Lights and Display Board Circuits

Light activated switch circuit

Here is a circuit diagram illustrating a light-activated switch utilizing the LM311 voltage comparator IC from National Semiconductors and a Light Dependent Resistor (LDR).

The circuit revolves around IC1 and operates on the principles of a voltage comparator circuit.

To provide a reference voltage of 6V to the non-inverting input of IC1, resistors R3 and R4 are employed. The voltage across the LDR, which varies with the surrounding light conditions, serves as the input for the inverting input. In dark conditions, the LDR exhibits high resistance, resulting in a higher voltage across it. This makes the voltage at the inverting input exceed the reference voltage at the non-inverting pin, causing the comparator’s output to be low (0V).

When the LDR is exposed to light, its resistance decreases along with the voltage across it. Consequently, the voltage at the inverting input falls below that of the non-inverting input, causing the comparator’s output to go high (12V). This, in turn, activates transistor Q1 and triggers the relay. Therefore, the intensity of light falling on the LDR directly influences the operation of the light-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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