Constant-Current Battery Charger - Diagramas dediagramas.diagramasde.com/otros/EFY 2009.pdf · 2012-05-03 · circuit ideas 114 • August 2009 • electronics for you w w w. e f

circuitideas

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

There are many ways of battery charging but constant-current charging, in particular, is a

popular method for lead-acid and Ni-Cd batteries. In this circuit, the battery is charged with a constant current that is generally one-tenth of the battery capacity in ampere-hours. So for a 4.5Ah battery, constant charging cur-rent would be 450 mA.

This battery charger has the follow-ing features:

1. It can charge 6V, 9V and 12V bat-teries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2.

2. Constant current can be set as per the battery capacity by using a potmeter and multimeter in series with the battery.

3. Once the battery is fully charged, it will attain certain voltage level (e.g.

13.5-14.2V in the case of a 12V battery), give indication and the charger will switch off automatically. You need not remove the battery from the circuit.

4. If the battery is discharged be-low a limit, it will give deep-discharge indication.

5. Quiescent current is less than 5 mA and mostly due to zeners.

6. DC source voltage (VCC) ranges from 9V to 24V.

7. The charger is short-circuit pro-tected.

D1 is a low-forward-drop schottky diode SB560 having peak reverse volt-age (PRV) of 60V at 5A or a 1N5822 diode having 40V PRV at 3A. Nor-mally, the minimum DC source volt-age should be ‘D1 drop+Full charged battery voltage+VDSS+ R2 drop,’ which is approximately ‘Full charged battery voltage+5V.’ For example, if we take full-charge voltage as 14V for a 12V battery, the source voltage should be

14+5=19V. For the sake of simplicity, this con-

stant-current battery charger circuit is divided into three sections: constant-current source, overcharge protection and deep-discharge protection sec-tions.

The constant-current source is built around MOSFET T5, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and potmeter VR1. Diode D2 is a low-temperature-coefficient, highly stable reference diode LM236-5. LM336-5 can also be used with reduced operating temperature range of 0 to +70°C. Gate-source voltage (VGS) of T5 is set by adjusting VR1 slightly above 4V. By setting VGS, charging current can be fixed depending on the battery capacity. First, decide the charging current (one-tenth of the battery’s Ah capacity) and then calculate the nearest standard value of R2 as follows:

R2 = 0.7/Safe fault current

Monoj Das

Constant-Current Battery Charger

s.c. dwivedi

circuitideas

electronics for you • August 2009 • 115w w w . e f y m A g . c o m

R2 and T1 limit the charging cur-rent if something fails or battery termi-nals get short-circuited accidentally.

To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly until the charging current reaches its required value.

Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All com-ponents in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any influ-ence from the charging current value. Set overcharge and deep-discharge voltage of the battery using potmeters VR1 and VR2 before charging the bat-tery.

In overcharge protection, zener

diode ZD1 starts conducting after its breakdown voltage is reached, i.e., it conducts when the battery voltage goes beyond a prefixed high level. Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor con-ducts. As a result, gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops.

Normally, zener diode ZD2 con-ducts to drive transistor T3 into con-duction and thus make transistor T4 cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V bat-tery, adjust potmeter VR3 such that transistor T3 is cut-off and T4 conducts.

LED2 will glow to indicate that the bat-tery voltage is low.

Values of zener diodes ZD1 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current pro-vided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 pack-age of IRF540 can handle up to 50W.

Assemble the circuit on a gen-eral-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount potmeters VR1, VR2 and VR3 on the front panel of the box.

circuitideas

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

At night when power fails, one finds it difficult to reach the generator to start it. Here is

the circuit for a generator room light that automatically turns on at night, facilitating easy access to the generator. During daytime, the light remains off.

Fig. 1 shows the circuit for gen-

erator room light, while Fig. 2 shows the battery charger circuit, which is optional and can be omitted if the gen-erator is self-start type and has built-in battery.

At the heart of the generator room light circuit (Fig.1) is a light-dependent resistor (LDR1) that senses the ambi-ent light as well as light from glowing LED1.

