

For charging Ni-Cd cells, shift switches S1 and S3 to ‘on’ position and use con-nector ‘B.’ Regulator IC 7806 (IC1) is wired as a constant-current source and its output is taken from the middle ter-minal (normally grounded). Using this circuit, a constant current goes to Ni-Cd cell for charging. A total of four 1.2V cells are used here. Resistor R2 limits the charging cur-rent.

For charging Li-ion battery (used in mobile phones), shift switches S1 and S2 to ‘on’ position and use con-nector ‘C.’ Regulator IC 7805 (IC2) pro-vides 5V for charging the Li-ion bat-tery. Using this circuit, you can charge a 3.6V Li-ion cell very easily. Resistor R3 limits the charging current.

Fig. 2 shows the circuit for a small LED-based lamp. It is simple and low-cost. Six 10mm white LEDs (LED2 through LED7) are used here. Just connect them in parallel and drive directly by a 3.6V DC source. You can use either pencil-type Ni-Cd batteries or rechargeable batteries as the power source.

Assemble the circuit on a general-purpose PCB and enclose in a small box. Mount RCA socket on the front panel of the box and wire RCA plug with cable for connecting the battery and LED-based lamp to the charger.

ELECTRONICS PROJECTS Vol. 24 167

Song number DiSplay Prabhash K.P.

Here’s a circuit to display the song number in an audio system for quick reference to songs. It also

serves the purpose of an extra visual indi-cator in modern audio systems.

When the power is switched on, the power-on-reset circuit comprising 3.3k resistor R20 and 1µF, 25V capacitor C6 resets the counters, showing ‘00’ in the dis-play. One can also reset the display to zero at any time by pressing reset switch S1.

When the first song starts playing, the output pins of IC1 (KA2281) go low and capacitor C5 starts charging. This forward biases transistor T1 and hence the input to IC3 at pin 1 goes to high state. As a

result, the output of the counter goes to the next state, showing 01 on the display. The counter remains in this state until the song is completed.

During the time gap before the next song starts playing, capacitor C5 discharges. After discharging of capacitor C5, the input to IC3 becomes low again. When the song starts, the process described above is repeated and the display shows 02. You can adjust VR3 to change the time gap setting. This must be set such that the circuit doesn’t respond to short gaps, if any, within a song and responds only to long gaps between different songs.

Transistor T2 helps in gap-delay ad-justment. The intensity of LED11 dimin-

ishes when a song is completed and the counter is ready to accept the next pulse.

Connect the input to the preamp output or equaliser output of the audio system. Adjust VR1 and VR2 to get the correct audio-level indication. If you are already using KA2281 for audio-level indication, just connect diodes D1 and D2 as shown in this circuit.

Note that the counter counts the songs by detecting the gaps. Therefore any long gap within a song may cause false trig-gering and the display will also be incre-mented. However, as this is very unlikely to happen, the circuit shows the correct song number almost all the time.

circuitideas

electronics for you • April 2010 • 93w w w . e f y m A g . c o m

W hat binoculars do to im-prove your vision, this personal sound enhancer

circuit does for listening. This light-weight gadget produces an adjustable gain on sounds picked up from the built-in high-sensitivity condenser microphone. So you can hear what you have been missing. With a 6V (4×1.5V) battery, it produces good results.

As shown in Fig. 1, a small signal amplifier is built around transistor BC547 (T1). Transistor T1 and the relat-

ed components amplify the sound signals picked up by the condenser microphone (MIC). The amplified signal from the preamplifier stage is fed to input pin 3 of IC LM386N (IC1) through capacitor C2 (100nF) and volume control VR1 (10-kilo-ohm log). A decoupling network comprising re-sistor R5 and capacitor C3 provides the preamplifier block with a clean supply voltage.

Audio amplifier IC LM386N (IC1)

T.K. HAREENDRAN

SPY EAR s.c. dwivedi is designed for operation

with power supplies in the 4-15V DC range. It is housed in a standard 8-pin DIL package, con-sumes very small quies-cent current and is ideal for battery-powered portable applications.

The processed out-put signal from capaci-tor C2 goes to one end of volume control VR1.

The wiper is taken to pin 3 of LM386N audio output amplifier. Note that the R6-C4 network is used to RF-decouple positive-supply pin 6 and R8-C7 is an optional Zobel network that ensures high frequency stability when feeding an inductive headphone load.

Capacitor C6 (22µF, 16V) wired between pin 7 and ground gives ad-ditional ripple rejection. The output of LM386N power amplifier can safely drive a standard 32-ohm monophonic headphone/earphone.

Assemble the circuit on a small general-purpose PCB and house in a suitable metallic enclosure with an integrated battery holder and head-phone/earphone socket as shown in Fig. 2. Fit the on/off switch (S1), vol-ume control (VR1) and power indicator (LED1) on the enclosure. Finally, fit the condenser microphone (MIC) on the front side of the enclosure and link it to the input of the preamplifier via a short length of the shielded wire.

Fig. 2: Compact unit of spy ear

Fig. 1: Circuit for spy ear

circuitideas

94 • December 2009 • electronics for you w w w . e f y m a g . c o m

Today telephone has become an integral part of our lives. It is the most widely used

communication device in the world. Owing to its immense popularity and

widespread use, there arises a need for call recording devices, which find ap-plication in call centres, stock broking firms, police, offices, homes, etc.

Here we are describing a call re-corder that uses very few components. But in order to understand its working, one must first have the basic knowl-

edge of standard telephone wiring and a stereo plug.

