EFY 1999

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

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

PRABHASH K.P.

RUPANJANA

The add-on circuit presented hereis useful for stereo systems. Thiscircuit has provision for connect-

ing stereo outputs from four differentsources/channels as inputs and only oneof them is selected/ connected to theoutput at any one time.

When power supply is turned ‘on’,channel A (A2 and A1) is selected. If noaudio is present in channel A, the cir-cuit waits for some time and then se-lects the next channel (channel B), Thissearch operation continues until it de-tects audio signal in one of the chan-nels. The inter-channel wait or delaytime can be adjusted with the help ofpreset VR1. If still longer time isneeded, one may replace capacitor C1with a capacitor of higher value.

Suppose channel A is connected toa tape recorder and channel B is con-nected to a radio receiver. If initially

channel A is selected, the audio fromthe tape recorder will be present at theoutput. After the tape is played com-pletely, or if there is sufficient pausebetween consecutive recordings, the cir-cuit automatically switches over to theoutput from the radio receiver. Tomanually skip over from one (selected)active channel, simply push the skipswitch (S1) momentarily once or more,until the desired channel inputs getsselected. The selected channel (A, B, C,or D) is indicated by the glowing of cor-responding LED (LED11, LED12,LED13, or LED14 respectively).

IC CD4066 contains four analogueswitches. These switches are connectedto four separate channels. For stereooperation, two similar CD4066 ICs areused as shown in the circuit. These ana-logue switches are controlled by ICCD4017 outputs. CD4017 is a 10-bit ring

counter IC. Since only one of its out-puts is high at any instant, only oneswitch will be closed at a time. ICCD4017 is configured as a 4-bit ringcounter by connecting the fifth outputQ4 (pin 10) to the reset pin. CapacitorC5 in conjunction with resistor R6 formsa power-on-reset circuit for IC2, so thaton initial switching ‘on’ of the powersupply, output Q0 (pin 3) is always‘high’. The clock signal to CD4017 is pro-vided by IC1 (NE555) which acts as anastable multivibrator when transistorT1 is in cut-off state.

IC5 (KA2281) is used here for notonly indicating the audio levels of theselected stereo channel, but also for for-ward biasing transistor T1. As soon asa specific threshold audio level is de-tected in a selected channel, pin 7 and/or pin 10 of IC5 goes ‘low’. This lowlevel is coupled to the base of transistorT1, through diode-resistor combinationof D2-R1/D3-R22. As a result, transis-tor T1 conducts and causes output ofIC1 to remain ‘low’ (disabled) as long asthe selected channel output exceeds thepreset audio threshold level.

Presets VR2 and VR3 have been in-cluded for adjustment of individual au-dio threshold levels of left stereo chan-nels, as desired. Once the multivibratoraction of IC1 is disabled, output of IC2does not change further. Hence, search-

CIRCUIT IDEAS

ELECTRONICS FOR YOU���APRIL '99

The circuit presented here can be-used as a lock for important elec-tronic/electrical appliances. When

card is inserted inside its mechanism,depending upon the position of punchedhole on the card, a particular appliancewould be switched on.

The card is inserted just like a floppydisk inside the disk drive. This cardshould be rectangular in shape with onlyone punched hole on it.

The circuit useseight photo-transistors(T1 through T8). Whenthere is no card in thelock, light from incan-descent lamp L1 (40-watt, 230V) falls on allthe photo- transistor de-tectors. Transistor T8 isused as enable detectorfor IC1 (74LS244).When light is incidenton it, it conducts and itscollector voltage goeslow. This makes transis-tor T16 to cut-off, andits collector voltage goeshigh. This logic high onits collector terminalwill inhibit IC1 as longas light is present onphoto-transistor T8.

Electronic Card-Lock SystemVIJAY D. SATHE

IC1 will get enabled only when thecard is completely inserted inside thelock mechanism. This arrangement en-sures that only the selected applianceis switched on and prevents false op-eration of the system.

You can make these cards using ablack, opaque plastic sheet. A small rect-angular notch is made on this card toindicate proper direction for insertion

of the card. If an attempt is made toinsert the card wrongly, it will not gocompletely inside the mechanism andthe system will not be enabled.

