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This equaliser uses low-costop-amps. Good-quality op-amps powered by a single volt-

age supply are readily available in themarket. The op-amp should have anoise density of less than 24nV/√Hz,slew rate of more than 5V/µs and gain-

bandwidth product greater than 3MHz. The NE5532 or LM833 used inthis circuit meets these requirements.

Equaliser circuits typically dividethe audio spectrum into separate fre-quency bands and have independentgain control for each band. The out-put of each band is mixed at IC4(A)and then fed to an audio power am-

plifier. Proper quality factor (Q) needsto be selected to avoid overlap in ad-jacent bands as this introducescolouration into the audio signal.

We have used the multiple-feed-back bandpass filter topology shownin left-most corner at the bottom of thefigure. This is a circuit for single-chan-nel bandpass filter. If the capacitors are

SOMEN GHOSH

5-BAND GRAPHIC EQUALISER R. SUNDARA KUMAR

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

of the same value, the calculations arefairly simple. For calculating the com-ponent values, use the following for-mulae:

Centre frequency (fo) : 1/2πC√(Ra||Rb)RcBandwidth (B) : 1/πCRcQuality factor (Q) : fo/B = πfoCRcGain (A) : –Rc/2Ra

These can be combined to give thefollowing formulae:

Ra = Q/2πfoACRb = Q/2πfoC (2Q2–A)Rc = Q/πfoC

Begin the calculations by choosinga large value of capacitance (~0.1F) andsmaller value of resistances. Increasingthe capacitance decreases resistances(Ra, Rb and Rc). Care must be taken toavoid overloading on the input bufferop-amp. Note that stray capacitanceson the board reduces the value of ‘C.’The bandwidth and gain do not de-pend on Rb. Hence, Rb can be used tomodify the mid-frequency without af-fecting the bandwidth and gain.

For equalisers, there are standardmid-frequencies that are normallyused. The exact frequencies depend on

the octave division, application andsome degree of manufacturers’ prefer-ence, but nearly all share the basic oc-tave boundaries that are based on acentre frequency of 1000 Hz.

A balance between the number offilters and bandwidth need to be ob-served. It is possible to use a widerbandwidth and fewer filters, or nar-rower bandwidth and more filters.Anything narrower than 1/3 octave israre, since the complexity of the filtersincreases for higher values of ‘Q.’ Thiscan get rather expensive and in realityis of limited use for most applicationsin audio systems.

National Semiconductor lists thefollowing mid-frequencies for a 10-band graphic equaliser: 32, 64, 125, 250,500, 1k, 2k, 4k, 8k and 16k. It also rec-ommends a ‘Q’ of 1.7 for equalisers.

The table lists the component val-ues for different centre frequencies ofthe equaliser. We used ‘Q’ of 1.7 andgain (A) of 4.

The circuit for the 5-band equaliseruses IC1 (A) LM833 as the buffer stagefor the equaliser. It is a non-inverting

amplifier with a gain of ‘2.’ The inputsignal is divided by ‘2’ by the resistivenetwork comprising R3 and R4. Hencethe net gain of this amplifier is unity.Two 100k resistors (R1 and R2) areused as a voltage divider and the junc-tion voltage is fed to itspositive input through R6. Thisdivider has enough power to feed allother op-amps directly. Resistor Ro(R8=R12=R16=R20=R24=R28=R30=100Ω)has the dual function of noise reduc-tion and resistive isolation of capaci-tive load. It may be varied between 50and 150 ohms depending on the noisein the circuit.

The potmeters (VR1 through VR5)are in the signal path and hence shouldbe of the best quality possible. Wrapthe body of the pots with bare copperwire and solder the other end of thewire to ground. Since the filters arevery sensitive, all resistances shouldbe metal-film type and the capacitorsshould be polyester type.

Each stage of the op-amp needs tobe capacitively coupled to the nextstage so that the DC does not getpropagated and amplified. For a goodlow-frequency response, this couplingcapacitor should be greater than 1 µF.A 10µF, 16V capacitor is used in eachstage of the circuit here.

The circuit is powered by a 12VDC regulated supply. A well-regulatedsupply using 7812 is recommended.Ground the Vcc pin of each op-ampwith a 0.1µF ceramic disk capacitor tobypass the noise.

Component Values for 5-band EqualiserCentre C Ra Rb Rc Gain (A) Quality (Q)frequency (μF) (kilo- (kilo- (kilo-fo (Hz) ohms) ohms) ohms)

60 C4=C5=0.1 R9=11 R11=27 R10=91 4.1 1.7

250 C7=C8=0.1 R14=2.7 R15=6.3 R13=22 4.1 1.7

1000 C10=C11=0.047 R18=1.5 R19=3.3 R17=11 3.7 1.6

4000 C13=C14=0.0022 R22=7.5 R23=18 R21=63 4.2 1.7

16000 C16=C17=0.0022 R26=2 R27=4.3 R25=15 4.2 1.7

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E L E C T R O N I C S F O R Y O U • J U N E 2 0 0 7 • 8 3W W W . E F Y M A G . C O M

D uring poor visibility, i.e.,when there is fog, or at dawnor dusk, or when your ve-

hicle gets stalled on a lonely stretch ofa highway, this flashing light will pro-vide safety and attract the attention ofpeople to help you out. It uses high-brightness yellow LEDs.

The circuit uses a dual binarycounter CD4520, quadruple 2-input

ASHOK K. DOCTOR

ANTI-COLLISION REAR LIGHT S.C. DWIVE

DI

NAND schmitt trigger CD4093, 8-stageshift-and-store bus register CD4094and some descrete components.

An oscillator is built around gate A,whose frequency can be varied throughpreset VR1 when required. The outputof the oscillator is fed to IC1 and IC3.When the circuit is switched on, theoscillator starts oscillating, the counterstarts counting through IC1 and thedata is shifted on positive-going clockthrough IC3. As a result, the four groups

of LEDs flash one by one.All the LEDs will then glow for

some time and switch off for sometime, and the cycle will repeat. Inputpins 12 and 13 of the unused gate Dmust be tied to ground and pin 11 leftopen. Preset VR1 should be of cermettype and used to change the flashingrate of each group of LEDs.

The circuit works off regulated12V. Assemble it on a general-purposePCB and house suitably.

circuitideas

98 • Mar ch 2010 • electronics for you w w w . e f y M a g . c o M

SureSh Kumar K.B.

antiSleep alarm for StudentS

s.c. dwivediThis circuit saves both time and electricity for students. It helps to prevent them from dozing

off while studying, by sounding a beep at a fixed time interval, say, 30 minutes. If the student is awake during the beep, he can reset the circuit to beep in the next 30 minutes. If the timer is not reset during this time, it means the student is in deep sleep or not in the room, and the circuit switches off the light and fan in the room, thus preventing the wast-age of electricity.

The circuit is built around Schmitt-trigger NAND gate IC CD4093 (IC1), timer IC CD4020 (IC2), transistors

BC547, relay RL1 and buzzer. The Schmitt-trigger NAND gate

(IC1) is configured as an astable multi-vibrator to generate clock for the timer (IC2). The time period can be calcu-lated as T=1.38×R×C. If R=R1+VR1=15 kilo-ohms and C=C2=10 µF, you’ll get ‘T’ as 0.21 second. Timer IC CD4020 (IC2) is a 14-stage ripple counter.

Around half an hour after the reset of IC1, transistors T1, T2 and T3 drive the buzzer to sound an intermediate beep. If IC2 is not reset through S1 at that time, around one minute later the output of gate N4 goes high and transistor T4 conducts. As the output of gate N4 is connected to the clock input (pin 10) of IC2 through diode

D3, further counting stops and relay RL1 energises to deactivate all the ap-pliances. This state changes only when IC1 is reset by pressing switch S1.

Assemble the circuit on a general-purpose PCB and enclose it in a suit-able cabinet. Mount switch S1 and the buzzer on the front panel and the relay at the back side of the box. Place the 12V battery in the cabinet for powering the circuit. In place of the battery, you can also use a 12V DC adaptor.

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

temperature, the diode generates 2mV output voltage. That is, at 5°C, it is 10 mV, which rises to 70 mV when the temperature is 35°C. This property is exploited in the circuit to sense the temperature variation in aquarium water. Fig. 1 shows the circuit diagram of the aquarium probe.