In India, landline telephones pri-marily use RJ11 wiring, which has two wires—tip and ring. While tip is the positive wire, ring is the negative one. And together they complete the

telephone circuit. In a telephone line, voltage between tip and ring is around 48V DC when handset is on the cradle (idle line). In order to ring the phone for an incoming call, a 20Hz AC cur-rent of around 90V is superimposed over the DC voltage already present in the idle line.

The negative wire from the phone line goes to IN1, while the posi-

tive wire goes to IN2. Further, the negative wire from OUT1 and the posi-tive wire from OUT2 are connected to the phone. All the resis-tors used are 0.25W carbon film resistors and all the capaci-

tors used are rated for 250V or more. The negative terminal of ‘To AUX IN’ is connected to pin 1 of the stereo jack while the positive terminal is con-nected to pins 2 and 3 of the stereo jack. This stereo jack, in turn, is con-nected to the AUX IN of any recording device, such as computer, audio cas-

sette player, CD player, DVD player, etc. Here we shall be connecting it to a computer.

When a call comes in, around 90V AC current at 20Hz is superimposed over the DC voltage already present in the idle line. This current is converted into DC by the diodes and fed to resis-tor R1, which reduces its magnitude and feeds it to LED1. The current is further reduced in magnitude by the resistor R2 and fed to the right and left channels of the stereo jack, which are connected to the AUX IN port of a computer.

Any audio recording software, such as AVS audio recorder (available at: http://www.avs4you.com/AVS-Audio-Recorder.aspx), Audacity audio recorder (http://audacity.sourceforge.net/), or audio recorder (http://www.audio-tool.net/audio_recorder_for _free.html), can be used to record the call. When a call comes in, one needs to launch the audio recording software and start recording.

For phone recording, simply con-nect the stereo jack to the AUX IN port of the PC. Install the Audacity audio recorder (different versions are available for free for different op-erating systems at http://audacity.sourceforge.net/) on your PC. Run the executable Audacity file. In the main window, you will find a drop-down box in the top right corner. From this box, select the AUX option. Now you are ready to record any call. As soon as a call comes in, press the record button found in the Audacity main window and then pick up the telephone receiver and answer the call. Press the stop button once the call ends. Now go to the file menu and select the ‘Export as WAV’ option and save the file in a desired location.

You may change the value of resis-

AlizishAAn KhAtri

telephone cAll recorder s.c. dwivedi

Fig. 1: Call recorder circuit

Fig. 2: Pin configuration of stereo jack

Fig. 3: RJ connector

circuitideas

electronics for you • December 2009 • 95w w w . e f y m a g . c o m

tor R2 if you want to change the output volume. You can use a variable resistor in series with R2 to vary the volume of the output. The recorded audio clip can be edited using different options in the Audacity software.

You can assemble the circuit on a

general-purpose PCB and enclose it in a small cabinet. Use an RJ11 connec-tor and stereo jack for connecting the telephone set and computer (for call recording). Telephone cords can be used to connect to the phone line and the circuit. Use of a shielded cable is

recommended to reduce disturbances in the recording. These can also be reduced by increasing the value of R2 to about 15 kilo-ohms.

EFY note. Audacity recording software is included in this month’s EFY-CD under ‘Utilities’ section.

C I R C U I T I D E A S

ELECTRONICS FOR YOU SEPTEMBER 2003

TELEPHONE RECEIVER S.C. DWIVEDI

S.K. ROUSHON

This simple telephone receiver with-out a dialling section can be con-nected in parallel to a telephone line.

It can be easily assembled on a small veroboard or a PCB. A geometry box made inthe shape of a telephone receiver will bean excellent cabinet for it. No external

power supply is needed, which makes thecircuit handy.

The ringer section comprises R1, C1,and a buzzer. If your telephone has aloud ringer, this circuit can be avoided. A

bridge rectifier consisting of diodes D1through D4 protects the circuit from anypolarity change in the telephone line. PNPtransistor MPS-A92 (T1) is the main inter-face transistor. The output of T1 is regu-lated by zener diode ZD and capacitor C2to get 6.8V for powering the amplifiersection. This power is also used to biasthe transmitter section.

The transmitter section comprises tran-sistor BC548 (T2) together with a fewdiscrete components and a condenser mi-crophone. The transmit signal is fed tothe base of interface transistor T1. The

voice input for the amplifier comes di-rectly from the positive end of the bridgerectifier.

The amplifier section is built aroundhigh-performance, low-wattage power am-plifier IC LM386. This circuit is designedas a high-gain amplifier. A small 8-ohmspeaker is good enough for the output.

After all soldering is done, adjust pre-

sets VR1 and VR2 to their middle positionand connect the circuit to the telephoneline in parallel. Adjust VR1 and VR2 foroptimum reception as well as transmis-sion.

circuitideas

96 • February 2008 • electronics for you w w w . e F y m a g . c o m

Here is a simple tester for check-ing the basic operations of an infrared remote control unit.

It is low-cost and easy to construct.The tester is built around infrared

receiver module TSOP1738. Operation of the remote control is acknowledged by a tone from the buzzer. The circuit is sensitive and has a range of approxi-

T.A. BABu

TesTer For remoTe ConTrol s.c. dwivedi

mately five metres. The integrated IR receiver detects, amplifies and de-modulates IR signals from the remote control unit. The piezobuzzer con-nected at its output sounds to indicate the presence of signal from the remote control unit.

