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.

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.

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

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

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.

circuitideas

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

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

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

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

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

Daniyal SyeD

nUMeRiC WaTeR-leVel inDiCaTOR

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

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

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

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

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

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

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

circuitideas

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

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

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

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

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

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

configuration.IC2 (CD4538B) is a dual,

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

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

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

T.K. Hareendran

SecuriTy SySTem SwiTcHer s.c. dwivedi

Fig. 1: Security system switcher

Fig. 2: Proposed cabinet

+5V ADAPTORFOR POWER

SUPPLY

CONNECTORFOR

SECURITYGADGET

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

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

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

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

circuitideas

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

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

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

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

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

Audio amplifier IC LM386N (IC1)

T.K. HAREENDRAN

SPY EAR s.c. dwivedi is designed for operation

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

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

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

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

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

Fig. 2: Compact unit of spy ear

Fig. 1: Circuit for spy ear

circuitideas

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

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

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

while the other hand is free.

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

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

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

Raju R. Baddi

VeRsatile PRoBe

s.c. dwivedi

Fig. 1: Circuit of versatile probe

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

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

Transistors C E X X UnusedpinisbaseAnytypepnpornpn E C X X Unusedpinisbase

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

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

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

Secondaryterminal1 Secondaryterminal2 Glowwith Glowwith BothLEDsglowingwithhighintensity highintensity highintensity Result:Secondaryside

Continuity Connectwith X On Off Indicatesshorting LEDsprobe

Fig. 2: Constructional detail of versatile probe

circuitideas

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

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

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

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

Assemble the circuit on a general-

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

circuitideas

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

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

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

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

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

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

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

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

Riju ThazhaThu VeeTTil

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

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

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

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

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

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

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

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