Since the output from the diode sensor is too low, a high-gain invert-ing DC amplifier is used to amplify the voltage. CA3140 (IC1) is the CMOS version op-amp that can operate down to zero-volt output. The highest output available from IC1 is 2.25V less than the input voltage at pin 7. With resistor

R 4 a n d VR2, the variation in diode v o l t a g e c a n b e

amplified to the required level. Resis-tor R1 restricts current flow through diode D1 and preset VR1 (1-kilo-ohm) sets the input voltage at pin 3. IC3 (7805) provides regulated 5 volts to the inputs of IC1, so that the input voltage is stable for accurate measurement of temperature.

The output from IC1 is fed to display driver LM3915 (IC2) through preset VR3 (50-kilo-ohm). With careful adjustments, the wiper of VR3 can provide 0-400 millivolts to the input of IC2. The highly sensi-tive input of IC2 accepts as low as 50 mV if the reference voltage at its pin 7 is adjusted using a variable resistor. To increase the sensitivity of IC2, preset VR4 is connected at one end to ‘reference voltage end’ pin 7 and its wiper is connected to ‘high end’ pin 6 of the internal resis-tor chain.

When approximately 70 mV is provided to the input of IC2 by adjust-ing preset VR3, LED1 (green) lights up to indicate that the temperature is approximately 35°C, which is the cross-ing point. When the input receives 100 mV, LED2 (red) lights up to indicate

approximately 50°C. Finally, the buzzer starts beeping if the input receives 130 mV corresponding to a temperature of 65°C.

In short, LEDs and the buzzer re-main standby when the temperature of the water is below 35°C (normal). With each step increase of 30 mV in the input (cor-responding to 15°C rise in temperature), LEDs and the buzzer become active.

Pin 16 of IC2 is used to drive the pi-

ezobuzzer through transistor T1. When pin 16 of IC2 becomes low, T1 conducts to beep the piezobuzzer. Resistor R7 keeps the base of transistor T1 high to avoid false alarm. IC4 provides regu-lated 9V DC to the circuit.

Assemble the circuit on a common PCB and enclose in a suitable case. Glass signal diode D1 is immersed in water to sense the temperature of water. Its leads should be coated with enamel paint to avoid shorting in water. Alternatively, enclose the diode in a small glass tube or test

D. Mohan KuMar

aquariuM Probe s.c. dwivedi

A number of environmental factors including light and temperature affect fish culture.

The temperature of water has profound effect because fish cannot breed above or below the critical temperature lim-its. Temperature between 24°C and 33°C is found to be the best to induce spawning in fishes. This particular temperature range is also necessary for the healthy growth of nursery fish fries (young fishes). Rise of water tem-perature due to sunlight may adversely affect the fish rearing process.

The circuit of aquatic probe de-scribed here can monitor the tem-perature of water and indicate the rise in temperature through audio-visual indicators. A readily available signal diode 1N34 is used in the circuit as the temperature sensing probe. The resistance of the diode depends on the temperature in its vicinity.

Typically, the diode can gener-ate around 600 mV when a potential difference is applied to its terminals. For each degree centigrade rise in

Fig. 2: Diode sensor assembly

Fig. 1: Circuit for aquarium probe

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w w w . e f y m a g . c o m electronics for you • july 2008 • 93

tube having sufficient internal space to fit the diode as shown in Fig. 2. Make the sensor assembly waterproof using wax.

Take care while calibrating and setting the circuit. With 5V DC sup-ply to diode D1 and an ambient tem-perature of about 35°C, D1 generates around 70 mV. Adjust VR3 until the voltage in its wiper increases to 70

mV, so that the input of IC2 (pin 5) receives 70 mV corresponding to the diode output voltage at 35°C. At this stage, green LED1 should turn on. If it doesn’t, adjust VR4 until LED1 just lights up. Immerse the diode in temperature-adjusted hot water (35°C) and adjust VR3 and VR4 until green LED1 lights up. Increase the water temperature to 50°C by adding

hot water. Now red LED2 will glow. At this position, the voltage at pin 6 of IC1 will be around 100 mV. When the temperature of water increases further to 65°C, the buzzer starts beeping. After calibration, immerse the diode assembly in the aquarium tank just below the water surface and fix it permanently to avoid float-ing.

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

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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

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

M.K. Chandra Mouleeswaran

autoMatiC light Controller using 7806

s.c. dwivedi

Voltage regulator ICs (78xx se-ries) provide a steady output voltage, as against a widely

fluctuating input supply, when the common terminal is grounded. Any voltage about zero volt (ground) con-nected in the common terminal is add-ed to the output voltage. That means the increase in the common terminal voltage is reflected at the output. On the other hand, if the common terminal is disconnected from the ground, the full input voltage is available at the output.

This characteristic is utilised in the present circuit. When the common terminal is connected to the ground, the regulator output is equivalent to the rated voltage, and as soon as the terminal is disconnected from the ground, the output increases up to the input voltage.

The common terminal is control-led by a transistor, which works as a switch on the terminal. For automatic control of light, a light-dependent re-sistor (LDR1) is connected to the base

of the transistor. In this way, the volt-age regulator is able to operate a light bulb automatically as per the ambient light.

To derive the power supply for the circuit, the 50Hz, 230V AC mains is stepped down by transformer X1 to deliver a secondary output of 12V, 250 mA. The secondary output of the transformer is applied to a bridge rec-tifier comprising diodes D1 through D4, filtered by capacitor C1 and fed to the input terminal of the regulator (IC1).

The common terminal (pin 2) of IC1 is connected to the ground line of the circuit through transistor BC557 (T1). The transistor is biased by R2, R3, VR1 and LDR1. The grounding of IC1 is controlled by transistor T1, while light is sensed by LDR1. Using preset VR1, you can adjust the light-sensing level of transistor T1.

The output of IC1 is fed to the base of transistor T2 (through resistor R4 and zener diode ZD1) and relay RL1. LED1 connected across the positive and ground supply lines acts as a power-‘on’ indicator.

Normally, the resistance of LDR1 is low during daytime and high dur-ing nighttime. During daytime, when light falls on LDR1, pnp transistor T1 conducts. The common terminal of IC1 connects to the ground and IC1 outputs 6V. As a result, transistor T2 does not conduct and the relay remains de-en-ergised. The light bulb remains ‘off’ as the mains connection is not completed through the relay contacts.

During nighttime, when no light falls on LDR1, it offers a high resist-ance at the base junction of transistor T1. So the bias is greatly reduced and T1 doesn’t conduct. Effectively, this removes the common terminal of IC1 from ground and it directs the full input DC to the output. Transistor T2 conducts and the relay energises to light up the bulb as mains connection completes through the relay contacts.

As LDR1 is in parallel to VR1+R3 combination, it effectively applies only half of the total resistance of the network formed by R3, VR1 and LDR1 to the junction at T1 in total darkness. In bright light, it greatly reduces the total effective resistance

at the junction. The circuit is

simple and can be assembled on a small g e n e r a l - p u r p o s e PCB. Use a heat-sink for IC1. Make sure that LDR1 and the light bulb are well separated.

The circuit can be used for streetlights, tubelights or any other home electri-cal lighting system that needs to be au-tomated.

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The mains power supply phase Ris stepped down by transformer X1 todeliver 12V, 300 mA, which is recti-fied by diode D1 and filtered by ca-pacitor C1 to produce the operatingvoltage for the operational amplifier(IC1). The voltage at inverting pin 2 ofoprational amplifier IC1 is taken fromthe voltage divider circuit of resistorR1 and preset resistor VR1. VR1 is usedto set the reference voltage accordingto the requirement. The reference volt-age at non-inverting pin 3 is fixed to5.1V through zener diode ZD1.

Till the supply voltage available inphase R is in the range of 200V-230V,

the voltage at inverting pin 2 of IC1remains high, i.e., more than referencevoltage of 5.1V, and its output pin 6also remains high. As a result, transis-tor T1 does not conduct, relay RL1 re-mains de-energised and phase ‘R’ sup-plies power to load L1 via normally-closed (N/C) contact of relay RL1.

As soon as phase-R voltage goesbelow 200V, the voltage at invertingpin 2 of IC1 goes below reference volt-age of 5.1V, and its output goes low.As a result, transistor T1 conducts andrelay RL1 energises and load L1 isdisconnected from phase ‘R’ andconnected to phase ‘Y’ through relayRL2.

Similarly, the auto phase-change ofthe remaining two phases, viz, phase‘Y’ and phase ‘B,’ can be explained.Switch S1 is mains power ‘on’/’off’switch.

Use relay contacts of proper ratingand fuses should be able to take-onthe load when transferred from otherphases. While wiring, assembly and in-stallation of the circuit, make sure thatyou:

1. Use good-quality, multi-strandinsulated copper wire suitable for your

MUHAMMAD AJMAL P.