When card for any appliance (sayappliance 1) is completely inserted inthe mechanism, the light will fall onlyon photo-transistor T1. So only T1 willbe on and other photo-transistors willbe in off state. When transistorT1 is on, its collector voltage falls,making transistor T9 to cut-off. As aresult, collector voltage of transistor T9as also pin 2 of IC1 go logic high. Thiscauses pin 18 (output Q1) also to gohigh, switching LED1 on. Simulta-neously, output Q1 is connected to pin1 of IC2 (ULN2003) for driving therelay corresponding to appliance 1.Similarly, if card for appliance 2 isinserted, only output pin 16 (Q2) ofIC1 will go high—making LED2 onwhile at the same time energising re-

A.P.S. DHILLON

CIRCUIT IDEAS

ELECTRONICS FOR YOU���APRIL '99

achieved with the help of diodes D1through D7 which provide a wired-ORconnection at their common-cathodejunction. When any of the outputs ofIC1 is logic high, the common-cathodejunction of diodes D1 through D7 also

lay for appliance 2 via ULN2003. Thesame is true for other cases/appliancesalso.

The time during which card ispresent inside the mechanism, the sys-tem generates musical tone. This is

goes logic high, enabling IC3 (UM66) togenerate a musical tone.

In this circuit IC1 (74LS244) is usedas buffer with Schmitt trigger. All out-puts (Q1 through Q7) of this IC are con-nected

CIRCUIT IDEAS

ELECTRONICS FOR YOU���AUGUST '99

Dual-Channel DigitalVolume ControlSHEENA K.

This circuit could be used for re-placing your manual volume con-trol in a stereo amplifier. In this

circuit, push-to-on switch S1 controls theforward (volume increase) operation ofboth channels while a similar switchS2 controls reverse (volume decrease)operation of both channels.

Here IC1 timer 555 is configured asan astable flip-flop to provide low-fre-

quency pulses to up/down clock inputpins of pre-setable up/down counter74LS193 (IC2) via push-to-on switchesS1 and S2. To vary the pulse width ofpulses from IC1, one may replace tim-ing resistor R1 with a variable resistor.

Operation of switch S1 (up) causesthe binary output to increment whileoperation of S2 (down) causes the bi-nary output to decrement. The maxi-

mum count being 15 (all outputs logic1) and minimum count being 0 (all out-puts logic 0), it results in maximum andminimum volume respectively.

The active high outputs A, B, C andD of the counter are used for control-ling two quad bi-polar analogue switchesin each of the two CD4066 ICs (IC3 andIC4). Each of the output bits, when high,short a part of the resistor network com-prising series resistors R6 through R9for one channel and R10 through R13for the other channel,and thereby control the output of theaudio signals being fed to the inputs ofstereo amplifier. Push-to-on switch S3is used for resetting the output ofcounter to 0000, and thereby turningthe volume of both channels to the mini-mum level.

A.P.S. DHILLON

CIRCUIT IDEAS

ELECTRONICS FOR YOU���AUGUST '99

Using this low-cost project onecan reproduce audio from TVwithout disturbing others. It

does not use any wire connection be-tween TV and headphones. In place ofa pair of wires, it uses invisible infra-red light to transmit audio signals from

TV to headphones. Without using anylens, a range of up to 6 metres is

possible. Range can be extended byusing lenses and reflectors with IR sen-sors comprising transmitters and re-ceivers.

IR transmitter uses two-stage tran-sistor amplifier to drive two series-con-nected IR LEDs. An

audio out-put trans-former isused (in re-verse) tocouple audiooutput fromTV to the IRtransmitter.TransistorsT1 and T2amplify theaudio sig-nals re-ceived from

TV through the audio transformer. Low-impedance output windings (lower

gauge or thicker wires) are used for con-nection to TV side while high-imped-ance windings are connected to IR trans-mitter. This IR transmitter can be pow-ered from a 9-volt mains adapter or bat-tery. Red LED1 in transmitter circuitfunctions as a zener diode (0.65V) aswell as supply-on indicator.