As shown in Fig. 1, output pin 3 of IR receiver module TSOP1738 (IRX1) normally remains high and the pi-ezobuzzer is in silent mode. When the

IR module I R X 1 r e -ceives an i n f r a r e d signal, its output goes low and, as a result, the piezobuzz-er sounds to indicate the recep-tion of sig-Fig. 1: Circuit diagram of remote tester

nal from the remote (such as TV remote control unit).

P o w e r supply for the circuit is derived from the mains us-ing a capaci-

tive potential dropper, a half-wave rec-tifier, a shunt regulator and associated components. Make sure that capacitor C1 is of X2 type. Use a suitably small enclosure to make the unit handy.

Assemble the circuit on a general-purpose PCB and enclose in a cabinet. Make sure that the IR receiver module is placed on the front panel of the cabi-net so that it can receive the IR signals easily. Before soldering/connecting the shunt regulator and IR module, refer Fig. 2 for the pin configuration.

Fig. 2: Pin configuration of TL431 and TSOP 1738

circuitideas

88 • august 2008 • electronics for you w w w . e f y m a g . c o m

R.G. ThiaGaRaj KumaR and P. Kasi Rajan

ThRee-Phase aPPliance PRoTecToR

s.c. dwivedi

Many of our costly appliances require three-phase AC sup-ply for operation. Failure

of any of the phases makes the appli-ance prone to erratic functioning and may even lead to failure. Hence it is of paramount importance to moni-tor the availability of the three-phase supply and switch off the appliance in the event of failure of one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations.

The complete circuit of a three-phase appliance protector is described here. It requires three-phase supply, three 12V relays and a timer IC NE555 along with 230V coil contactor having

four poles.Relays RL1 and RL2 act as a

sensing devices for phases Y and B, respectively. These relays are connected such that each acts as an enabling device for the subsequent relay. Therefore the combination of the relays forms a logical AND gate connected serially.

The availability of phase R en-ergises relay RL1 and its normally-opened (N/O) contacts close to connect phase Y to the input of transformer X2. The availability of phase Y energises relay RL2 and its N/O contacts close to connect phase B to the input of transformer X3, thus applying a triggering input to timer IC NE555 (IC1).

Therefore the delay timer built around NE555 triggers only when all the phases (R, Y and B) are avail-able. It provides a delay of approxi-

mately four seconds, which energises relay RL3 and its N/O contact closes to connect the line to the energising coil of four-pole contactor relay RL4. Contactor RL4 closes to ensure the availability of the three-phase sup-ply to the appliance.

The rating of contactor RL4 can be selected according to the full-load cur-rent rating of the appliances. Here the contact current rating of the four-pole contactor is up to 32A. The availability of phases R, Y and B is monitored by appropriate LEDs connected across the secondary windings of transform-ers X1, X2 and X3, respectively. Hence this circuit does not require a separate

circuitideas

electronics for you • august 2008 • 89w w w . e f y m a g . c o m

indicator lamp for monitoring the availability of the three phases. When phase R is available, LED1 glows. When phase Y is available, LED2 glows. When phase B is available, LED3 glows.

The main advantage of this protec-tor circuit is that it protects three-phase appliances from failure of any of the

mounted on the backside of cabinet. Connect the appliance through exter-nal wires.

Caution. To avoid the risk of elec-tric shock, ensure that AC mains is disconnected during assembly of the circuit and double check everything before connecting your circuit to the mains.

phases by disconnecting the power supply through the contactor and automatically restores the three-phase supply to the appliance (with reason-able time delay) when all the phases are available.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a cabinet with the relays and contactor

circuitideas

94 • january 2009 • electronics for you w w w . e f y m a g . c o m

Ashok k. Doctor

trAffic BAton s.c. dwivedi

other uses bright LEDs. Both the cir-cuits operate off a 6V, 4.5Ah recharge-

able battery, w h i c h i s c l ipped to the police-man’s waist-band.

F i g . 1 shows the circuit of the LED flasher. It is wired as an astable multivibra-tor. The ‘on’ time of the LED cluster is about 108 milliseconds and ‘off’ time is around 105 milliseconds. The frequen-cy is around 5 Hz. A di-ode is used

in series with the base of BD140 to increase the forward voltage in order to ensure that when BD139 conducts, BD140 is cut-off. Select the LED which consumes low current (20 mA or so) but flashes bright.

Fig. 2 shows the circuit of the bulb flasher. Timer NE555 is wired as an astable multivibrator. The ‘on’ period of flashing bulb is around 344 milli-seconds and ‘off’ period is around 329 milliseconds. The frequency is around 1.5 Hz. Bulb-driver transistors 2N3053/BD139 and 2N2905/BD140 are used to light up the lamp. Two diodes are used in series with the base of 2N2905 to increase the forward voltage in order to ensure that when BD139 is conduct-ing, BD140 is cut-off. Slide switch S2 is used to change the colour status of the

In small towns, there are no traffic lights and the police regulates the traffic with hand signals. Since

Fig. 1: Circuit of LED flasher

their hand signals may not be visible at night, it is necessary to have some illuminated direction indicator.

Here we present two circuits for the same. One uses 6V bulbs and the

Fig. 2: Circuit of bulb flasher

circuitideas

electronics for you • january 2009 • 95w w w . e f y m a g . c o m

Fig. 3: Traffic baton for LED flasher

flashing bulb.Assemble the LED flasher and

bulb flasher circuits on separate gen-eral-purpose PCBs. Enclose the LED flasher in a transparent acrylic pipe as shown in Fig. 3. The bulb flasher can be enclosed in another transparent acrylic pipe as shown in Fig. 4. Slide switches and red and green acrylic sheets are used for appropriate colour emissions. Now your traffic baton is ready to use.