D u r i n g daytime, sun-light or light f r o m L E D 1 reduces the r e s i s t a n c e of LDR1. As a result, the voltage drop across LDR1 decreases and npn transistor T1 does not conduct. The collector of T1 and therefore pins 2 and 6 of

IC1 remain high, making output pin 3 of IC1 low and transistor T2 cut-off. So lamp L1 connected between the collec-tor of T1 and the positive terminal of 12V supply does not glow.

As the ambient light fades dur-ing sunset, the resistance of LDR1 increases. As a result, the voltage drop across LDR1 increases and npn transis-

tor T1 conducts. Pins 2 and 6 of IC1 go low to make its output pin 3 high, and lamp L1 glows.

You can replace incan-descent lamp L1 with bright white LEDs using proper current-limiting resistors.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Install the unit near the gen-erator. Arrange LED1 and LDR1 such that during the availability of mains, light emitted from LED1 falls di-rectly on LDR1. Also, make sure that during daytime the ambient light falls on the LDR.

For powering the battery charger circuit (Fig. 2), 15V AC secondary voltage is

derived from step-down transformer X1. For fast charging of the battery, you may increase the current rating of transformer X1.

The charger charges the battery through a thyristor (SCR1) when the battery voltage is low. The thyristor gets a regulated gate voltage from the zener diode, and goes to tickle charg-ing mode when the battery voltage nears the zener voltage.

Assemble the charger circuit on a general-purpose PCB and enclose in a suitable cabinet. Use two crocodile clips (red for positive and black for negative) for connecting the battery terminal to the charger circuit.

Manuj Paul

Generator rooM liGht s.c. dwivedi

Fig. 1: Circuit for generator room light

Fig. 2: Battery charger circuit (optional)

circuitideas

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

circuitideas

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.

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

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.

circuitideas

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

Raj K. GoRKhali

DuoPhone s.c. dwivedi

This simple circuit of a duo-phone allows you to access two telephone lines through

one telephone set. Each telephone con-versation will remain entirely separate unless you choose to combine the two lines through a conference switch. Its unique feature is a three-party conver-sation/conference facility.

The entire circuit is divided into three main sections—the ringer, hold and conferencing. The telephone set is connected to line 1 under normal conditions. The ringer is used for in-

dicating a call on line 2 that is not con-nected to the telephone receiver. When you have a call on line 2, the ringer will buzz. The telephone receiver can then be connected to line 2 through the telephone changeover switch S4 to receive the call.

The ringer section is built around IC3 and its associated components. Its circuit uses IC 1240 to detect the ring signal and keeps the buzzer ringing for an incoming call on line 2. The sup-ply voltage for the ringer is obtained from the phone line’s AC ring (80V AC RMS) signal and is regulated inside the IC so that the noise on the line does not

affect operation of the IC. The two-tone frequencies generated are switched by an internal oscillator in a fast sequence, which appear at the output amplifier and drive the piezo buzzer element directly.

The hold section is built around IC1 and IC2 . Switch S1 is used to hold line 1 and S2 is used to put line 2 on hold. Since one telephone set is used for two separate lines, provision is thus made to hold the first call while the telephone set is connected to make or receive the second call.

The circuit comprises two identi-cal hold circuits, each with its own flashing LED to maintain the holding current. Each hold circuit has a timer LM555 (IC1 or IC2) connected as a free-running oscillator operating at a frequency of 2 Hz. The output pin 3 of each timer is used for driving an

LED that flashes twice in a second. The hold circuit is powered by the telephone lines through manually-operated hold switches (S1 and S2). Resistors R2 and R6 are placed in the hold circuits to ensure that suf-ficient current is drawn from the telephone line to prevent a discon-nection.