IR receiver uses 3-stage transistoramplifier. The first two transistors (T4and T5) form audio signal amplifierwhile the third transistor T6 is used todrive a headphone. Adjust potmeter VR2for max. clarity.

Direct photo-transistor towards IRLEDs of transmitter for max. range. A

9-volt battery can be used with receiverfor portable operation.

Infrared CordlessHeadphonePRADEEP G.

G.S. SAGOO

ELECTRONICS PROJECTS Vol. 19 143

This instrument displays the speed of the vehicle in kmph. An

opaque disc is mounted on the spindle attached to the front wheel of the vehicle. The disc has ten equidistant holes on its periphery. On one side of the disc an infrared LED is fixed and on the opposite

DIGITAL SPEEDOMETERNARENDRA WADHWANI

side of the disc, in line with the IR LED, a phototransistor is mounted. IC LM324 is wired as a comparator.

When a hole appears between the IR LED and phototransistor, the pho-totransistor conducts. Hence the voltage at collector of the phototransistor and in-

verting input of LM324 go ‘low’, and thus output of LM324 becomes logic ‘high’. So rotation of the speedometer cable results in a pulse (square wave) at the output of LM324. The frequency of this waveform is proportional to the speed.

Let ‘N’ be the number of pulses in time ‘t’ seconds and numerically equal to the number of kilometres per hour (kmph). For a vehicle such as LML Vespa, with a wheel circumference of 1.38 metres, and number of pulses equal to 10 per revolution, we get the relationship:

Therefore, time ‘t’ in seconds= 0.4968 second.

As shown in the timing diagram, at t=0, output of astable flip-flop IC1(a) i.e. ½556 goes low and trig-gers monostable multivibrator IC1(b) i.e. ½556. Pulse width of monostable IC1(b) = 0.5068 sec. For IC1(a), t(on) = 0.51 sec. and t(off)= 0.01 sec. The outputs of IC1(a) and IC1(b), and the signal from the transducer section are ANDed. The number of pulses counted during the gating period (0.4968 sec.) is the speed N in kmph (kilometres per hour).

At the end of the gating period, output ‘B’ of monostable IC1(b) goes low and B goes high. The rising edge of B is used to enable the quad ‘D’ flip-flops IC6 and IC7.

At this instant, i.e. at t=0.5068 sec., the number (speed) N will be latched corresponding to the ‘D’ flip-flops and displayed. At t=0.52 sec., output of astable flip-flop IC1(a)

Nx1000x10 pulses per second3600x1.38

=

N pulsest

= N kmph

Nx1000 metres per second3600x1.38

=

ELECTRONICS PROJECTS Vol. 19144

goes low and remains low for 0.01 sec. This waveform is inverted and applied to the reset terminals of all counters (active high).

Thus the counters are re-set and counting begins afresh at t=0.53 sec. up to the time t=0.52+0.2068 sec. However the ‘D’ flip-flops are not enabled and the previous speed is displayed. The new speed is displayed at t=0.52 + 0.5068 sec. In this way the speed will be updated every 0.52 sec.

This speedometer can measure up to 99 kmph with a resolution of 1 kmph. The range can be in-creased up to 999 kmph by adding another stage consisting of one each of ICs 7490, 74175, 7447 and

a 7-segment display. The voltage supply required for the operation of the circuit is derived from the vehicle power supply (12V).

The calculations shown above are for LML Vespa and Kinetic Honda. The calculations for using this speedom-eter for Yamaha, whose circumference of wheel = 1.8353m, can be obtained in a similar fashion. The gating period will simply vary in direct proportion to the wheel diameter. It will be 0.6607 sec. for Yamaha.

The same speedometer can be used for other vehicles by making similar calculations. In all the calculations it has been assumed that the speedometer cable makes one revolution for every revolution of the wheel of the vehicles. Note that

on/off periods of the wave-forms have to be precise. High quality multiturn pots and low temperature coefficient components should be used in the timer ICs.

CIRCUIT IDEAS

The circuit shown here is that ofthe IC controlled emergencylight. Its main

features are: automaticswitching-on of the lighton mains failure and bat-tery charger with over-charge protection.