AUTOMATIC PHASE CHANGERS.C.

DWIVEDI

In three-phase applications, if lowvoltage is available in any one ortwo phases, and you want your

equipment to work on normal voltage,this circuit will solve your problem.However, a proper-rating fuse needsto be used in the input lines (R, Y andB) of each phase. The circuit providescorrect voltage in the same power sup-ply lines through relays from the otherphase where correct voltage is avail-able. Using it you can operate all yourequipment even when correct voltageis available on a single phase in the

building.The circuit is built around a trans-

former, comparator, transistor and re-lay. Three identical sets of this circuit,one each for three phases, are used.Let us now consider the working ofthe circuit connecting red cable (call it‘R’ phase).

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current requirement.2. Use good-quality relays with

proper contact and current rating.3. Mount the transformer(s) and re-

lays on a suitable cabinet. Use a TagBlock (TB) for incoming/outgoing con-

nections from mains.EFY Note: 1. During testing in the

lab, we used a 12V, 200-ohm, single-phase changeover relay with 6A cur-rent rating. Similarly, ampere-ratedfuses were used.

2. If the input voltage is low in twophases, loads L1 and L2 may also beconnected to the third phase. In thatsituation, a high-rating fuse will be re-quired at the input of the third phasewhich is taking the total load.

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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

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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.

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84 • DECEmb Er 2008 • electronics for you w w w . E f y m a g . C o m

Aniruddh K.S.

BAttery-level indicAtors.c. dwivedi

Normally, in mobile phones, the battery level is shown in dot or bar form. This lets you

easily recognise the battery level. Here we present a circuit that lets you know the battery level of a device from the number of LEDs that are glowing. It uses ten LEDs in all. So if three LEDs glow, it indicates battery capacity of

30 per cent. Unlike in mobile phones where the battery-level indicator func-tion is integrated with other functions, here only one comparator IC (LM3914) does it all.

The LM3914 uses ten comparators, which are internally assembled in the voltage divider network based on the current-division rule. So it divides the battery level into ten parts.

The circuit derives the power supply for its operation from the battery of the device itself. It uses ten LEDs wired in a 10-dot mode. The use of different coloured LEDs

makes it easier to recognise the voltage level on the basis of the calibration made. Red LEDs (LED1 through LED3) indicate battery capacity of less than 40 per cent. Orange LEDs (LED4 through LED6) indicate battery capacity of 40 to less than 70 per cent and green LEDs (LED7 through LED10) indi-cate battery capacity of 70 to under 100 per cent. The brightness of the

LEDs can be adjusted by varying the value of preset VR2 between pins 6 and 7.

Diode D1 prevents the circuit from reverse-polarity battery con-nection. The tenth LED glows only when the battery capacity is full, i.e., the battery is fully charged. When the battery is fully charged, relay-driver transistor T1 conducts to energise relay RL1. This stops the charging through normally-open (N/O) contacts of relay RL1.

For calibration, connect 15V vari-able, regulated power supply and

initially set it at 3V. Slowly adjust VR1 until LED1 glows. Now, increase the input voltage to 15V in steps of 1.2V until the corresponding LED (LED2 through LED10) lights up.

Now the circuit is ready to show any voltage value with respect to the maximum voltage. As the number of

LEDs is ten, we can easily consider one LED for 10 per cent of the maximum voltage.

Connect the voltage from any battery to be tested at the input probes of the circuit. By examining the number of LEDs glowing you can easily know the status of the bat-tery. Suppose five LEDs are glowing. In this case, the battery capacity is 50 to 59 per cent of its maximum value.

Assemble the circuit on a general-purpose PCB. Calibrate it and then en-close in a box.

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w w w . e f y m a g . c o m electronics for you • july 2008 • 95

Fig. 1: Circuit of bi-cycle indicator

Fig. 3: Suggested enclosure (indicators)

Fig. 2: Suggested enclosure (master unit)

T.K. Hareendran

Bicycle indicaTor s.c. dwivedi

The electronic bicycle signaling unit described here uses low-cost components and is a good

substitute to many commercially avail-able versions. It works in an extremely different manner and is convenient to operate.

The circuit works off a 9V PP3 (alkaline-type) battery and is basically a set of two independent free-running oscillators (astable multivibrators) built around four low-power transistors and a few passive components. Both

the square-wave oscillators (one built around T1 and T2 and the other built around T3 and T4) drive four red LEDs (LED1 and LED2, and LED5 and LED6, respectively), which blink to indicate

the direction of turn. Ad-ditional steady-glow LEDs (LED3 and LED4) are in-corporated to indicate the working status.

T h e w o r k i n g o f t h e c i r -cuit is s traightforward. When switch S1 is flipped to ‘on’ po-s i t i o n , D C s u p p l y f r o m the battery is extended to the oscil-lator circuit formed by transistors T1 and T2. Now the left-side oscil-lator starts oscillating and the visual indicators at the front left (FL) and rear left (RL) start blinking at a rate

determined by timing capacitors C1 and C2. Resistors R2 and R3 limit the

operating current of LEDs (LED1 and LED2). At the same time, the green LED (LED3) starts glowing to indicate the present direction status.

Similar action happens in the next oscillator circuit built around transistors T3 and T4 when switch S2 is flipped to ‘on’ position. Indi-cators at the front right (FR) and rear right (RR) start blinking, and at the same time the green LED (LED4) glows to indicate the direc-tion status.

Switch S3 is used for emergency indication. When it is flipped to ‘on’ position, both the oscillators get power supply through diodes D1 and D2. As a result, LED1 through LED6 start working simultaneous-

ly. In this condition, all the LEDs blink, except LED3 and LED4, which glow steadily.

After assembling the circuit on a general-purpose PCB, enclose it in a suitable cabinet as shown in Fig. 2 and mount on the handle bar of the bicycle, pref-erably at the mechani-cal centre point. Con-

nect switch S1 at the left-hand side, S2 at the right-hand side and emergency switch S3 in the middle of the master unit. Now place this master unit at the top of the handle bar and do the essential interconnections using flex-ible wires. Connect the front indica-tors (LED1 and LED5) to the left and right side of the handle and similarly rear indicators (LED2 and LED6) can be mounted in the carrier frame of the bicycle. For the direction indica-tor, you can use the symbol shown in Fig. 3 and place it at the centre of the handle.

ELECTRONICS PROJECTS Vol. 2212

blown fuse indicatorGenerally, when an equipment in-

dicates no power, the cause may be just a blown fuse. Here is a

circuit that shows the condition of fuse through LEDs. This compact circuit is very useful and reliable. It uses very few components, which makes it inexpensive too.

Under normal conditions (when fuse is alright), voltage drop in first arm is 2V + (2 x 0.7V) = 3.4V, whereas in

second arm it is only 2V. So current flows through the second arm, i.e. through the green LED, causing it to glow; whereas the red LED remains off.

When the fuse blows off, the supply to green LED gets blocked, and because only one LED is in the circuit, the red LED glows. In case of power failure, both LEDs remain ‘off’.

This circuit can be easily modified to produce a siren in fuse-blown condition

(see Fig. 2). An optocoupler is used to trigger the siren. When the fuse blows, red LED glows. Simultaneously it switches ‘on’ the siren.

In place of a bicolour LED, two LEDs of red and green colour can be used. Similarly, only one diode in place of D1 and D2 may be used. Two diodes are used to increase the voltage drop, since the two LEDs may produce different voltage drops.

circuitideas

w w w . e f y m a g . c o m electronics for you • may 2008 • 75

Here is an easy-to-build car anti-theft guard. The circuit, shown in Fig. 1, is simple and

easy to understand. When key-oper-ated switch S2 of the car is turned on, 12V DC supply from the car battery is extended to the entire circuit through polarity-guard diode D5. Blinking LED1 flashes to indicate that the guard circuit is enabled. It works off 12V power supply along with current-limit-ing resistor R4 in series.

When the car door is closed, door switch S1 is in ‘on’ position and 12V power supply is available across resis-

T.K. Hareendran

Car anTi-THefT Guard s.c. dwivedi

Fig. 1: Circuit of car anti-theft guard

Fig. 2: Wiring diagram for door switch (S1)

tor R1, which prevents transistor T1 from conducting. In this position, anti-theft guard cir-cuit is in sleep mode.

W h e n someone opens the car door, switch S1 be-comes ‘off’ as shown in Fig. 2. As a result, transistor T1 conducts to fire r e l a y - d r i v e r SCR1 (BT169) after a short delay introduced by ca- pacitor C1. Electromagnetic relay RL1

energises and its N/O contact connects the power supply to pi-ezobuzzer PZ1, which starts sounding to in-dicate that someone is trying to steal your car. To reset the circuit, turn off switch S2 using car key. This will cut-off the power supply to the circuit and stop the buzzer sound.