Fig. 4: Traffic baton for bulb flasher

circuitideas

80 • November 2008 • electronics for you w w w . e f y m a g . c o m

N.R. PaRaNjaPe

TRaffic coNTRolleR s.c. dwivedi

This simple traffic controller can be used to teach children rudi-ments of traffic rules.

The circuit (shown in Fig. 1) uses readily available components. It main-ly comprises rectifier diodes (1N4001), a 5V regulator 7805, two timers IC 555, two relays (5V, single-changeover), three 15W, 230V bulbs and some dis-crete components.

Mains power is stepped down by transformer X1 to deliver a secondary output of 9V, 300 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7805 (IC1).

IC2 is wired as a multivibrator with ‘on’ and ‘off’ periods of approximately 30 seconds each with the component values selected. As soon as mains power is switched on, pin 3 of IC2 goes high for 30 seconds. This, in turn, ener-gises relay RL1 through transistor T1 and the red lamp (B1) glows through its normally-open (N/O) contact. At the same time, mains power is discon-nected from the pole of relay RL2.

As the ‘on’ time of IC2 ends, a

Fig. 1: Circuit of traffic controller

Fig. 2: Construction details of traffic controller unit

high-to-low pulse at its pin 3 triggers IC3 through C5. IC3 is configured as a monosta-ble with ‘on’ time of about 4 seconds, which means pin 3 of IC3 will remain high for this period and energise relay RL2 through driver transistor T2. The amber lamp (B2) thus lights up for 4 seconds.

As soon as 4-second time period of timer IC3 at pin 3 lapses, relay RL2 de-ener-gises and the green lamp (B3) lights up for the rest of ‘off’ period of IC2, which is about 26 seconds. The green lamp is activated through the nor-mally closed (N/C) contacts of relay RL2.

So when mains power is switched on, red light glows for 30 seconds, amber for 4 seconds and green for 26 seconds.

You can assemble this cir-cuit on a general-purpose PCB and enclose in an insulated box. The box should have enough space for mounting transformer X1 and two re-lays. It can be fixed near 230V

circuitideas

electronics for you • November 2008 • 81w w w . e f y m a g . c o m

AC, 50Hz power supply or mounted on the PVC tube used in construction of the traffic light container.

Construction of the traffic light container box is shown in Fig. 2. A stout cardboard box of 30x15x10cm3 is required for housing the lamps. To en-sure strength, use a 10x45cm2 plywood plate having 1.5cm thickness and se-cure onto it three light sockets and the

box using nuts and bolts or screws.Make three tubes of thin alumini-

um sheet, which is readily available in hardware shops. The inner diameter of aluminium tubes should be such that these can snugly fit on the light sock-ets. Using a sharp knife, make holes opposite the sockets carefully. Wire the sockets at the back and take the wires out through the PVC tube.

First, fix three 15W bulbs (B1 through B3) and then press on the tubes. Support the other ends of the tubes in the holes made on the front panel of cardboard box. Sandwich gela-tine papers of the three colours between two sheets of cardboard and fix over the tubes. The visibility of red, amber and green lights improves with their mount-ing on the tubular shape.

circuitideas

electronics for you • Mar ch 2009 • 81w w w . e f y M a g . c o M

Sandip Trivedi and p.d. LeLe

TripLe power SuppLy s.c. dwivedi

in positive and negative regulated power supplies. LED1 glows to indi-cate that +5V is available, while LED2 indicates that –5V is available.

Switch S1 is used for mains ‘on’/‘off’. Using switches S2 through S4, any of the three supplies can be independently turned off when not required in a particular experiment. This reduces unnecessary power dis-sipation and increases the life and reliability of the power supply. Since the circuit uses three terminal regula-tors, only capacitors are required at the input and output. The use of few components makes the circuit very simple. The three terminal regulators have heat-sink provision to directly deliver 1A output current. To ensure the maximum output, do not forget to

This low-cost, multipurpose power supply fulfils the re-quirements of almost all labora-

tory experiments. Nonetheless, it can be easily fabricated by hobbyists.

A single transformer is used to build this triple power supply. Regula-tor IC LM317 generates variable power supply of 1.25 to 20V, 1A. The dual ±12V, 1A power supply is generated by regulators 7812 and 7912. Similarly, dual ±5V, 1A power supply is gener-ated by regulators 7805 and 7905. ‘On’/‘off’ switches (S2 through S4) select the required power supply. Vari-able power supply is used to study the characteristics of devices. Fixed +5V power supply is used for all digital, microprocessor and microcontroller experiments. Dual ±12V power supply

is used for op-amp-based analogue circuit experiments.