When mains is ab-sent, relay RL2 is in de-energised state, feedingbattery supply to in-verter section via its N/C contacts and switchS1. The inverter sectioncomprises IC2 (NE555)which is used in stablemode to produce sharppulses at the rate of 50Hz for driving theMOSFETs. The output ofIC3 is fed to gate ofMOSFET (T4) directlywhile it is applied toMOSFET (T3) gate afterinversion by transistorT2. Thus the power am-plifier built aroundMOSFETs T3 and T4functions in push-pullmode.

The output acrosssecondary of transformerX2 can easily drive a230-volt, 20-watt fluores-cent tube. In case lightis not required to be onduring mains failure,simply flip switch S1 tooff position.

inverting pin is held at a reference volt-age of approximately 6.9 volts which isobtained using diode D5 (1N4148) and6.2-volt zener D6. The inverting pin ofIC1 is connected to the positive termi-nal of battery. Thus when mainssupply is present, IC1 comparator out-put is high, unless battery voltage ex-ceeds 6.9 volts. So transistor T1 is nor-mally forward biased, which energisesrelay RL1. In this state the battery re-mains on charge via N/O contacts ofrelay RL1 and current limiting resistorR2. When battery voltage exceeds 6.9volts (overcharged condition), IC1 out-put goes low and relay RL1 gets de-energised, and thus stops further charg-ing of battery.

MOSFETs T3 and T4 may bemounted on suitable heat sinks.

IC ControlledEmergency Lightwith Charger R. RAINA

A.P.S. DHILLON

Battery overcharge preventer circuitis built around IC1 (LM308). Its non-

ELECTRONICS PROJECTS Vol. 19196

INFRARED ELECTRONIC SHOOTING GAME

T.K. HAREENDRAN

Just trigger an infrared electronic gun and there goes one invisible bullet hitting the bull’s eye, if timed

properly. The circuit is very simple, inex-

pensive and easy to construct. The game offers hours of fun and excitement.

The target screen consists of a number of LEDs moving rapidly in a circular fash-ion. All the LEDs are red except one—the real target located in centre of the screen which is green. When a shot is fired by triggering the gun, all LEDs go off except one. If it happens to be the target (green LED) then you have made a hit which is indicated by lighting up of another green LED accompanied by a pleasing musical tone. After a short delay the game restarts automatically.

Infrared gun (transmitter) for this electronic game is built around IC1 timer (NE555) wired as an astable multivibra-

ELECTRONICS PROJECTS Vol. 19 197

tor with a centre frequency of about 35 kHz. The frequency is determined by the timing components comprising resistors R1 and R2 and capacitor C2. When push-to-on trigger switch S1 is pressed, the astable multivibra-tor starts modulating the infrared beam with short pulses (See output waveform). The whole circuit can be enclosed in a toy gun for giving it a professional look as illustrated in the figure. The infrared LED has to be fit-ted with a suitable reflector to ensure good sensitivity.

When power switch S2 in the receiv-er is turned on, astable multivibra-tor wired around IC3 (NE555) generates clock pulses which are fed to clock input (pin 14) of decade counter IC4 (CD4017B). This IC has ten outputs, and each one goes high sequentially on the rising edge of successive clock pulse. As a result, LEDs connected to the output appear to move from one to the other rapidly.

You would notice that only nine outputs are used for driving LEDs. The tenth output (Q9) at pin 11 is connected to reset pin 15.

When gun is fired, infrared bursts are received by the integrated infrared mod-ule and its output at pin 2 goes low. The resulting falling edge triggers monostable

IC2 and its output (pin 3) goes high. This makes clock enable (CE) pin 13 of IC4 to go high (normally held at a low potential via resistor R8) and it starts counting. When mono pulse ends, and if the last lit LED happens to be the target LED then both inputs of NAND gate N1 become high. As a result, the output of gate N2 also goes high. This in turn switches on transistor T2; thereby the ‘HIT’ LED lights up and the buzzer also sounds. At the end of the mono pulse period (about 5 sees), decided by resistor R5 and capacitor C5, the mono IC2 is again ready to receive another trigger pulse.

