Light sensitive switch

Light sensitive switch is a special type of switch which properties depends upon the light falling in it. Here is a simple, inexpensive and easy to use light sensitive switch. The working of this circuit is truly based on light sensing, i.e. automatic turn it on or off when light fall on it.

Circuit Description

As this is a switching circuit so, for more detail we can divide this circuit into two section i.e. power supply and switching circuit.

In this power supply section the work of step-down transformer is done by register R1 and further rectification to change into 10V dc is by zener diode ZD1. The output voltage across zener diode is further filtered by capacitor C1.

Another section is switching section built around light-dependent register LDR1 with the help of operational amplifier IC 741, where LDR is used as sensor of the switching circuit. By changing the position of switch we can made this circuit both light sensitive and dark sensitive (i.e. turn on in light and turn in dark respectively). LDR sense the light and change their resistance corresponding to light and given to pin 2 which is further adjusted by preset VR1. Output is obtained from pin 6 of IC1 and given to base of transistor T1 through resistor R6 where resistor R7 is used as current limiter. The output is from collector of transistor T1 is given to gate of TRIAC1 which further turn on the light.

circuit diagram of light sensitive switch

As light Sensor

By connecting point 1 and 2 of switch SW2 and point 2 and 3 of switch SW1 then the circuit is work as light sensor (i.e. turn on the bulb when light fall on it).

As dark Sensor

By connecting point 2 and 3 of switch SW2 and point 1 and 2 of switch SW1 then the circuit is work as dark sensor (i.e. turn on the bulb in absence of light).


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 100 KΩ/1W

R2, R3 = 100 KΩ

R4 = 4.7 KΩ

R5, R6 = 220 KΩ

R7 = 68 KΩ

R8 = 33 KΩ

VR1 = 100 KΩ (preset)


C1 = 100 µF/16


IC1 = LM741 (Op-amp)

T1 = BC547

TR1 = 10GD (Triac)

D1 – D4 = 1N4004

ZD1 = 10V /100 mW


LDR1 = Light – Dependent – Resistor

B1 = 200W bulb

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