The conferencing section is built around the audio coupling trans-former X1. Switch S3 enables three-way conversation through both the telephone lines. The transformer couples the audio signals from one telephone line to the other. At the same time, complete DC isolation is maintained between both the telephone lines. Capacitors C1 and C3 are used for preventing any DC from flowing into the transformer windings. Resistor R1 provides a holding current on line 1 when the telephone set is connected to line 2 during a conference call. Once the three-way conversation is es-tablished through the double-pole single-throw (DPST) switch S3, the hold circuits and flashing LED indi-cators are turned off. LED3, which gets illuminated by the holding cur-rent through R1, provides a visual

circuitideas

electronics for you • November 2009 • 93w w w . e f y m a g . c o m

indication of the conferencing.The working of the circuit is sim-

ple. To check if the wiring of switch S4 is correct, connect the telephone set to line 1. Now lift up the handset and dial the number of line 2. The ringer would sound. Now discon-nect line 1 and connect line 2 through switch S4. You would get the dial tone from line 2.

To check a conference call, you would need the help of two friends. First connect switch S4 to line 1 and make a call to friend 1. Now flip the DPST switch S3 to the ‘on’ position. This puts on hold friend 1 on line 1 and the conference LED3 lights up. Connect switch S4 to line 2 and dial friend 2. When the call on line 2 is answered, a three-way conversation can be made.

When the duophone is not in use,

connect switch S4 to line 1. All other switches should be in the ‘off’ mode and all LEDs should be unlit. This permits the telephone ringer to be activated if a call comes on line 2. For making calls using line 1 or line 2, you can simply connect switch S4 to the desired line.

Assemble the circuit on a gen-eral purpose PCB and enclose it in a suitable cabinet. Fix the switches S1 through S4 on the front side of the cabinet. Also fix the LEDs on the front of the cabinet and the buzzer at the back of the cabinet. It would be better if you use telephone sockets for the telephone lines. Sockets are relatively inexpensive and save time when troubleshooting needs to be done. Use modular plugs to connect the circuit and the two telephone lines. By using such ‘quick discon-

nect’ plugs, you can easily remove the unit from the telephone lines. Check the polarity of the telephone lines with a multimeter and connect it to the circuit accordingly.

To check the circuit after complet-ing the wiring, connect a 6V regulated power supply to line 1. When you switch S1 to the ‘on’ position, LED1 blinks at a rate of 2 Hz. If you flip switch S1 to the ‘off’ position and switch S3 to the ‘on’ position, LED1 stops blinking and LED3 starts glow-ing, indicating that the conferencing facility is being used. Now disconnect line 1 from the 6V power supply, con-nect it to line 2 and flip switch S2 to the ‘on’ position. Now LED2 blinks at a rate of 2 Hz. Before connecting the circuit to the telephone lines, flip each hold switch to the ‘off’ position. Now your circuit is ready to be used.

circuitideas

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).

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

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.

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.

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.

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

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

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

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

electronics for you • OctOber 2009 • 99w w w . e f y m a g . c O m

Here is a simple circuit that starts playing the car horn whenever your car is in re-

verse gear. The circuit (refer Fig. 1) employs dual timer NE556 to generate the sound. One of the timers is wired

as an astable multivibrator to generate the tone and the other is wired as a monostable multivibrator.

Working of the circuit is simple. When the car is in reverse gear, re-verse-gear switch S1 of the car gets shorted and the monostable timer triggers to give a high output. As a result, the junction of diodes D1

Ashok k. Doctor

cAr-rEVErsING horN WIth FLAshEr

s.c. dwivediand D2 goes high for a few seconds depending on the time period de-veloped through resistor R4 and capacitor C4. At this point, the asta-ble multivibrator is enabled to start oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker,

in turn, produces sound until the output of the mon-ostable is high.

When the junc-tion of diodes D1 and D2 is low, the astable multivibrator is disabled to stop oscillating. The output of the asta-ble multivibrator is fed to the speaker through capacitor C6. The speaker, in turn, does not produce sound.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect the circuit to the car

Fig. 1: Car reverse horn

Fig. 2: Flasher circuit

reverse switch through two wires such that S1 shorts when the car gear is reversed and is open otherwise. To

power the cir-cuit, use the car battery.

The flasher circuit (shown in Fig. 2) is built around timer NE555, which is wired as an astable multi-vibrator that outputs square wave at its pin 3. A 10W auto

bulb is used for flasher. The flashing rate of the bulb is decided by preset VR1.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The flasher bulb can be mount-ed at the car’s rear side in a reflector or a narrow painted suitable enclosure.