Assembly and component layout is not very critical. The circuit may be as-sembled on a veroboard using IC sockets. A well regulated power supply is required for powering the unit. In place of the IR transmitter it is also possible to make use of the remote control used for TVS or VCPs/VCRs.

CIRCUIT IDEAS

ELECTRONICS FOR YOU���JULY '99

Many digital code lock circuitshave been published in thismagazine. In those circuits a

set of switches (conforming to code) arepressed one by one within the specifiedtime to open the lock. In some othercircuits, custom-built ICs are used andpositive and negative logic pulses are

keyed in sequence as per the code bytwo switches to open the lock.

A low-cost digital code lock circuitis presented in this article. Here thekeying-in code is rather unique. Sixswitches are to be pressed to open thelock, but only two switches at a time.Thus a total of three sets of switcheshave to be pressed in a particular se-quence. (Of these three sets, one set isrepeated.) The salient features of thiscircuit are:

1. Use of 16 switches, which sug-gests that there is a microprocessor in-side.

2. Elimination of power amplifiertransistor to energise the relay.

3. Low cost and small PCB size.

A. JEYABAL

Simple Low-CostDigital Code Lock

An essential property of this elec-tronic code lock is that it works inmonostable mode, i.e. once triggered,the output becomes high and remainsso for a period of time, governed by thetiming components, before returing tothe quiescent low state. In this circuit,timer IC 555 with 8 pins is used. The

IC is inexpensive and easily available.Its pin 2 is the triggering input pinwhich, when held below 1/3 of the sup-ply voltage, drives the output to highstate. The threshold pin 6, when heldhigher than 2/3 of the supply voltage,drives the output to low state. By ap-plying a low-going pulse to the resetpin 4, the output at pin 3 can bebrought to the quiescent low level. Thusthe reset pin 4 should be held high fornormal operation of the IC.

Three sets of switches SA-SC, S1-S8 and S3-S4 are pressed, in that or-der, to open the lock. On pressing theswitches SA and SC simultaneously, ca-pacitor C3 charges through the poten-tial divider comprising resistors R3 and

R4, and on releasing these two switches,capacitor C3 starts discharging throughresistor R4. Capacitor C3 and resistorR4 are so selected that it takes aboutfive seconds to fully discharge C3.

Depressing switches S1 and S8 inunison, within five seconds of releasingthe switches SA and SC, pulls pin 2 toground and IC 555 is triggered. The ca-pacitor C1 starts charging through re-sistor R1. As a result, the output (pin3) goes high for five seconds (i.e. thecharging time T of the capacitor C1 tothe threshold voltage, which is calcu-lated by the relation T=1.1 R1 x C1 sec-onds).

Within these five seconds, switchesSA and SC are to be pressed momen-tarily once again, followed by the de-pression of last code-switch pair S3-S4.

These switches connect the relay to out-put pin 3 and the relay is energised.The contacts of the relay close and thesolenoid pulls in the latch (forming partof a lock) and the lock opens. The re-maining switches are connected betweenreset pin 4 and ground. If any one ofthese switches is pressed, the IC is re-set and the output goes to its quiescentlow state. Possibilities of pressing thesereset switches are more when a codebreaker tries to open the lock.

LED D5 indicates the presence ofpower supply while resistor R5 is a cur-rent limiting resistor.

The given circuit can be recoded eas-ily by rearranging connections to theswitches as desired by the user.

RUPANJANA

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

S.C. DWIVEDI

While travelling by a train or bus,we generally lock our luggageusing a chain-and-lock arrange-

ment. But, still we are under tension, ap-prehending that somebody may cut thechain and steal our luggage. Here is asimple circuit to alarm you whensomebody tries to cut the chain.

Transistor T1 enables supply tothe sound generator chip when thebase current starts flowing throughit. When the wire (thin enameledcopper wire of 30 to 40 SWG, usedfor winding transformers) looparound the chain is broken by some-body, the base of transistor T1,which was earlier tied to positiverail, gets opened. As a result, tran-

sistor T1 gets forward biased to extendthe positive supply to the alarm circuit.In idle mode, the power consumption ofthe circuit is minimum and thus it can beused for hundreds of travel hours.