Assemble the cir-cuit on a general-pur-pose PCB and house in a small box. Con-nect switch S1 to the car door and keep pi-ezobuzzer PZ1 at an appropriate place in the car.

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9 6 • F E B R U A R Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

W ith this simple clock-controlled timer, you willnever again miss your

favourite TV or radio programme. TheTV or radio will switch on automati-cally at the time preset by you andwill remain ‘on’ until the power sup-ply fails or is disconnected.

The circuit uses the AC signals gen-erated at the buzzer terminals of analarm clock. The AC signals are am-plified by transistors T1 and T2 andthe amplified output from the emitterof T2 is fed to the inverting input ofnegative-voltage comparator IC LM311(IC1). The non-inverting input of IC1gets a presettable voltage through pre-

set VR1. The inverting and non-invert-ing inputs of LM311 are different fromother op-amps and it outputs sink cur-rent through pin 7 or source currentthrough pin 1.

When pin 3 of IC1 is at a highervoltage than pin 2, its output sinksas indicated by LED1. This gives ashort negative pulse to the monostablewired around timer NE555. ResistorR5 keeps trigger pin 2 of IC2 high.The short-interval monostable outputsa high signal for a brief period to thegate of SCR1 (BT169) and relay RL1energises. The latching action of SCR1keeps the relay pulled even when theoutput of the monostable turns low.The relay can be de-energised by dis-connecting the supply to the circuit

D. MOHAN KUMAR

CLOCK TIMERS.C.

DWIVEDI

via switch S1.The circuit works off a 9V bat-

tery. Assemble it on a general-pur-pose PCB and enclose in a suitablecabinet. Provide an AC outlet in thecabinet to switch on the appliance us-ing the circuit. As mentioned earlier,the input signal is obtained from thebuzzer terminals of the clock. Removethe small buzzer of the clock and con-nect point ‘A’ to the positive termi-nal and point ‘B’ to the negative ter-minal of the buzzer. Connect themains AC terminal outlet to the nor-mally-opened (N/O) contact of relayRL1. So when the relay energises,230V AC operates the connected ap-pliance.

Set the desired time in the clock byadjusting the alarm set-up and switchon the circuit. When the set timereaches, the appliance will switch onautomatically. The circuit can also beconnected to digital clocks.

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9 4 • D E C E M B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

H igh-wattage appliances likeelectric irons, ovens and heat-ers result in unnecessary

power loss if left ‘on’ for hours unno-ticed. Here is a circuit that senses theflow of current through the appliancesand gives audible beeps every fifteenminutes to remind you of power-’on’status.

This is a non-contact version of cur-rent monitor and can sense the flowof current in high-current appliancesfrom a distance of up to 30 cm . It uses

a standard step-down transformer (0-9V, 500mA) as the current sensor. Itssecondary winding is left open, whilethe primary winding ends are used todetect the current. The primary endsof the transformer are connected to afull-wave bridge rectifier comprisingdiodes D1 through D4. The rectifiedoutput is connected to the non-invert-ing input of IC CA3140 (IC1).

IC CA3140 is a 4.5MHz BIMOS op-erational amplifier with MOSFET in-put and bipolar transistor output. Ithas gate-protected MOSFET (PMOS)

D. MOHAN KUMAR

CURRENT SENSORS.C.

DWIVEDI

transistors in the input to provide veryhigh input impedance (1.5 T-ohms),very low input current (10 pA) andhigh-speed switching performance.

The inverting input of IC1 is pre-set with VR1. In the standby mode,the primary of the transformer acceptse.m.f. from the instrument or sur-rounding atmosphere, which results inlow-voltage input to IC1. This lowvoltage at the non-inverting inputkeeps the output of IC1 low. Thus tran-sistor T1 doesn’t conduct and pin 12of IC2 goes high to disable IC2. As aresult, the remaining part of the cir-

cuit gets inactivated.When a high-current appliance is

switched on, there will be a currentdrain in the primary of the transformerto the negative rail due to an increasein the e.m.f. caused by the flow of cur-rent through the appliance. This resultsin voltage rise at the non-inverting in-put and the output of IC1 becomes high.This high output drives transistor T1into conduction and the reset pin ofIC2 becomes low, which enables IC2.

IC CD4060 (IC2) is a 14-stage ripplecounter. It is used as a 15-minute timer

by feeding Q9 output to thepiezobuzzer for aural alarm throughthe intermediate circuitry. Resistors R5and R6 along with capacitor C1 main-tain the oscillations in IC2 as indicatedby blinking LED1. The high outputfrom IC2 is used to activate a simpleoscillator comprising transistors T2and T3, resistors R8 and R10, and ca-pacitor C2.

When the Q9 output of IC2 be-comes high, zener diode ZD1 provides3.1 volts to the base of transitor T2.

Since transistor T2 is biased by a high-value resistor (R8), it will not conductimmediately. Capacitor C2 slowlycharges and when the voltage at thebase of T2 increases above 0.6 volt, itconducts. When T2 conducts, the baseof T3 turns low and it also conducts.The piezobuzzer connected to the col-lector of T3 gives a short beep as ca-pacitor C2 discharges. This sequenceof IC2 output at Q9 becoming high andconduction of transistors T2 and T3 re-sulting in beep sound repeats at shortintervals.

ELECTRONICS PROJECTS Vol. 22174

DARKROOM TIMERsistor T2 is coupled to a small speakerthrough a transistor-radio type outputtransformer.

The 22-kilo-ohm value of resistor R3represents a compromise between toneduration and intensity. You can use re-sistors having a value anywhere between10 kilo-ohms and 25 kilo-ohms for differ-ent durations and intensities of the out-put signals.

Since the unijunction transistor isfunctioning as the oscillator trigger,changing the values of one or more com-ponents in the UJT circuit will changethe rate of the tone burst. The tone fre-quency can be varied by changing thevalue of any or more of capacitors C2through C4 and resistors R5 and R6 inthe phase-shift network.

The primary winding of transformerX1 can be tuned for a slight increase inthe output, using capacitor values between0.05 and 0.25 μF for C5 by trial-and-errormethod. Tone pulses should begin aboutten seconds after the unit is turned on.After a minute or so, adjust preset VR1for 1-second beats by comparing the tim-ing of the beats with the seconds needleon your wristwatch.

The timer circuit described here pro-vides a pleasant musical tone inyour darkroom at 1-second inter-

vals. The circuit takes up very little spaceand can be easily converted into a metro-nome.

Unijunction transistor (UJT) T1 func-tioning as a relaxation oscillator triggersthe phase-shift audio oscillator circuit

built around transistor T2, turning it onand off. As capacitor C1 is chargedthrough preset VR1 and resistor R1, theemitter voltage of UJT rises toward thesupply voltage.

When the emitter voltage becomes suf-ficiently positive, the emitter becomes for-ward biased and discharges capacitor C1through the emitter-base 1 (B1) junction

and resistor R2.The voltage dropacross R2 forwardbiases transistorT2 and turns it on.As capacitor C1becomes dis-charged, the cur-rent through re-sistor R2 dropsand transistor T2is cut off.

A tone signalis generated bytransistor T2 andR-C coupledphase-shift oscilla-tor. Part of the sig-nal taken from thecollector of tran-

CIRCUIT

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9 8 • S E P T E M B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

Fig. 1: Circuit of desktop power supply

Fig. 2: Pinconfiguration ofLM317

Fig. 3: Suggested powersupply box

T.K. HAREENDRAN

DESKTOP POWER SUPPLYS.C.

DWIVEDI

U seful for electronics hobbyists,this linear workbench powersupply converts a high input

voltage (12V) from the SMPS of a PCinto low output voltage (1.25 to 9volts). An adjustable three-pin voltageregulator chip LM317T (IC1) is usedhere to provide the required voltages.The LM317T regulator, in TO-220 pack,can handle current of up to 1 amp inpractice.

Fig. 1 shows the circuit of the desk-

top power supply.Regulator IC LM317Tis arranged in its stan-dard application. Di-ode D1 guards againstpolarity reversal andcapacitor C1 is an ad-ditional buffer. Thegreen LED (LED1) in-dicates the status of thepower input. Diode D2prevents the output

voltage from rising above the inputvoltage when a capacitive or induc-

tive load isconnected atthe output.Similarly, ca-pacitor C3 sup-presses any re-sidual ripple.

