Circuit Description

The circuit diagram of light activated switch circuit has been described here. The core part of this circuit is the operational amplifier IC LM 311which acts as a voltage comparator here. This circuit also consists of an LDR which senses light intensity. A transistor Q1 (BC 107) is included in this circuit for driving the relay.

The non-inverting terminal of comparator is given with 6 V fixed voltage by using resistors R3 and R4 (10K). The voltage at the inverting terminal is the voltage across LDR which is variable according to the intensity of light falling on it. When light intensity is low (darkness), the resistance of LDR will be high and hence the voltage across it. Then the output of comparator will become low and relay becomes inactive. If light intensity increases, the resistance of LDR decreases. As a result of this comparator output will become high and it turns on relay. So relay switches according to the light intensity and which is the desired output of this circuit.

Circuit Diagram

Light Activated Switch

Light Activated Switch


  • Use a good quality PCB to assemble this circuit.
  • Adjust potentiometer R1 to set the desired light intensity for switching the relay.
  • Use a 12 V battery or 12V DC regulated power supply for powering this circuit.
  • The chance of oscillation in this circuit can be minimized by shorting the pins 5 and 6 of IC LM 311.