Fig. 1 shows the circuit of the triple power supply, while Fig. 2 shows the pin configuration of the regulators used in the circuit. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. The transformer output is rectified by full-wave bridge rectifier BR1, filtered by capacitors C1, C2, C3, C7 and C8, and regulated by IC1 through IC5. Regulator IC1 (LM317) provides vari-able voltages (1.25 to 20V), while IC2 and IC4 provide regulated +12V and –12V, respectively. The output of IC2 is fed to regulator IC3 (7805), which pro-vides fixed +5V. Similarly, the output of IC4 is fed to regulator IC5 (7905), which provides fixed –5V. Capacitors C4 through C6, and C9 through C11, are used for further filtering of ripples

Fig. 1: Tripple power supply

OUTIN IC378051

23

GNDF1

1.5AFUSE

S1ON/OFFSWITCH

S2

230V AC50Hz

L

N

X1

BR1W04

C11000µ35V

C510µ16V

C4100µ25V

C9100µ25V

C20.1µ

C30.1µ

C60.1µ

C80.1µ

C110.1µ

C1010µ16V

C71000µ35V

S3

S4

R2330

R1120

R3330

LED1

LED2

GND

GND

+5V

–5V

OUTIN IC278121

2

3

GND

OUTIN

IC47912

2

1

3

GND

OUTIN

IC57905

2

1

3

GND

OUTIN IC1LM3173

1

2

ADJ.

VR12.2K

+1.25 TO 20V

+12V

–12V

X1 = 230V ACPRIMARY TO 18V-0-18V,

1.5A SECONDARYTRANSFORMER

BR1-W041.5A, BRIDGE

RECTIFIER

GND

BR1W04

HEAT SINK

HEAT SINK

HEAT SINK HEAT SINK

HEAT SINK

S2 = FOR VARIABLE VOLTAGE

S3 = FOR +12V AND +5V

S4 = FOR –12V AND –5V

S1-S4 = ON/OFF SWITCH

POT

circuitideas

82 • Mar ch 2009 • electronics for you w w w . e f y M a g . c o M

use heat-sinks for the regulators. The three-terminal regulators are

almost non-destructible. These have inbuilt protection circuits including the thermal shutdown protection. Even if there is overload or shorting of the output, the inbuilt overload protection circuit will limit the current and slowly reduce the output voltage to zero. Similarly, if the temperature increases beyond a certain value due to excessive load and heat dissipation, the in-built thermal shutdown circuit will reduce the output current and the output volt-age (gradually) to zero. Thus complete protection is provided to the circuitry.

Assemble the circuit on a general-purpose PCB and enclose in a box as shown in Fig. 3.

The step-by-step procedure to build the triple power supply for the labora-tory follows:

Fig. 2: Pin configurations of regulators

Fig. 3: Proposed cabinet for power supply

1. Collect all the components shown in the circuit diagram.

2. Connect switch S1, fuse, trans-former and mains cord to the assem-bled PCB as well as the box.

3. Keep the multimeter in DC volt-age range (more than 25V DC) and measure the DC voltage across ca-pacitors C1 and C7 (1000 µF, 35V). This voltage should be around 18V×1.41=25 to 26V DC. Check both positive and negative voltages with respect to ground.

4. It is advisable to use three-wire mains cable and plug. If you are using any metallic box, earthing wire/pin of the mains plug should be soldered to the body of the metallic box using an

earthing tag. 5. If the 18V-0-18V

transformer is replaced with 15V-0-15V trans-former, the output voltage of the variable supply using LM317 will be correspond-ingly lower.

6. If proper voltages are available, go to step 7. Otherwise, check the connections.

7. Connect variable regulator LM317 to the circuit and check 1.25V to 20V output by varying the 2.2-kilo-ohm linear potentiometers.

8. Now connect ICs 7812, 7912, 7805 and 7905 to the circuit and check their output voltage.

9. Connect terminals, potmeter, switches and indicator LED on the front panel of the box and complete the connections. Close the box by us-ing screws.

Precaution. At the primary side of the transformer, 230V AC could give lethal shocks. So be careful not to touch this part. EFY will not be responsible for any resulting loss or harm to the user.

circuitideas

w w w . e f y m a g . c o m electronics for you • april 2008 • 87

T.A. BABu

uSB Power BooSTer s.c. dwivedi

power signal from the PC (+5V) is re-ceived through socket A, LED1 glows, opto-diac IC1 conducts and TRIAC1 is triggered, resulting in availability of mains supply from the primary of transformer X1. Now transformer X1 delivers 12V at its secondary, which is rectified by a bridge rectifier compris-ing diodes D1 through D4 and filtered by capacitor C2.

Regulator 7805 is used to stabilise

The USB serial bus can be con-figured for connecting several peripheral devices to a single

PC. It is more complex than RS232, but faster and simpler for PC expansion.

Since a PC can supply only a lim-ited power to the external devices con-nected through its USB port, when too many devices are connected simultane-ously, there is a possibility of power shortage. Therefore an external power source has to be added to power the external devices.

In USB, two different types of con-nectors are used: type A and type B. The circuit presented here is an add-on unit, designed to add more power to a USB supply line (type-A). When Fig. 2: Pin configurations of moc3021, bt136 and 5v regulator 7805

Fig. 1: Circuit of the usb power booster

the rectified DC. Capacitor C3 at the output of the regulator bypasses the ripples present in the rectified DC out-put. LED1 indicates the status of the USB power booster circuit.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Bring out the +5V, ground and data points in the type-A socket.

Connect the data cables as assigned in the cir-cuit and the USB power booster is r e a d y t o function.

circuitideas

electronics for you • april 2009 • 89w w w . e f y m a g . c o m

T.K. Hareendran

USB Power SocKeT s.c. dwivedi

Today, almost all computers con-tain logic blocks for implement-ing a USB port. A USB port, in

practice, is capable of delivering more than 100 mA of continuous current at 5V to the peripherals that are connected to the bus. So a USB port can be used, without any trouble, for powering 5V DC operated tiny electronic gadgets.