Connect astandard digi-tal voltmeter inparallel with the output leads to accu-rately set the desired voltage with thehelp of variable resistor VR1. You can

also use your digital multimeterif the digital voltmeter is notavailable. Switch on S1 and setthe required voltage throughpreset VR1 and read it on thedigital voltmeter. Now thepower supply is ready for use.

The circuit can be wired ona common PCB. Refer Fig. 2 forpin configuration of LM317 be-fore soldering it on the PCB. Af-ter fabrication, enclose the cir-cuit in a metallic cover asshown in Fig. 3. Then open thecabinet of your PC and connectthe input line of the gadget to afree (hanging) four-pin drivepower connector of the SMPScarefully.

CIRCUIT

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

Decoded Segment Outputsfor Counts 0 through 9

EFY LAB

DICE WITH 7-SEGMENT DISPLAY

A digital dice circuit can be eas-ily realised using an astableoscillator circuit followed by

a counter, display driver and a dis-play.

Here we have used a timer NE555as an astable oscillator with a fre-quency of about 100 Hz. Decadecounter IC CD4026 or CD4033 (which-ever available) can be used as counter-cum-display driver. When using

CD4026, pin 14 (cascading output) isto be left unused (open), but in case ofCD4033, pin 14 serves as lamp test pinand the same is to be grounded.

The circuit uses only a handful ofcomponents. Its power consumption isalso quite low because of use of CMOSICs, and hence it is well suited for bat-tery operation. In this circuit two tac-tile switches S1 and S2 have been pro-vided. While switch S2 is used for ini-tial resetting of the display to ‘0,’ de-pression of S1 simulates throwing of

the dice by a player.When battery is con-

nected to the circuit, thecounter and display sectionaround IC2 (CD4026/4033)is energised and the displaywould normally show ‘0’, asno clock input is available.Should the display showany other decimal digit, youmay press re-set switch S2so that display shows ‘0’. Tosimulate throwing of dice,the player has to pressswitch S1, briefly. This ex-tends the supply to the

astable oscillator configured aroundIC1 as well as capacitor C1 (throughresistor R1), which charges to the bat-tery voltage. Thus even after switchS1 is released, the astable circuitaround IC1 keeps producing the clockuntil capacitor C1 discharges suffi-

ciently. Thus for du-ration of depressionof switch S1 and dis-charge of capacitorC1 thereafter, clockpulses are producedby IC1 and applied toclock pin 1 of counterIC2, whose count ad-vances at a frequencyof 100 Hz until C1discharges suffi-ciently to deactivateIC1.

When the oscilla-tions from IC1 stop,the last (random)count in counter IC2can be viewed on the7-segment display.This count wouldnormally lie between0 and 6, since at theleading edge of every

7th clock pulse, the counter is reset tozero. This is achieved as follows.

Observe the behavior of ‘b’ seg-ment output in the Table. On reset, atcount 0 until count 4, the segment ‘b’output is high. At count 5 it changesto low level and remains so duringcount 6. However, at start of count 7,the output goes from low to high state.A differentiated sharp high pulsethrough C-R combination of C4-R5 isapplied to reset pin 15 of IC2 to resetthe output to ‘0’ for a fraction of apulse period (which is not visible onthe 7-segment display). Thus, if theclock stops at seventh count, the dis-

S.C. DWIVE

DI

CIRCUIT

IDEAS

9 8 • N O V E M B E R 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

play will read zero. There is a prob-ability of one chance in seven that dis-play would show ‘0.’ In such a situa-tion, the concerned player is given an-

other chance until the display is non-zero.

Note. Although it is quite feasibleto inhibit display of ‘0’ and advance

the counter by ‘1,’ the same makes thecircuit somewhat complex and there-fore such a modification has not beenattempted.

circuitideas

electronics for you • July 2010 • 99w w w . e f y m a g . c o m

Raj K. GoRKhali

DiGital theRmometeR s.c. dwivedi

This digital thermometer can measure temperatures up to 150°C with an accuracy of ±1°C.

The temperature is read on a 1V full scale-deflection (FSD) moving-coil voltmeter or digital voltmeter.

Operational amplifier IC 741 (IC3) provides a constant flow of current

through the base-emitter junction of npn transistor BC108 (T1). The volt-age across the base-emitter junction of the transistor is proportional to its temperature. The transistor used this way makes a low-cost sensor. You can use silicon diode instead of transistor. The small variation in voltage across the base-emitter junction is amplified by second operational amplifier (IC4),

before the temperature is displayed on the meter. Preset VR1 is used to set the zero-reading on the meter and preset VR2 is used to set the range of temperature measurement.

Operational amplifiers IC3 and IC4 operate off regulated ±5V power sup-ply, which is derived from 3-terminal

positive voltage regula-tor IC 7805 (IC1) and negative low-dropout regulator IC 7660 (IC2). The entire circuit works off a 9V battery.

Assemble the cir-cuit on a general-pur-pose PCB and enclose in a small plastic box. Calibrate the thermom-eter using presets VR1 and VR2. After calibra-tion, keep the box in the vicinity of the object whose temperature is to be measured.

circuitideas

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

This simple circuit automatically activates or deactivates an elec-tronic device at the time of alarm

preset in a clock. When the alarm rings, the tone burst generated at the terminal of the buzzer triggers the circuit and the relay energises or de-energises to switch on or switch off the load.

The circuit is built around ICs CD40106 (IC1) and CD4017 (IC2) and a few discrete components. IC1 is a hex Schmitt trigger, while IC2 is a decade counter. The circuit works off regulat-ed 6V power supply, while the alarm clock runs off its own 1.5V battery.

The tone burst generated at the

piezobuzzer is tapped from its connec-tion points. The positive terminal of the clock buzzer is connected to the base of transistor T1 and the negative terminal is connected to ground of the circuit.

When the alarm clock sounds, the signal from the clock buzzer makes transistor T1 conduct. As a result, pin 1 of gate N1 goes low and it outputs high at pin 2. This low-to-high transi-tion clocks the counter (IC2) at pin 14 through diode D1 and gate N2. In this way, IC2 advances by one at each clock produced due to the sounding alarm.

There are two situations where this

circuit can be used:1. You want an appliance or gadget

to switch on automatically at a preset time

2. You switch on an appliance or gadget manually at a particular time and want it to switch off automatically at a preset time

Let us see how it works when you want your appliance to switch on at a preset time, say, 3 pm. Set the alarm in your clock to 3 pm and slide switch S3 towards Q1. When the alarm sounds at 3 pm, Q0 output of IC2 advances to Q1 and relay RL1 energises to connect the load (appliance) to mains power supply through its contacts. The load remains ‘on’ until you reset IC2 by

momentarily pressing S1. At this time, you need to pause the alarm using pause switch of the clock.

Now suppose you manually start the load at 3 pm and want it to stop automatically at 6 pm. First, reset IC2 by momentarily pressing S1 and slide switch S3 towards Q2. Set the alarm in your clock to 6 pm. To start the load, press switch S2 momentarily at 3 pm. The Q0 output of IC2 advances to Q1 and relay RL1 energises to connect the load to mains power supply through its contacts. When the alarm sounds at 6 pm, Q1 output of IC2 advances to Q2

and relay RL1 de-energises to discon-nect the load from mains power supply through its contacts. At this time, you need to pause the alarm using pause switch of the clock.

When you press reset switch S1, LED1 glows to indicate that the circuit is ready to work. When you press start switch S2, LED2 glows to indicate start mode. Glowing of LED3 means that the counter has stopped counting and needs to be reset before use.

When the counter is in stop mode, Q2 output of IC2 remains high. As this pin is connected to the clock-enable input (pin 13) of IC2, the clock input is inhibited. In this condition, any tone

burst signal arriving from the clock has no effect on IC2 and therefore the circuit remains in stop mode. You can now set the alarm time in the clock.

Assemble the circuit on a general-purpose PCB and enclose in a small cabinet. Connect the base of transistor (T1) to positive terminal of the alarm clock and negative terminal to ground of the circuit. Put the alarm clock at a convenient place. If you do not want to use a 6V battery, replace it with a 6V adaptor to power the circuit. Mount the LEDs and the pushbutton on the front panel of the cabinet.

Raj K. GoRKhali

DiGital timeR enhancement s.c. dwivedi

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w w w . e f y m a g . c o m electronics for you • mar ch 2008 • 105

Dr C.H. VitHalani

Drinking Water alarm s.c. dwivedi

The State Jal Boards supply water for limited duration in a day. Time of water supply is

decided by the management and the public does not know the same. In such a situation, this water alarm circuit will save the people from long wait as it will inform them as soon as the water supply starts.