To enable generation of different

DHURJATI SINHA

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Select 1 Select 2 Sound effect(Pin6) (Pin1)X X Police sirenVDD X Fire-engine sirenVSS X Ambulance siren“-” VDD Machine-gun soundNote: X = no connection; “-” = do not care

alarm sounds, connections to pin 1 and 6may be made as per the table.

CIRCUIT IDEAS

ELECTRONICS FOR YOU���MARCH '99

Wiper SpeedControllerAcontinuously working wiper in

a car may prove to be a nui-sance, especially when it is not

raining heavily. By using the circuit de-scribed here one can vary sweeping rateof the wiper from once a second to oncein ten seconds.

The circuit comprises two timerNE555 ICs, one CD4017 decade counter,one TIP32driver tran-sistor, a2 N 3 0 5 5power tran-sistor (orT I P 3 0 5 5 )and a fewother dis-crete compo-nents.

T i m e rIC1 is con-figured as amono- stablemultivibratorwhich pro-duces apulse whenone pressesswitch S1m o m e n -tarily. Thispulse acts asa clock pulse

for the decade counter (IC2) which ad-vances by one count on each successiveclock pulse or the push of switch S1.

Ten presets (VR1 through VR10),set for different values by trial and er-ror, are used at the ten outputs of IC2.But since only one output of IC2 is highat a time, only one preset (at selectedoutput) effectively comes in series with

timing resistors R4 and R5 connected inthe circuit of timer IC3 which functionsin astable mode.

As presets VR1 through VR10 areset for different values, different timeperiods (or frequencies) for astablemultivibrator IC3 can be selected. Theoutput of IC3 is applied to pnp drivertransistor T1 (TIP32) for driving the fi-nal power transistor T2 (2N3055) whichin turn drives the wiper motor at theselected sweep speed. The power supplyfor the wiper motor as well as the circuitis tapped from the vehicle’s battery it-self. The duration of monostablemultivibrator IC1 is set for a nearly onesecond period.

PRADEEP G.

Y. KATARIA

CIRCUIT IDEAS

ELECTRONICS FOR YOU���SEPTEMBER '99

Abattery is a vital element of anybattery-backed system. In manycases the battery is more expen-

sive than the system it is backing up.Hence we need to adopt all practicalmeasures to conserve battery life.

As per manufacturer’s data sheets,a 12V rechargeable lead-acid batteryshould be operated within 10.1V and13.8V. When the battery charges higherthan 13.8V it is said to be overcharged,and when it discharges below 10.1V itcan be deeply discharged. A single eventof overcharge or deep discharge canbring down the charge-holding capacityof a battery by 15 to 20 per cent.

It is therefore necessary for all con-cerned to monitor the charge level of

their batteries continuously. But, inpractice, many of the battery users areunable to do so because of non-avail-ability of reasonably-priced monitoringequipment. The circuit idea presentedhere will fill this void by providing acircuit for monitoring the charge levelof lead-acid batteries continuously. Thecircuit possesses two vital features:

• First, it reduces the requirementof human attention by about 85 per cent.

• Second, it is a highly accurate andsophisticated method.

Input from the battery under test isapplied to LM3914 IC. This applied volt-age is ranked anywhere between 0 and10, depending upon its magnitude. Thelower reference voltage of 10.1V is ranked

‘0’ and the upper voltage of 13.8V is rankedas ‘10.’ (Outputs 9 and 10 are logicallyORed in this circuit.) This calibration ofreference voltages is explained later.

IC 74LS147 is a decimal-to-BCD pri-ority encoder which converts the outputof LM3914 into its BCD complement.The true BCD is obtained by using thehex inverter 74LS04. This BCD outputis displayed as a decimal digit after con-version using IC5 (74LS247), which is aBCD-to-seven-segment decoder/driver.The seven-segment LED display (LTS-542) is used because it is easy to readcompared to a bar graph or, for thatmatter, an analogue meter. The chargestatus of the battery can be quickly cal-culated from the display. For instance,if the display shows 4, it means that thebattery is charged to 40 per cent of itsmaximum value of 13.8V.