EFY note. A higher-wattage bulb may reduce the intensity of the head-light. You can enclose both the car-reversing horn and flasher circuits together or separately in a cabinet in your car.

circuitideas

electronics for you • September 2009 • 119w w w . e f y m a g . c o m

A logic probe is a handheld pen-like probe used for analysing and troubleshooting the logi-

cal states ‘0’ or ‘1’ of a digital circuit. While most logic probes are powered by the circuit under test, some probes use batteries. These can be used for ei-ther TTL (transistor-transistor logic) or CMOS (complementary metallic oxide semiconductor) integrated circuit de-vices. The circuit described here can be used for TTL logic only, and shows the presence of either state (logic 1 or 0).

Raju R. Baddi

TRansisToRised Logic PRoBe foR TTL s.c. dwivedi

As shown in Fig. 1, the logic probe circuit is built around four transis-tors and a few passive components. It uses two LEDs that show logic states 1 (green) and 0 (red). The green LED glows when the voltage at probe tip exceeds about 2.4V and the red LED glows when this voltage falls below about 1.2V, which is adequate to detect the normal TTL levels. When no LED glows, it means the probe is in sus-pended state, i.e., it is neither showing logic 1, nor logic 0.

Working of the logic probe circuit is simple and can be divided into three

states: The probe is suspended, the probe’s tip is at logic 1 and the probe’s tip is at logic 0.

W h e n t h e probe is in sus-pended condi-tion, the junction voltage of resis-tors R1 and R2 is almost equal to that obtained when R1 and R2 alone form

a voltage divider bias for the supply voltage. Under this condition, the base-emitter junction of transistor T1 is for-ward biased, so the voltage drop across resistor R5 (>0.6V) is enough to drive transistor T3 into conduction, which, in turn, cuts off transistor T4. As a result, the red LED (LED2) does not glow. The current flowing through R3 is nearly equal to that through R5. However, this does not produce enough voltage drop (0.3V-0.6V) across resistor R3 to make transistor T2 conduct. As a result, the green LED (LED1) does not glow. Thus neither of the LEDs glows when the probe is in suspended condition.

When the probe tip is at logic 1, transistor T3 forward biases to cut-off transistor T4. But, this voltage should exceed 2.4V (normally TTL logic 1 is well above this voltage), this will cause a greater current to flow through emit-ter resistor R5 of T1, which also flows through collector resistor R3 causing a larger voltage drop across R3. Thus transistor T2 conducts and the green LED (LED1) glows to indicate the pres-ence of logic 1 (high) at the probe tip.

When the probe tip is at logic 0, transistor T3 cuts off to make transistor Fig. 1: Circuit of transistorised logic probe

Fig. 2: Proposed arrangement for compact logic probe

circuitideas

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

T4 conduct. As a result, the red LED (LED2) glows to indicate logic 0 (low). It can be easily seen from the circuit diagram that a voltage of more than 1.2 volts is required at the probe tip to forward-bias transistors T1 and T3 whose base-emitter junctions appear in series in the circuit. Any voltage less than this at the probe tip will not for-

ward-bias T3, resulting in the red LED (LED1) glowing.

Assemble the circuit on a general-purpose PCB and insert in a small plas-tic tube, say, a glue stick whose inner mechanism has been removed. For the probe tip, use the front portion of a gel-pen refill. The proposed arrangement for the compact logic probe is shown

in Fig. 2. Before checking the logic of any circuit, connect the black clip to the ground of the circuit and the red clip to the positive terminal of the circuit.

The main feature of the circuit is that when the supply voltage is less than about 4.0V, the red LED (LED2) glows to indicate non-standard TTL voltage supply.

circuitideas

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

Pallabi Sarkar and anirban SenguPta

automated alarm CirCuitS s.c. dwivedi

Two alarm circuits are presented here. One produces bird-chirp-ing sound and the other British

police siren tone. Fig. 1 shows the circuit of the bird-

chirping-sound alarm unit along with the circuit of the control unit. Fig. 2 shows the circuit of only the British police siren tone generator, which has to be integrated with the control circuit portion of Fig. 1 at points A and B to complete the circuit diagram of auto-mated alarm.