Nowadays, many handheld de-vices (for instance, portable reading lamps) utilise this facility of the USB port to recharge their built-in bat-tery pack with the help of an internal circuitry. Usually 5V DC, 100mA cur-rent is required to satisfy the input power demand.

Fig. 1 shows the circuit of a versatile USB power socket that safely converts the 12V battery voltage into stable 5V. This circuit makes it possible to power/recharge any USB power-operated de-vice, using in-dash board cigar lighter socket of your car.

The DC supply available from the cigar lighter socket is fed to an adjust-able, three-pin regulator LM317L (IC1).

Capacitor C1 buffers any disorder in the input supply. Resistors R1 and R2 regulate the output of IC1 to steady 5V, which is available at the ‘A’ type female USB socket. Red LED1 indicates the out-put status and zener diode ZD1 acts as a protector against high voltage.

Fig. 1: Circuit of USB power socket

Assemble the circuit on a gen-eral-purpose PCB and enclose in a slim plastic cabi-net along with the indicator and USB socket. While wir-ing the USB out-let, ensure correct

polarity of the supply. For intercon-nection between the cigar plug pin and the device, use a long coil cord as shown in Fig. 2. Pin configuration of LM317L is shown in Fig. 3.

Fig. 3: Pin configuration of LM317L (To-92 package)

CIGARPLUG COIL CORD

USB POWER SOCKETWITH INDICATOR

Fig. 2: Interconnection of cigar plug and USB power socket using a coil cord

circuitideas

106 • February 2010 • electronics for you w w w . e F y m a g . c o m

You can use this versatile probe for continuity testing and iden-tification of transistor type and

transformer windings. The n-side or p-side of a transistor can be identified quickly in one go. You can make two contacts with the probe in one hand

while the other hand is free.

Fig. 1 shows the circuit of the probe. The operation

of the circuit is simple. It is driven by an alter-nating current flow-ing through two LEDs (LED1 and LED2). So one LED corresponds to forward direction of current flow, while the other shows reverse di-rection of current flow. This helps to detect ori-entation of the p-n junc-tion with respect to the probes. The LEDs can be arranged near the probes to glow either for the p-side or the n-side as per your choice.

The frequency is de-termined by capacitor C1 and preset VR1 con-nected between gates G1

Raju R. Baddi

VeRsatile PRoBe

s.c. dwivedi

Fig. 1: Circuit of versatile probe

Testing Results for Different ComponentsComponent Probe D Probe C Red LED Green LED Result

Diode 1stterminal 2ndterminal Off On ProbeDsideisanode(p)andprobeC 1stterminal 2ndterminal On Off sideiscathode(n) ProbeDsideis‘n’andprobeCsideis‘p’

Transistors C E X X UnusedpinisbaseAnytypepnpornpn E C X X Unusedpinisbase

npn-typetransistor B E On Off p-njunction B C On Off p-njunction Result:‘p’iscommon,sonpntransistor

pnp-typetransistor B E Off On n-pjunction B C Off On n-pjunction Result:‘n’iscommon,sopnptransistor

Step-downtransformer Primaryterminal1 Primaryterminal2 Glowwith Glowwith BothLEDsglowingwithlowintensity lowintensity lowintensity Result:Primaryside

Secondaryterminal1 Secondaryterminal2 Glowwith Glowwith BothLEDsglowingwithhighintensity highintensity highintensity Result:Secondaryside

Continuity Connectwith X On Off Indicatesshorting LEDsprobe

Fig. 2: Constructional detail of versatile probe

circuitideas

electronics for you • February 2010 • 107w w w . e F y m a g . c o m

and G2. The frequency can be varied using preset VR1. Higher frequency results in more sensitivity to inductive reactance. The preset is trimmed so that when the probes are shorted, both the LEDs glow equally.

Fig. 2 shows the probe arrangement for testing. Most of the battery power is consumed only when the LEDs glow. The probes have been constructed to provide a good grip on the components

under testing. One probe’s tip has been widened. (Drop the empty refill of a ball-pen from some height to remove the ball, then insert a sharp needle or something similar into the tip. Slowly push the needle inside and widen the tip so that a component lead can be inserted into it during testing.) Slightly unequal probe lengths help to make easy contacts.

Assemble the circuit on a general-

purpose PCB which is as compact as possible and put it inside a glue stick tube (whose inner mechanism has been removed) at its centre. The metallic disk and metallic strips can be cut out from any tin container. For the probes, use the spring mechanism of gel ball pens. Probes C and D are the points representing the probe terminals. Two button cells (CR2032) are used to power the probe circuit.

ELECTRONICS PROJECTS Vol. 20

Water LeveL IndIcator

WIth aLarmVIJay D. SaThE

Here is a simple, versatile circuit which indicates the level of water in a tank. This circuit pro-

duces alarm when water level is below the lowest level L1 and also when water just touches the highest level L12. The circuit is designed to display 12 differ-ent levels. However, these display levels can be increased or decreased depending upon the level resolution required. This can be done by increasing or decreasing the number of level detector metal strips (L1 through L12) and their associated components.

In the circuit, diodes D1, D2 and D13 form half-wave rectifiers. The rectified output is filtered using capacitors C1 through C3 respectively.

Initially, when water level is below strip L1, the mains supply frequency oscillations are not transferred to diode D1. Thus its output is low and LED1 does not glow. Also, since base voltage of tran-sister T1 is low, it is in cut-off state and its collector voltage is high, which enables melody generating IC1 (UM66) and alarm is sounded.

