Light Operated Switch is a very commonly used circuit to control outside lighting or used for driveways and garage entrances. Light Operated Switch is very simple and inexpensive design, which uses timer IC555 (IC1) and Light Dependent Resistor or LDR as a sensor. LDR has unique feature that in presence of light it has very low resistance. However, when light falls on it is interrupted, its resistance increases to a very large value. This feature is used to trigger IC1, which operates in monostable mode.
About the Circuit :
In presence of light, LDR or R3 as shown in the circuit diagram has very low resistance. Hence, voltage at trigger pin2 of IC1 is above 1/3Vcc. In this case, the output of IC1 at pin3 remains low and relay is not energized.
As soon as light falls on LDR gets obstructed due to the movement of some objects, the voltage at pin2 goes below 1/3Vcc. At this moment, IC1 is triggered and the circuit is switched on. The output at pin3 becomes high and relay is energized. The output remains high as long as light is obstructed.

Here diodes D3 and D4 are used as a rectifier diode and capacitor C3 as a filter.
Calibration :
The preset VR1 is used to adjust the sensitivity of the circuit. Initially make lights off on LDR and slightly go on adjusting VR1, till the circuit is switched on.

Light Operated Switch is a very commonly used circuit to control outside lighting or used for driveways and garage entrances. Light Operated Switch is very simple and inexpensive design, which uses timer IC555 (IC1) and Light Dependent Resistor or LDR as a sensor. LDR has unique feature that in presence of light it has very low resistance. However, when light falls on it is interrupted, its resistance increases to a very large value. This feature is used to trigger IC1, which operates in monostable mode.
About the Circuit :
In presence of light, LDR or R3 as shown in the circuit diagram has very low resistance. Hence, voltage at trigger pin2 of IC1 is above 1/3Vcc. In this case, the output of IC1 at pin3 remains low and relay is not energized.
As soon as light falls on LDR gets obstructed due to the movement of some objects, the voltage at pin2 goes below 1/3Vcc. At this moment, IC1 is triggered and the circuit is switched on. The output at pin3 becomes high and relay is energized. The output remains high as long as light is obstructed.

Here diodes D3 and D4 are used as a rectifier diode and capacitor C3 as a filter.
Calibration :
The preset VR1 is used to adjust the sensitivity of the circuit. Initially make lights off on LDR and slightly go on adjusting VR1, till the circuit is switched on.