The control unit is built around ICs CD4047 and CD4027 (as shown on the left side of the dotted line in Fig. 1). As mentioned earlier, it is common to both the alarm circuits. IC CD4047 (IC1) is wired in positive-edge-triggering monostable multivibrator mode to set and reset IC CD4027 (IC2). The output pulse width of IC1 depends on the values of capacitor C2 and resistor R3 connected to its pins 1, 2 and 3.

Normally, when the door is closed, reed switch S1 is closed, transistor T1 conducts and the monostable multivi-brator (IC1) remains in standby mode with ‘low’ output at pin 10.

When the door is opened, reed switch S1 gets disconnected, T1 stops conducting and low-to-high pulse at pin

Fig. 1: Alarm circuit that generates bird-chirping sound

Fig. 2: Alarm circuit that generates police siren tone

circuitideas

electronics for you • February 2009 • 97w w w . e F y m a g . c o m

ing sound.For the chirping-sound alarm gen-

erator, assemble the circuit shown in Fig. 1 on a separate general-purpose PCB and enclose in a small box. And if you want an alarm circuit with British police siren tone, assemble the circuit shown in Fig. 2 on another general-purpose PCB and connect it to points A and B of the control unit shown in Fig. 1 after removing the circuit on the right side of the dotted line. Use a 9V, 500mA standard adaptor to power the circuit.

This circuit may be used as a secu-rity alarm in banks, households and motorcars.

8 of IC1 triggers the monostable and a short-duration positive pulse of about 10 seconds is available as Q output at pin 10. At the same time, complementary output Q goes low at pin 11. The output from IC1 is used to set and reset IC2.

IC2 is a low-power, dual J-K mas-ter/slave flip-flop having independ-ent J, K, set, reset and clock inputs. The flip-flops change states on the positive-going transition of the clock pulses. IC2 is wired such that its Q output turns ‘high’ when reset pin 4 receives a high pulse. When set pin 7 receives a high pulse, Q output goes low and Q output goes high. This lights up LED2 and drives transistor

T2 (BC548), which enables the alarm circuit.

The output at point A is used to enable the alarm tone generator circuit (on the right side of the dotted line) consisting of two 555 timer ICs marked as IC3 and IC4. The R-C network de-termines the frequency of the sound produced. The triangular waveform of the astable multivibrator is taken out from the junction of pins 2 and 6 of IC3. This waveform is fed as the control voltage at pin 5 of IC4 through resistor R18. The output received from pin 3 of IC4 is fed to the base of transis-tor T3 to drive an 8-ohm loudspeaker (LS1), which generates the bird-chirp-

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

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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),

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electronics for you • May 2009 • 85w w w . e f y M a g . c o M

The electronic bicycle lock de-scribed here is a worthwhile alternative for bicycle own-

ers who want to make their bicycles ‘intelli-gent’ at reason-able cost. One of the benefits of building it yourself is that the circuit can be used for vir-

tually any make of bicycles.In the circuit, input jacks J1 and

T.K. Hareendran

eLeCTrOnIC BICYCLe LOCK

s.c. dwivedi

Fig. 1: Circuit of electronic bicycle lock

Fig. 2: Lock box

Fig. 3: Lock fitted on the bicycle

J2 are two standard RCA sockets. A home-made security loop can be used to link these two input points. Around 50cm long, standard 14/36 flexible wire with one RCA plug per end is

enough for the security loop.Fig. 1 shows the circuit of the

electronic bicycle lock. It is powered by a compact 9V battery (6F22). Key lock switch S1 and smoothing capacitor C2 are used for connect-ing the power supply. A connected loop cannot activate IC1 and there-fore the speaker does not sound. When the loop is broken, zener diode ZD1 (3.1V) receives operating power supply through resistor R2 to enable

tone generator UM3561 (IC1). IC1 remains enabled until power to the circuit is turned off using switch S1 or the loop is re-plugged through J1 and J2.