When water just touches level detector strip L1, the supply frequency oscillations are transferred to diode D1. It rectifies the supply voltage and a positive DC voltage develops across capacitor C1, which lights up LED1. At the same time base voltage for transistor T1 becomes high, which makes it forward biased and its collector voltage falls to near-ground potential. This disables IC1 (UM66) and alarm is inhibited.

Depending upon quantity of water present in the tank, corresponding level indicating LEDs glow. It thus displays intermediate water levels in the tank in bar-graph style.

When water in the tank just touches the highest level detector strip L12, the DC voltage is developed across capacitor C2. This enables melody generating IC1 (UM66) and alarm is again sounded.

ELECTRONICS PROJECTS Vol. 22158

joydeep kumar chakraborty

Water-LeveL ControLLer

In most houses, water is first stored in an underground tank (UGT) and from there it is pumped up to the

overhead tank (OHT) located on the roof. People generally switch on the pump when their taps go dry and switch off the pump when the overhead tank starts overflow-ing. This results in the unnecessary wast-age and sometimes non-availability of water in the case of emergency.

The simple circuit presented here makes this system automatic, i.e. it switches on the pump when the water level in the overhead tank goes low and switches it off as soon as the water level reaches a pre-determined level. It also prevents ‘dry run’ of the pump in case the level in the underground tank goes below the suction level.

In the figure, the common probes con-necting the underground tank and the overhead tank to +9V supply are marked ‘C’. The other probe in underground tank, which is slightly above the ‘dry run’ level, is marked ‘S’. The low-level and high-level probes in the overhead tank are marked ‘L’ and ‘H’, respectively.

When there is enough water in the underground tank, probes C and S are connected through water. As a result, transistor T1 gets forward biased and starts conducting. This, in turn, switches transistor T2 on. Initially, when the overhead tank is empty, transistors T3 and T5 are in cut-off state and hence pnp transistors T4 and T6 get forward biased via resistors R5 and R6, respectively.

As all series-connected transistors T2, T4, and T6 are forward biased, they conduct to energise relay RL1 (which is also connected in series with transistors T2, T4, and T6). Thus the supply to the pump motor gets completed via the lower set of relay contacts (assuming that switch S2 is on) and the pump starts filling the overhead tank.

Once the relay has energised, tran-sistor T6 is bypassed via the upper set of contacts of the relay. As soon as the water level touches probe L in the over-head tank, transistor T5 gets forward

biased and starts conducting. This, in turn, reverse biases transistor T6, which then cuts off. But since transistor T6 is bypassed through the relay contacts, the pump continues to run. The level of water continues to rise.

When the water level touches probe H, transistor T3 gets forward biased and starts conducting. This causes reverse biasing of transistor T4 and it gets cut off. As a result, the relay de-energises and the pump stops. Transis-tors T4 and T6 will

ELECTRONICS PROJECTS Vol. 22 159

be turned on again only when the water level drops below the position of L probe.

Presets VR1, VR2, and VR3 are to be adjusted in such a way that transis-tors T1, T3, and T5 are turned on when the water level touches probe pairs C-S, C-H, and C-L, respectively. Resistor R4 ensures that transistor T2 is ‘off’ in the absence of any base voltage. Similarly, resistors R5 and R6 ensure that transis-tors T4 and T6 are ‘on’ in the absence of any base voltage. Switches S1 and S2 can

be used to switch on and switch off, respec-tively, the pump manually.

You can make and install probes on your own as per the requirement and facilities available. However, we are de-scribing here how the probes were made for this prototype.

The author used a piece of non-metallic conduit pipe (generally used for domestic wiring) slightly longer than the depth of the overhead tank. The common wire C goes up to the end of the pipe

through the conduit. The wire for probes L and H goes along with the conduit from the outside and enters the conduit through two small holes bored into it as shown in Fig. 2.

Care has to be taken to ensure that probes H and L do not touch wire C directly. Insulation of wires is to be removed from the points shown. The same arrangement can be followed for the underground tank also. To avoid any false triggering due to interfer-ence, a shielded wire may be used.

circuitideas

electronics for you • May 2010 • 101w w w . e f y M a g . c o M

This water-level indicator uses a 7-segment display, instead of LEDs, to indicate the water

level (low, half and full) in the tank. Moreover, a buzzer is used to alert you of water overflowing from the tank. The circuit shows the water level by displaying L, H and F for low, half and full, respectively.

The circuit uses five sensors to sense the different water levels in the

tank. Sensor A is connected to the negative terminal (GND) of the power supply. The other four sensors (B through E) are connected to the inputs of NOT gate IC 7404. When there is

a high voltage at the input pin of the NOT gate, it outputs a low voltage. Similarly, for a low voltage at the input pin of the NOT gate, it outputs a high voltage.

When the tank is empty, the input pins of IC 7404 are pulled high via a 1-mega-ohm resistor. So it outputs a low voltage. As water starts filling the tank, a low voltage is available at the input pins of the gate and it outputs a high voltage.

When the water in the tank rises to touch the low level, there is a low voltage at input pin 5 of gate N3 and high output at pin 6. Pin 6 of the gate is connected to pin 10 of gate N9, so pin

Riju ThazhaThu VeeTTil

WaTeR-leVel indicaToR using 7-segmenT display s.c. dwivedi

10 also goes high. Now as both pins 9 and 10 of gate N9 are high, its output pin 8 also goes high. As a result, posi-tive supply is applied to DIS3 and it shows ‘L’ indicating low level of water in the tank.