At the heart of this circuit is a small water sensor. For fabricating this water sensor, you need two foils—an aluminium foil and a plastic foil. You can assemble the sensor by rolling aluminium and plastic foils in the shape of a concentric cylinder. Connect one end of the insulated flexible wire on the aluminium foil and the other end to resistor R2. Now mount this sensor inside the water tap such that water can flow through it uninterrupted. To complete the circuit, connect another wire from the junction of pins 2 and 6 of IC1 to the water pipeline or the water tap itself.

The working of the circuit is sim-ple. Timer 555 is wired as an astable multivibrator. The multivibrator will

work only when water flows through the water tap and completes the cir-cuit connection. It oscillates at about 1 kHz. The output of the timer at pin 3 is connected to loudspeaker LS1 via capacitor C3. As soon as water starts flowing through the tap, the speaker starts sounding, which indicates re-sumption of water supply. It remains ‘on’ until you switch off the circuit with switch S1 or remove the sensor

from the tap. The circuit works off a 9V battery supply.

Assemble the circuit on any gen-eral-purpose PCB and house in a suitable cabinet. The water sensor is inserted into the water tap. Connect the lead coming out from the junc-tion of 555 pins 2 and 6 to the body of the water tap. Use on/off switch S1 to power the circuit with the 9V PP3 battery.

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

electronics for you • september 2010 • 117w w w . e f y m a g . c o m

Fig. 2: Guitar preamplifier circuit

Fig. 1: A typical example of mounting the guitar pickup

Headstock

pickup

jack

T.K. HAREENDRAN

ElEcTRic GuiTAR PREAmPlifiERsani theo

up attached to a guitar headstock is shown in Fig. 1. The pickup device has a transducer on one end and a jack on the other end. The jack can be plugged into a preamplifier circuit and then to a power amplifier system.

The pickup device captures me-chanical vibrations, usually from stringed instruments such as guitar or violin, and converts them into an electrical signal, which can then be amplified by an audio amplifier. It is most often mounted on the body of the instrument, but can also be at-tached to the bridge, neck, pickguard or headstock.

The first part of this preamplifier circuit shown in Fig. 2 is a single-tran-

sistor common-emitter amplifier with degenerative feedback in the emitter and a boot-strapped bias divider to

Here is the circuit of a guitar preamplifier that would ac-cept any standard guitar

pickup. It is also versatile in that it has two signal outputs.

A typical example of using a pick-

secure optimal input impedance. With the component values shown here, the input impedance is above 50 kilo-ohms and the peak output voltage is about 2V RMS. Master-level-control potmeter VR1 should be adjusted for minimal distortion.

The input from guitar pickup is fed to this preamplifier at J1 terminal. The signal is buffered and processed by the op-amp circuit wired around IC TL071 (IC1). Set the gain using preset VR2. The circuit has a master and a slave control. RCA socket J2 is the master signal output socket and socket J3 is the slave.

It is much better to take the signal from J2 as the input to the power amplifier sys-tem or sound mixer. Output signals from J3 can be used to drive a standard headphone amplifier. Using potmeter VR3, set the slave output sig-nal level at J3.

House the circuit in a metallic case. VR1 and VR3 should preferably be the types with metal enclosures. To prevent hum, ground the case and the enclosures. A well-regulated 9V DC power supply is crucial for this circuit. How-ever, a standard 9V alkaline

manganese battery can also be used to power the circuit. Switch S1 is a power-on/off switch.

circuitideas

110 • September 2008 • electronics for you w w w . e f y m a g . c o m

Here is the circuit of a simple electric window charger. With a couple of minor circuit

variations, it can be used as an electric fence charger too. A standard 12V, 7Ah sealed maintenance-free (SMF) UPS battery is required for powering the entire unit.

Any component layout and mount-ing plan can be used. However, try to keep the output terminals of transformer X1 away from the circuit board. Timer NE555 (IC1) is wired as a free-running oscillator with narrow negative pulse at the output pin 3. The pulse frequency is determined by resis-

T.K. HAREENDRAN

ELECTRIC WINDOW/FENCE CHARGER

s.c. dwiveditors R2 and R3, preset VR1 and capaci-tor C3. The amplitude of the output pulse can be varied to some extent by adjusting variable resistor VR1. You can vary the frequency from 100 Hz to 150 Hz.

X1 is a small, iron-core, step-down transformer (230V AC primary to 12V, 1A secondary) that must be reverse connected, i.e., the secondary winding terminals of the transformer should be connected between the emitter and ground and the output taken across the primary winding. Switch S1 is used for power ‘on’/‘off’ and LED1 works as a power-‘on’ indicator. LED2 is used to indicate the pulse activity.

The output pulse from pin 3 of IC1

drives pnp transistor T1 into conduc-tion for the duration of the time period. The collector of T1 is connected to the base of driver transistor T2 through re-sistor R5. When transistor T1 conducts, T2 also conducts. When T2 conducts, a high-current pulse flows through the secondary winding of transformer X1 to generate a very high-voltage pulse at the primary winding.

This dangerously high voltage can be used to charge the window rails/fences. Ordinary silicon diode

D1 (1N4001) protects T2 against high-voltage peaks generated by X1 inductance during the switching time.

You can replace X1 with another trans-former rating, and, if necessary, replace T2 with another higher-capacity transistor. The circuit can be used to charge a 1km fence with some minor mod-ifications in the output section.

Caution. Take all the relevant electri-cal safety precautions when assembling, test-ing and using this high-voltage generator.

circuitideas

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.

circuitideas

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

Here is a simple circuit that can produce the effect of candle light in a normal

electric bulb. A candle light, as we all know, resembles a randomly flickering light. So, the objective of this project activity is to produce a randomly flickering light effect in an electric bulb.

To achieve this, the entire circuit can be divided into three parts. The first part comprises IC1 (555), IC2 (74LS164), IC3 (74LS86), IC4 (74LS00) and the as-sociated components. These generate a randomly changing train of pulses.

The second part of the circuit consists of SCR1 (C106), an electric bulb connected between anode of SCR1 and mains live wire, and

Raj K. GoRKhali

ElEctRonic candlEs s.c. dwivedi

gate trigger circuit components. It is basically half-wave AC power being supplied to the electric bulb.

The third part is the power sup-ply circuit to generate regulated 5V DC from 230V AC for random signal generator. It comprises a stepdown transformer (X1), full-wave rectifier (diodes D3 and D4), filter capacitor (C9), followed by a regulator (IC5).

The random signal generator of the circuit is built around an 8-bit serial in/parallel out shift register (IC2). Different outputs of the shift register IC pass through a set of logic gates (N1 through N5) and final out-

put appearing at pin 6 of gate N5 is fed back to the inputs of pins 1 and 2 of IC2. The clock signal appears at pin 8 of IC2, which is clocked by an astable multivibrator configured around timer (IC1). The clock fre-quency can be set using preset VR1 and VR2. It can be set around 100 Hz

to provide better flickering effect in the bulb.

The random signal triggers the gate of SCR1. The electric bulb gets AC power only for the period for which SCR1 is fired. SCR1 is fired only during the positive half cycles. Conduction of SCR1 depends upon the gate triggering pin 3 of IC2, which is random. Thus, we see a flickering effect in the light output.

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

case. Fix bulb and neon bulb on the front side of the cabinet. Also, connect a power cable for giving AC mains supply to the circuit for operation. The circuit is ready to use.

Warning. Since the circuit uses 230V AC, care must be taken to avoid electric shock.

Fig. 1: Circuit diagram for electronic candle

Fig. 2: Pin configurations of C106 and 7805

ELECTRONICS PROJECTS Vol. 22 13

ElEctronic card- lock systEm

The circuit presented here can be used as a lock for important electronic/electrical appliances.

When card is inserted inside its mecha-nism, depending upon the position of punched hole on the card, a particu-lar appliance would be switched on.

The card is inserted just like a floppy disk inside the disk drive. This card should be rectangular in shape with only one punched hole on it.

T h e c i r c u i t u s e s eight photo-transistors (T1 through T8). When there is no card in the lock, light from incandes-cent lamp L1 (40-watt, 230V) falls on all the photo- transistor detec-tors. Transistor T8 is used as enable detector for IC1 (74LS244). When light is incident on it, it conducts and its collector voltage goes low. This makes transistor T16 to cut-off, and its collector voltage goes high. This logic high on its collector terminal will inhibit IC1 as long as light is present on photo-transistor T8.

IC1 will get enabled only when the card is completely inserted inside the lock mechanism. This arrangement en-sures that only the selected appliance is switched on and prevents false operation of the system.

