Lights and Display Board Circuits

Light activated switch circuit

This is a circuit diagram for a light-activated switch using the comparator IC LM 311 from National Semiconductors and an LDR.

The circuit is built around IC 1 and is based on a voltage comparator circuit.

Using resistors R3 and R4, the non inverting in put of IC1 is provided with a reference voltage of 6V. The voltage across the light-dependent LDR will be used as the input to the inverting input. The resistance of the LDR will be high in the dark, as will the voltage across it.The voltage at the inverting input will be higher than the reference voltage at the non inverting pin, and the comparator’s output will be low (o V). When the LDR is turned on, the resistance of the device decreases, as does the voltage across it. The voltage at the inverting input is now lower than at the non-inverting input, and the comparator’s output is high (12 V). This turns on transistor Q1 and activates the relay. As a result, the intensity of the light shining on the LDR causes the relay tolight-activated-switch-circuit .

Light Activated Switch Circuit Diagram with Parts List .

Light activated
Light Activated Switch Circuit Diagram


  • To select the required light intensity for switching the relay, adjust POT R1. To do so, shine a light of the desired intensity on the LDR. Either the relay will be on or off. Slowly adjust POT R1 until the status of the relay changes. That is all there is to it. The circuit is now configured for the specified light intensity.
  • Assemble the circuit on a high-quality PCB or a standard circuit board.
  • A 12 V battery or a well-regulated and filtered 12V DC mains-operated power supply can be used.
  • To reduce the probability of oscillations, pins 5&6 (Balance & Balance/Strobe ) of IC LM311 are shorted.
  • Along with the circuit diagram, the pin out of the LM311 is also provided.

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