Similarly, when water in the tank touches the half level, pins 4 and 5 of AND gate N8 become high. As a result, its output also goes high and DIS2 shows ‘H’ indicating half level of water in the tank. At this time, pin 9 of

gate N9 also goes low via gate N4 and DIS3 stops glowing.

When the water tank becomes full, the voltage at pin 1 of gate N1 and pin 3 of gate N2 goes low. Output pin 3 of gate N7 goes high and DIS1 shows ‘F’ indi-cating that the water tank is full.

When water starts overflowing the tank, pin 13 of gate N6 goes low to make output pin 12. The buzzer sounds to indicate that water is over-flowing the tank and you need to switch off the motor pump.

Assemble the cir-cuit on a general-pur-pose PCB and enclose

in a suitable box. Use a non-corrosive material such as steel strip for the five sensors and hang them in the water tank as shown in the circuit diagram. Use regulated 5V to power the circuit.

circuitideas

98 • June 2010 • electronics for you w w w . e f y m a g . c o m

Raj K. GoRKhali

WeeKly RemindeR s.c. dwivedi

This circuit reminds you of all the important tasks that are due on a specific day every

week. So be it returning your library book, switching on your favourite TV programme, putting the dustbin out or cleaning the car, it automatically flashes an LED that very day to alert you of something to be done. The LED keeps flashing until you press the re-set button. The circuit consumes very little power.

The circuit can count the days. The rising of the Sun is detected by a light-dependent resistor (LDR1). When the sun rises, the ambient light level reduces the resistance of LDR1. The voltage level at pin 13 of gate N4 goes low. Since the other input (pin 12) of gate N4 is high, its output also goes high. This is invert-ed by gate N10 whose output goes low to make counter IC3 advance by one count. This way each day is counted. Similarly, the counter ad-vances by one every morning until it counts seven days.

In the morning of the seventh day, all the inputs of gate N9 become high, making its output low. The low output of gate N9 is inverted by gate N11 to trigger the pulse generator built around gates N5 and N7, and it produces a short-duration pulse to trigger the flip-flop built around gates N6 and N8. As a result, the output of the flip-flop goes high to enable the astable multivibrator built around gate N3. The astable mul-tivibrator produces 2Hz frequency to flash LED1 as a reminder. LED1 flashes until you press reset switch S2 momentarily.

When the enable input (pin 8) of gate N3 is low, the output of the asta-ble multivibrator remains high. Gate N2 inverts this high level into low and the transistor does not conduct. So LED1 doesn’t flash when the astable multivibrator is disabled.

Counter IC3 also resets when the pulse generator triggers because its reset pin 11 is connected to the output of gate N11.

When the counter IC3 resets, its output becomes low and it’s ready

to begin day counting for the next week.

Suppose you require a reminder for four days. Then first cover the sensor and press increment switch S1 thrice momentarily and leave it. Now your reminder (flashing of LED1) starts after four days.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a suitable cabinet. After assembling the circuit, proper setting is required. First of all, switch on the power. LED1 flashes. Press switch S2 to stop it from flashing. Cover LDR1 and press S1 several times until LED1 flashes again. The counter is now set at a count of 0 and is ready to start weekdays counting. Press S2 to stop flashing.

Do not uncover the sensor im-mediately after pressing S2. Else, the counter will register arrival of the next day and LED1 will flash after six days. To make it flash weekly on a particular day, keep the LDR1 in dark until night.

circuitideas

100 • october 2008 • electronics for you w w w . e f y m a g . c o m

Normally, home appliances are controlled by means of switch-es, sensors, etc. However,

physical contact with switches may be dangerous if there is any shorting.

The circuit described here requires no physical contact for operating the appliance. You just need to move your hand between the infrared LED (IR LED1) and the phototransistor (T1). The infrared rays transmitted by

IR LED1 is detected by the phototran-sistor to activate the hidden lock, flush system, hand dryer or else.

This circuit is very stable and sensi-tive compared to other AC appliance control circuits. It is simple, compact and cheap. Current consumption is low in milliamperes.

The circuit is built around an IC CA3140, IRLED1, phototransistor and other discrete components. When regu-

Navpreet SiNgh tuNg

WireLeSS SWitCh s.c. dwivedi

lated 5V is connected to the circuit, IR LED1 emits infrared rays, which are received by phototransistor T1 if it is properly aligned. The collector of T1 is connected to non-inverting pin 3 of IC1. Inverting pin 2 of IC1 is connected to voltage-divider preset VR1. Using preset VR1 you can vary the reference voltage at pin 2, which also affects sen-sitivity of the phototransistor.

Op-amp IC1 amplifies the signal received from the phototransistor. Resistor R3 controls the base current of transistor BC548 (T2). The high output of IC1 at pin 6 drives transistor T2 to energise relay RL1 and switch on the appliance, say, hand dryer, through the relay contacts.

The working of the circuit is simple. In order to switch on the appliance, you simply interrupt the infrared rays falling on the phototransistor through your hand. During the interruption, the appliance remains on through the relay. When you remove your hand from the infrared beam, the appliance turns off through the relay.

Assemble the circuit on any gen-eral-purpose PCB. Identify the resistors through colour coding or using the multimeter. Check the polarity and pin configuration of the IC and mount it using base. After soldering the circuit, connect +5V supply to the circuit.