You can make these cards using a black, opaque plastic sheet. A small rectangular notch is made on this card to indicate proper direction for insertion of the card. If an attempt is made to insert the card wrongly, it will not go completely inside the mechanism and the system will not be enabled.

When card for any appliance (say appliance 1) is completely inserted in the mechanism, the light will fall only on photo-transistor T1. So only T1 will

be on and other photo-transistors will be in off state. When transistor T1 is on, its collector voltage falls, making transistor T9 to cut-off. As a result, col-lector voltage of transistor T9 as also pin 2 of IC1 go logic high. This causes

pin 18 (output Q1) also to go high, switching LED1 on. Simultaneously, output Q1 is connected to pin 1 of IC2 (ULN2003) for driving the relay cor-responding to appliance 1. Similarly, if card for appliance 2 is inserted, only output pin 16 (Q2) of IC1 will go high-making LED2 on while at the same time energising relay for appliance 2 via ULN2003. The same is true for other cases/appliances also.

The time during which card is pres-ent inside the mechanism, the system generates musical tone. This is achieved with the help of diodes D1 through D7 which provide a wired-OR connection at their common-cathode junction. When any of the outputs of IC1 is logic high, the commoncathode junction of diodes D1 through D7 also goes logic high, enabling IC3 (UM66) to generate a mu-sical tone.

In this circuit IC1 (74LS244) is used

as buffer with Schmitt trigger. All outputs (Q1 through Q7) of this IC are connected to IC2 (ULN2003) which is used as relay driver. IC2 consists of seven highcurrent relay drivers having integral diodes. Ex-ternal free-wheeling diodes are therefore

not required.When an input of this IC is made logic

high, the corresponding output will go logic low and relay connected to that pin gets energised. This switches on a specific appliance and the corresponding LED.

Once a specific card is inserted to switch on a specific relay, that relay gets latched through its second pair of contacts. Thus even when the card is removed, the specific appliance remains on. The same holds true for all other re-lays/appliances as well. The only way to deenergise a latched relay after removal of the corresponding card is to switch off the corresponding switch (S1 through S7) which would cut-off the supply to the desired relay.

The +5V and +12V supplies can be obtained with conventional arrangement using a step-down transformer followed by rectifier, filter and regulator (using 7805 and 7812 etc).

ELECTRONICS PROJECTS Vol. 2214

circuitideas

electronics for you • June 2010 • 105w w w . e f y m a g . c o m

This simple circuit demonstrates the capability of an AT89C2051 microcontrol-

ler chip to function as a random number generator based on the flying counter principle. The pro-gram in the chip con-stantly updates the

DebDoot Sheet

electronic Dice USing At89c2051

sani theocounter variable, which, on being inter-rupted by an external trigger, latches the counter value and displays a ran-dom number through its output ports. This method is similar to the one used in PCs or calculators for generating ran-dom numbers at any instance.

The application of this dice is similar to the one used in a game of dice. The random numbers generated are not displayed numerically, but

represented by the number of glowing LEDs. The LEDs are the dot repre-sentation on the face of a dice. Suggested LED arrangement for the electronic dice display is shown in Fig. 1.

The use of IC AT89C2051 (IC1) module in the design is quite simple. It operates off 3-5.5V DC supply

and uses an 18MHz crystal to generate the clock (refer Fig. 2). Switch S1 con-nected at pin 1 is used as a reset switch. Interrupt occurs at pin 6 of IC1 on logic 0. Switch S2 connected to pin 6 (INT0) of IC1 is used to trigger an external in-terrupt to make pin 6 low. It is used as input to generate the random number. The random number is indicated by glowing of the LEDs (LED1 through LED7) connected to port pins P1.2-P1.7 and P3.7.

TL0 and TH0 act as free-wheeling counters in auto-increment mode and constantly count up from the initial value. When the interrupt occurs, the value from the counter is latched and glowing LEDs indicate the random number generated by the microcontrol-ler chip. Assembly language is used for programming the chip. The Assembly code listing is self-explanatory.

EFY note. The source code is in-cluded in this month’s EFY-CD and Fig. 2: Circuit for electronic dice using AT89C2051

Fig. 1: Suggested LED arrangement for electronic dice display

CIRCUIT

IDEAS

E L E C T R O N I C S F O R Y O U • J A N U A R Y 2 0 0 7 • 1 0 9W W W . E F Y M A G . C O M

comparator. A2 and A3 togetherwork as a ‘wandering voltage gen-erator’ op-amp. Op-amp A4 is wiredas a buffer and its output providesbase current to npn transistor T2. npntransistor T2 and audio output trans-former X1 form a voltage-controlledoscillator.

When power is switched on, a ba-sic tone is generated by transistor T2and transformer X1, which is fre-quency-modulated by the wanderingvoltage generator, which, in turn, isinfluenced by the low-frequencysquarewave generator.

The circuit works off regulated

9V. To generate several differenttones, connect its point A1 to pins1, 3, 4, 5, 8, 9, 10, 11, 12 and 13 of IC1and point A2 to pins 1, 2, 3, 6, 8, 11and 13.

The circuit can be used as an auto-mobile horn by using about 10W au-dio amplifier.

H ere’s a simple circuit of anelectronic horn that is builtaround quadruple op-amp

ASHOK K. DOCTOR

ELECTRONIC HORNS.C.

DWIVEDI

IC LM3900 (IC1). IC LM3900 has fourindependent op-amps (A1 through A4)with a large output voltage swing. Itcan work at up to 32V DC.

The first op-amp (A1) is wired as

a low-frequency squarewave genera-tor. Op-amp A2 works as an integra-tor, while op-amp A3 works as a

CIRCUIT

IDEAS

1 2 6 • J A N U A R Y 2 0 0 8 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

H ere is a white-LED-basedemergency light that offersthe following advantages:

1. It is highly bright due to the useof white LEDs.

2. The light turns on automaticallywhen mains supply fails, and turns offwhen mains power resumes.

3. It has its own battery charger.When the battery is fully charged,charging stops automatically.

The circuit comprises two sections:charger power supply and LED driver.The charger power supply section is

built around 3-terminal adjustableregulator IC LM317 (IC1), while theLED driver section is built aroundtransistor BD140 (T2).

In the charger power supply sec-tion, input AC mains is stepped downby transformer X1 to deliver 9V, 500mA to the bridge rectifier, which com-prises diodes D1 through D4. Filtercapacitor C1 eliminates ripples. Un-regulated DC voltage is fed to inputpin 3 of IC1 and provides charging

current through diode D5 and limit-ing resistor R16. By adjusting presetVR1, the output voltage can be ad-justed to deliver the required charg-ing current.

When the battery gets charged to6.8V, zener diode ZD1 conducts andcharging current from regulator IC1finds a path through transistor T1 toground and it stops charging of thebattery.

The LED driver section uses a to-tal of twelve 10mm white LEDs. Allthe LEDs are connected in parallelwith a 100-ohm resistor in series witheach. The common-anode junction of

all the twelve LEDs is connected tothe collector of pnp transistor T2 andthe emitter of transistor T2 is directlyconnected to the positive terminal of6V battery. The unregulated DC volt-age, produced at the cathode junc-tion of diodes D1 and D3, is fed tothe base of transistor T2 through a 1-kilo-ohm resistor.

When mains power is available, thebase of transistor T2 remains high andT2 does not conduct. Thus LEDs are

off. On the other hand, when mainsfails, the base of transistor T2 becomeslow and it conducts. This makes allthe LEDs (LED1 through LED12) glow.

The mains power supply, whena v a i l a b l e ,charges the bat-tery and keepsthe LEDs off astransistor T2 re-mains cut-off.During mainsfailure, thecharging sec-tion stopsworking andthe battery sup-ply makes theLEDs glow.

A s s e m b l ethe circuit on ag e n e r a l - p u r -pose PCB andenclose in acabinet withenough spacefor battery and

switches. Mount the LEDs on the cabi-net such that they light up the room.A hole in the cabinet should be drilledto connect 230V AC input for the pri-mary of the transformer.

EFY lab note. We have tested thecircuit with twelve 10mm white LEDs.You can use more LEDs provided thetotal current consumption does not ex-ceed 1.5A. Driver transistor T2 can de-liver up to 1.5A with proper heat-sinkarrangement.