The use of digital principles enablesus to employ a buzzer that sounds when-ever there is an overcharge or deep dis-charge, or there is a need to conservebattery charge. A buzzer is wired in thecircuit such that it sounds whenever bat-tery-charge falls to ten per cent. At thispoint it is recommended that unneces-sary load be switched off and the re-maining charge be conserved for moreimportant purposes.

Another simple combinational logic

SIDDHARTH SINGH AND SRINIVAS REDDY PINGLE

Charge Monitor for12V RechargeableLead-acid Battery

RUPANJANA

CIRCUIT IDEAS

ELECTRONICS FOR YOU���SEPTEMBER '99

circuit can also be designed that willsound the buzzer when the displayshows 9. Further charging should bestopped at this point in order to pre-vent overcharge.

The circuit is powered by the bat-tery under test, via a voltage regulatorIC. The circuit takes about 100 mA forits operation.

For calibrating the upper and lowerreference levels, a digital multimeter

and a variable regulated power supplysource are required. For calibrating thelower reference voltage, follow the stepsgiven below:

• Set the output of power supplysource to 10.1V.

• Connect the power supply sourcein place of the battery.

• Now the display will show somereading. At this point vary preset VR2until the reading on the display just

changes from 1 to 0.The higher reference voltage is cali-

brated similarly by setting the powersupply to 13.8V and varying preset VR1until reading on the display just changesfrom 8 to 9.

After the calibration is completed,the circuit may be housed in a suitableenclosure. The cost of all the compo-nents, including the enclosure, wouldbe around Rs 200.

CIRCUIT IDEAS

ELECTRONICS FOR YOU���MAY '99

The given circuit, when connectedin parallel to a telephone, dis-plays the number dialled from

the telephone set using the DTMF mode.This circuit can also show the numberdialled from the phone of the calledparty. This is particularly helpful forreceiving any number over the phonelines.

The DTMF signal—generated by thephone on dialling a number—is decodedby DTMF decoder CM8870P1 (IC1),which converts the received DTMF sig-nal into its equivalent BCD number thatcorresponds to the dialled number. Thisbinary number is stored sequentially in10 latches each time a number is dialledfrom the phone. The first number isstored in IC5A (1/2 of CD4508) whilethe second number is stored in IC5Band so on. The binary output from IC1for digit ‘0’ as decoded by IC1 is 10102(=1010), and this cannot be displayed bythe seven-segment decoder, IC10. There-fore the binary output of IC1 is passedthrough a logic-circuit which convertsan input of ‘10102’ into ‘00002’ withoutaffecting the inputs ‘1’ through ‘9’. Thisis accomplished by gates N13 throughN15 (IC11) and N1 (IC12).

The storing of numbers in respec-tive latches is done by IC2 (4017). Thedata valid output from pin 15 of IC1 isused to clock IC2. The ten outputs ofIC2 are sequentially connected to thestore and clear inputs of all the latches,except the last one, where the clear in-put is tied to ground. When an outputpin of IC2 is high, the correspondinglatch is cleared of previous data andkept ready for storing new data. Then,on clocking IC2, the same pin becomeslow and the data present at the inputsof that latch at that instant gets storedand the next latch is cleared and keptready. The similar input and output pinsof all latches are connected together to

Telephone NumberDisplayBHASKAR BANERJEE

R. RAINA

CIRCUIT IDEAS

ELECTRONICS FOR YOU���MAY '99

form two separate input and outputbuses.