Assemble the circuit on a general-purpose PCB and house in a small tin-plate enclosure. Fit the system key lock switch (S1) on the front side of the en-closure as shown in Fig. 2. Place RCA sockets (J1 and J2) at appropriate posi-tions. Now, mount the finished unit in place of your existing lock (as shown in Fig. 3) by using suitable clamps and screws.

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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

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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

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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.

circuitideas

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

D. Mohan KuMar

Pyroelectric fire alarMs.c. dwivedi

Here is an ultra-sensitive fire sensor that exploits the direct piezoelectric property of an

ordinary piezo element to detect fire. The lead zirconate titanate crystals in the piezo element have the property to deform and generate an electric

potential when heated, thus convert-ing the piezo element into a heat sen-sor. The circuit described here is very sensitive. It gives a warning alarm if the room temperature increases more than 10°C. The entire circuit has two sections—the sensor and the power supply section.

Sensor side circuit. Fig. 1 shows the fire sensor circuit. The front end of the circuit has a sensitive signal amplifier built around IC1 (CA3130). It gives a high output when temperature near the piezo element increases. IC CA3130 is a CMOS operational amplifier with

gate protected p-channel MOSFETs in the inputs. It has high speed of performance and low input current re-quirements. There are two inputs—the non-inverting input (pin 3) connected to the piezo element through diode D7 (OA71) that carries the voltage signal from the piezo element and the invert-ing input (pin 2) that gets a preset volt-

age through VR1.

By ad-justing VR1, it is easy to set the ref-erence volt-age leve l at pin 2. In normal con-dition, IC1 gives a low

output and the remaining circuitry is in a standby state. Capacitor C2 keeps the non-inverting input of IC1 stable, so that even a slight change in volt-age level in the inputs can change the output to high.

Normally, IC1 gives a low output, keeping transistor T1 non-conduct-ing. Reseting pin 12 of IC2 (CD4060) connected to the collector of transistor T1 gets a high voltage through R5 and IC2 remains disabled. When the piezo element gets heat from fire, asymmetry in its crystals causes a potential change, enabling capacitor C2 to discharge. It

momentarily changes the voltage level at pin 3 of IC1 and its output swings high. Transistor T1 conducts taking the reset pin 12 of IC2 to ground. IC2 is now enabled and starts oscillating. With the shown values of the oscil-lating components C3 (0.22µ) and R6

(1M), the first output (Q3) turns high after a few seconds and a red LED2 starts flashing. If heat near the piezo persists, Q7 (pin 14) output of IC2 be-comes high after one minute, and the alarm starts beeping. If heat contin-ues, Q9 (pin 15) turns high after four minutes and turns on the relay driver transistor T2. At the same time, diode D8 conducts and IC2 stops oscillating and toggles.

The solenoid pump connected to the N/O (normally opened) contact of the relay starts spraying the fire-ceas-ing foam or water to the possible sites of fire.

Power supply circuit. Power supply section (Fig. 2) comprises a 0-12V, 1A step-down transformer with a standard full-wave rectifier formed by D1 through D4 and filter capacitor C1. A battery backup is provided if the mains supply is cut-off due to short-circuit and fire. A 12V, 4.5Ah rechargeable battery is used for backup to give sufficient current to the solenoid pump. When mains

Fig. 1: Pyroelectric fire sensor

Fig. 2: Power supply with battery backup

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electronics for you • December 2009 • 93w w w . e f y m a g . c o m

power is available, diode D5 forward biases. It provides power to the circuit and also charges the battery through resistor R2, and it limits the charging current to 120 mA. When power fails, diode D5 reverse biases and diode D6 forward biases, giving instant backup

to the circuit. LED1 indicates the avail-ability of mains power.

Assemble the circuit on a general-purpose PCB and enclose it in a suit-able case. Connect the piezo element to the circuit using a thin insulated wire. Glue the flat side of the piezo el-

ement on a 30×30cm aluminium sheet to increase its sensitivity. Fix the sheet with the piezo sensor to the site where protection is needed. The remaining circuit can be fixed at a suitable place. If only the alarm generator is needed, omit the relay driver section.