S.C. DWIVEDI

AUTOMATIC LOW-POWEREMERGENCY LIGHT

SUNIL KUMA

R

Fig. 1: Automatic high intensity LED-based emergency light

Fig. 2: Pin configurations of LM317, BD140and BC548

circuitideas

electronics for you • November 2010 • 115w w w . e f y m a g . c o m

Here is the circuit for a port-able electric lamp-cum-LED flasher. It uses a 25W, 230V

AC bulb and nine LEDs. When the bulb glows all the LEDs remain ‘off,’ and when the LEDs glow the bulb remains ‘off.’

The circuit is built around timer IC 555 (IC1), which is wired as an astable multivibrator generating square wave.

The output of IC1 drives transistor T1. Working of the circuit is simple.

When output pin 3 of IC1 goes high, transistor T1 conducts to fire TrIAC1 and the bulb glows. Bulb L1 turns off when output pin 3 of IC1 goes low.

The collector of transistor T1 is con-nected to anodes of all the LEDs (LED1 through LED9). So when T1 is cut-off the LEDs glow, and when T1 conducts the LEDs go off. Current-limiting resis-tor R4 protects the LEDs from higher

Sunil Kumar

FlaSher For Deepawali s.c. dwivedi

currents.In brief, the bulb and the LEDs

flash alternately depending on the frequency of IC1. Flashing rates of the bulb as well as LEDs can be varied by adjusting potmeter Vr1. Connect the power supply line (L) of mains to bulb L1 via switch S1 and neutral (N) to MT1 terminal of TrIAC1.

A 12V, 200mA AC adaptor is used to power the circuit. Using switch S1, you can switch off the bulb permanently if you do not want it to flash.

Assemble the circuit on a general-purpose PCB and en-close in a circular plastic cabinet keeping the bulb at the centre and LEDs at the circumference. Drill holes for mounting the ‘on’/‘off’ switch. Use a bulb holder for bulb L1 and LED holders for the LEDs. Also use an IC socket for timer IC 555.

Warning. While assembling, testing or repairing, take care to avoid the lethal electric shock.

CIRCUIT

IDEAS

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

T his unidentified flying object(UFO) is nothing but an elec-tronic toy depicting the fantacy.

It comprises three separate sections,viz, rim flasher, dome flasher andsound generator.

The rim flasher is a simple sequen-tial circuit built around timer IC 555(IC1) and decade counter IC CD4017(IC2) as shown in Fig. 1. IC1 is wiredas an astable multivibrator whose out-put is fed to clock pin 14 of decadecounter IC2. All the eight outputs ofIC2 are connected with two LEDs each.These 16 LEDs (LED1 through LED16)are arranged round the rim of a fly-ing-saucer-like toy. The colour of LEDsused may be yellow, pink orange oreven white to give a good colour ef-fect.

The dome flasher circuit is builtaround a 14-stage ripple-carry binarycounter and oscillator IC CD4060 (IC3)as shown in Fig. 2. Three outputs areused here. Three groups of LEDs withsix LEDs in each are arranged such thateach group flashes at a different rate.Preset VR1 (47-kilo-ohm) is used tovary the flash cycle.

These 18 LEDs (LED17 throughLED34) are arranged around the grove(disk) of a general-purpose PCB orveroboard, which is covered by atransparent dome. Use different-coloured LEDs for each group to cre-ate the required light effect. Red, blue,yellow or green LEDs will create a niceeffect. If a transparent dome is not pos-sible, drill holes around the top to fixthe LEDs.

The sound generator is builtaround two 555 timers, two transis-tors and some discrete components asshown in Fig. 3. Timer IC5 is config-

ASHOK K. DOCTOR

FLYING SAUCER

S.C. DWIVE

DI

Fig. 1: Rim flasher

Fig. 2: Dome flasher

Fig. 3: Sound generator

CIRCUIT

IDEAS

9 6 • J U L Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

ured as an astable multivibrator. Thecharge-discharge cycle of capacitor C8(47µF) generates a sawtooth waveformwhich rises rapidly but falls slowly.This waveform is fed to the base oftransistor T2 (BC327), which is an emit-ter follower. Its output is used to con-trol frequency modulation. It is fed to

Fig. 4: Fittings of LEDs on rim

Fig. 5: Assemble unit of unidentify bird

pin 5 of timer IC4. The rectangu-lar-wave output at pin 3 of timer

IC5 is fed to transistor BC548 (T1) tooperate timer IC4, which is also anasymmetrical multivibrator. If a 75-ohm-impedance speaker is available,there is no need to use resistor R16 (68ohms).

For assembling the circuit, use twodeep, plastic bowls of about 20 cm di-

ameter each. Make surethat bowls have rims tofacilitate fixing of LEDswith small screws. Forfixing the LEDs, refer toFig. 4. Assemble the rimflasher, dome flasherand sound generator cir-cuits on separate gen-eral-purpose PCBs andmount these on the deepbowls along with batter-ies and speaker. PCB1,

PCB2 and PCB3 are for rim flasher,dome flasher and sound generator, re-spectively.

The assembled flyingsaucer is shown in Fig. 5. When youswitch on the circuit, rim LEDs anddome LEDs flash, and at the sametime, a sound is generated. This givesthe simulated effect of an unidentifiedflying object.

circuitideas

electronics for you • Mar ch 2010 • 103w w w . e f y M a g . c o M

the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which de-

livers RF power to a 50-ohm horizontal dipole or ground plane antenna.

Use a heat-sink with transistor 2N3866 for heat dissipation. Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88-108 MHz. Also adjust trimmers VC2 through VC7 to get maximum output at maximum range.

Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the

PradeeP G.

Four-StaGe FM tranSMitter s.c. dwivedi

This FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around

transistor BF494 (T1), a preamplifier

built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A con-denser microphone is connected at the input of the oscillator.

Working of the circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1. The FM signals are am-plified by the VHF preamplifier and

frequency generated. You can also use a 12V battery to power the circuit.

Assemble the circuit on a general-purpose PCB. Install the antenna prop-

erly for maximum range. Coils L1 through L5 are made with

20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively.

EFY note. This transmitter is meant only for educational purposes. use of this transmitter with outdoor antenna is illegal in most parts of the world. The author and EFY will not be responsible for any misuse of this transmitter.

CIRCUIT

IDEAS

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

DR C.H. VITHALANI

FULLY AUTOMATIC EMERGENCY LIGHT

S.C. DWIVE

DI

T his simple automatic emer-gency light has the followingadvantages over conventional

emergency lights:1. The charging circuit stops auto-

matically when the battery is fullycharged. So you can leave the emer-gency light connected to AC mainsovernight without any fear.

2. Emergency light automaticallyturns on when mains fails. So youdon’t need a torch to locate it.

3. When mains power is available,emergency light automatically turnsoff.

The circuit can be divided into in-verter and charger sections. The in-verter section is built around timerNE555, while the charger section is

built around 3-terminal adjustableregulator LM317.

In the inverter section, NE555 iswired as an astable multivibrator thatproduces a 15kHz squarewave. Out-put pin 3 of IC 555 is connected to theDarlington pair formed by transistorsSL100 (T1) and 2N3055 (T2) via resis-tor R4. The Darlington pair drives fer-rite transformer X1 to light up thetubelight.

For fabricating inverter transformerX1, use two EE ferrite cores (of25×13×8mm size each) along with plas-tic former. Wind 10 turns of 22 SWGon primary and 500 turns of 34 SWGwire on secondary using some insula-tion between the primary and second-ary.

To connect the tubelight to ferritetransformer X1, first short both ter-

minals of each side of the tubelightand then connect to the secondary ofX1. (You can also use a Darlington pairof transistors BC547 and 2N6292 for a6W tubelight with the same trans-former.)

When mains power is available, re-set pin 4 of IC 555 is grounded viatransistor T4. Thus, IC1 (NE555) doesnot produce squarewave and emer-gency light turns off in the presenceof mains supply.

When mains fails, transistor T4does not conduct and reset pin 4 getspositive supply though resistor R3. IC1

(NE555) starts producing squarewave and tubelight turns on viaferrite transformer X1.

In the charger section, inputAC mains is stepped down bytransformer X2 to deliver 9V-0-9V AC at 500 mA. Diodes D1and D2 rectify the output of thetransformer. Capacitors C3 andC4 act as filters to eliminateripples. The unregulated DCvoltage is fed to IC LM317 (IC2).By adjusting preset VR1, theoutput voltage can be adjustedto deliver the charging voltage.

When the battery getscharged above 6.8V, zener diodeZD1 conducts and regulator IC2stops delivering the chargingvoltage.

Assemble the circuit on ageneral-purpose PCB and en-close in a cabinet with enoughspace for the battery andswitches. Connect a 230V ACpower plug to feed chargingvoltage to the battery andmake a 20W tube outlet in thecabinet to switch on thetubelight.

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)