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Light Beam Controlled Remote Switch Circuit

Updated on August 31, 2013


Switching on and off of electrical appliances by a remote control can be fun besides being safe and convenient. Since there need not be any physical contact between the operator and the appliance, the hazard of any electrical shock is eliminated by use of this switch. The “magic’ toggling switch described here is light beam controlled.

By showing a light beam the electronic switch is made to toggle on and off alternately. Therefore this switch can operate just about anything which can be switched on or off. It may be used even to changeover from one gadget to another for example, from radio to tape recorder or from a bright lamp to a night lamp. The circuit is simple and straightforward, and uses a single IC. It needs a regulated supply of + 5V to power the IC.

Circuit Description

The circuit may be powered with the 15V DC supply (shown as V1 in Fig. 2) Transformer X1 steps down the AC mains voltage to a lower value and the bridge rectifier D1- D4 rectifies it. Capacitor C1 filters ripple in supply voltage. The unregulated voltage V2 may be used directly for driving the relay. Phototransistor T4 is used here as a light controlled resistor.

It acts as a low resistance when bright light falls on it and as a high resistance under darker conditions. T4 and P1 form a potential divider, the voltage at whose junction varies in proportion to the light’s intensity. This voltage is smoothed by R2-C2 combination before application to transistor T1 as its base bias.

When there is no light falling on T4, only a negligible base bias is fed to T1, and T1 is cut off. When light beam falls on T4, the base bias voltage increases, Diode D5 provides additional back bias to T1. When the base bias exceeds the limit, then only T1 conducts. Potentiometer Pl sets the light sensitivity of the circuit. The smoothing circuit comprising C2-R2 ensures that the base bias voltage corresponds to the mean value determined over a time period. The switching action of the circuit is thus unaffected by sudden variations in the light level.

Also, the delay provided by R2-C2 combination prevents multiple triggering from a slowly changing light source. Thus this delay feature is very important for the circuit. When T1 is off, T2 is also off. When T1 conducts, T2 gets heavily forward biased and gets saturated so that about 4 volts appear at its collector. Therefore whenever light beam falls on T4, T2‘s output goes “high’ or to logic low condition, and as soon as the light beam is removed, T2’s output goes ‘low’ or to logic ‘0’ condition.

This output is fed to IC1 which a digital TTL logic integrated circuit is containing dual JK master slave flip-flop whose one part only is utilized here. JK flip-flop is connected as a toggling circuit. J and K inputs are tied to logic and the input is given to the clock. IC1 responds to the negative transitions of the pulse, i.e. whenever the light beam is focused and removed it switches once. Thus ICI gives output of logic 1 or logic 0 alternately and drives the base of T3, which energizes the relay. Therefore` the relay is made to close or open its contacts by alternate light beams.

Useful Hints

1. Use a 4V, 20mA relay so that the complete circuit can be powered from any single 5V regulated power supply.

2. Use any readily available relay and correspondingly adjust the unregulated voltage V2 in Fig. 2.

3. IC1 needs a precise voltage, strictly within the limits of 4.5 and 5.5V. Therefore utmost care is to be taken to use proper voltage and regulation. In Fig. 2, zener voltage should be within the limits under all load conditions. Better to use a low tolerance zener.

4. A photocell (LDR) may be substituted in place of T4 to give better sensitivity and longer distance remote control.

5. Normal flashlight or torchlight can be used for remote controlling. There is no need of any special light beam.

6. Make a light shield around T4, using a thick black paper, to prevent random ambient light from affecting it.

7. The preferred direction of incident light is perpendicular to the plane containing the emitter and collector leads and is on the collector side of the transistor’s crystal.

8. Use about 10-kilohm resistance instead of P1 for nominal sensitivity. If necessary, test selects it. If it is required to control the sensitivity, use a 20-kilohm potentiometer. But do not set the potentiometer to its minimum any time, or you will damage transistor T4. To prevent this limitation, use a fixed resistor of about 2 kilohm in series with the potentiometer Pl.

9. Relay with multiple contacts can be useful for complex functions and simultaneous switching.

10. The output is taken at pin 12 of the IC in this circuit. For reverse functions, use the output at the IC’s pin 13. Both the outputs can also be used simultaneously with independent output circuits.

Parts List

IC1 : 14-pin DIP plastic integrated
circuit SMC 7473N (SEM)
T1 : BC108c transistor (Hindustan)
T2 : BC177 transistor (BEL).
T3 : BC108c transistor (Hindustan)
T4 : AC132 phototransistor (BEL).
D1-D4 : DR·50 diodes (BEL), or any
rectifier bridge with 200mA rating.
D5,D6 : CD28 diode (CDIL).
R1 : 120-ohm, 3W resistor.
R2, R3, R4, R8 : 5 kilohm, 1/2W resistor.
R5 : 510-ohm, 1/2W resistor.
R6, R7 : 910 kilo Ohm, 1/2W resistor.
C1 :l000p.F, 25V electrolytic capacitor
C2 : 100pF, GV electrolytic capacitor
P1 : 20-kilohm potentiometer (trimpot).
R : Relay 9V, 20mA, with changeover contacts
having 3A rating
X1 : Stepdown transformer, 230Vto 12V,
250mA secondary rating.
Z1 1 5V, 1W zener,


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