There is only one 7-segment decoder/driver IC10 for all the ten displays. Thisnot only reduces size and cost but re-duces power requirement too. The out-put from a latch is available only whenits disable pins (3 and 15) are broughtlow. This is done by IC3, IC12 and IC13.IC3 is clocked by an astablemultivibrator IC4 (555). IC3 also drivesthe displays by switching correspond-ing transistors. When a latch is enabled,its corresponding display is turned onand the content of that latch, after de-

coding by IC10, gets displayed in thecorresponding display. For instance, con-tents of IC5A are displayed on display‘DIS1,’ that of IC5B on ‘DIS2’ and soon. The system should be connected tothe telephone lines via a DPDT switch(not shown) for manual switching, oth-erwise any circuit capable of sensinghandset’s off-hook condition and therebyswitching relays, etc. can be used forautomatic switching. The power-supplyswitch can also be replaced then. Suchcircuits, under different captions, canbe found in EFY’s back issues. Thoughthis circuit is capable of showing a maxi-

mum of ten digits, one can reduce thedisplay digits as required. For doingthis, connect the reset pin of IC2, say,for a 7-digit display, with S6 output atpin 5.

The present circuit can be built on averoboard and housed in a suitable box.The displays are common-cathode type.To make the system compact, small, 7-segment displays can be used but withsome extra cost. Also, different colourdisplays can be used for the first threeor four digits to separate the exchangecode/STD code, etc. The circuit can besuitably adopted for calling-line display.

CIRCUIT IDEAS

ELECTRONICS FOR YOU���MARCH '99

Time Switch Y. KATARIA

This circuit is especially designedfor those who often need to wakeup early in the morning. Ordi-

nary alarms in electronic watches arenot loud enough andvery often they fail towake up. The switchcircuit described herewill come handy; it canbe used to switch on aTV, radio or tape re-corder etc, which willnot allow even the la-ziest amongst us to ig-

nore their sound for too long. Besides,this time switch can also be used to

switch on/off any other electric or elec-tronic gadget at any time. What youneed is a simple analogue electronicclock with alarm facility and a smallcircuit to implement the time switch.

This time switch has two modes.One is ‘time-on’ mode and the other is‘time-off ’ mode. In time-on mode, youset up the alarm in your clock as pernormal procedure and at the set timethis switch turns on the gadget con-nected at the output socket-1. In time-off mode, it turns your gadget off at theset time. The optional output socket-2is wired in such a way that when youuse this socket, the mode changes with-

out having to flip the mode switch (i.e.mode switch can be omitted).

Please refer to the back panel dia-gram of a typical analogue clock andthe audio jack, to see how the existing

buzzer ofthe clock isrequired tobe wired tothe audioo u t p u tfrom theclock. This

will ensure that when plug is insertedin the audio jack, the clock’s buzzer willremain off and not consume any power

unnecessarily.The audio alarm output from the

clock is coupled to the AF detector builtaround low-power switching transistorT1. During alarm, the collector of tran-sistor T1 will fluctuate around groundlevel and Vcc. During absence of audioalarm input, the collector of transistorT1 is held at Vcc potential.

The next stage consists of an S-Rlatch built around NAND gates N1 andN2. Capacitor C2 and resistor R4 areused for power-on-reset. On switchingthe power supply, gate N2 output willacquire logic 1 and that of gate N1 logic0. This is the initial state, irrespective of

the position of mode switch. At the timeof alarm, when point A connected to col-lector of transistor T1 passes throughlogic 0 state, the output logic state ofboth the gates will toggle.

Assuming that mode switch isflipped to ‘Mode Off’ position at power-on-reset (when point D is at logic 1),initially diode D1 would be in blockingstate and transistor T2 would be for-ward biased via resistor R5 anddiodes D2 and D3. As a result, therelay is in energised state, which makesoutput power available at output socket-1 and cuts it off from socket-2. At alarmtime, the audio signal toggles logicoutput states of both gates N1 and N2.As a result, point D goes to logic 0state. Diode D1 conducts, taking thevoltage at junction of diodes D1 andD2 to near about 1 volt. Diode D3ensures that its series combination with

diode D2 puts them in blocking mode.Capacitor C3 meanwhile dischargesvia resistor R6 and the voltage at baseof transistor T2 approaches towardsground level, cutting off transistorT2 and de-energising relay RL1. Nowthe power at output socket-1 would becut off while it becomes available insocket-2.

If the above operation is repeatedwith switch S1 in ‘Mode On,’ the powerwould initially not be available insocket-1 (but available in socket-2). Butafter the alarm, the power would be-come available in socket-1 and not insocket-2.

AVNISH PUNDIR