Mar 242023

Light activated switch circuit

This light activated switch circuit with LDR and an Operational Amplifier has many applications. It could act as a photocell, to switch off the light in a room or turn on the radio when it is dawning, etc.

How the Light activated switch circuit works?

The LDR value in ohms varies depending on the amount of incident light upon it. A network of two resistors (R1 and R2) of equal value, causes that the voltage at the non-inverting terminal of the operational amplifier be 6 or 4.5 volts depending on the battery voltage (12 V or 9 V)

When the LDR is not lit, its resistance is high, causing the voltage at the non-inverting terminal of the op amp to drop below 6 / 4.5 volts. The output of the op amp will be high, transistor Q1 will not conduct, the relay will not receive voltage and will not turn on.

Light activated switch circuit with LDR and Op Amp

When the LDR is illuminated, the value of the photoresistor decreases and the voltage between its terminals also decreases. This decrease in voltage causes the voltage at the inverting terminal of the operational to increase beyond 6 / 4.5 volts.

The op amp output goes low causing transistor Q1 to conduct (saturate), the relay receives voltage activating any device such as lights, radio, coffee pot, etc.

LDR (Light Dependent Resistor)

Photoresistor – LDR

The value of the LDR is not critical. The potentiometer P is regulated so that it has the same resistive value as the LDR under normal conditions. The battery can be 12 or 9 volts.

Another light activated circuit you can see is the light operated relay circuit using LDR.

List of circuit components

  • 1 741 operational amplifier (IC1)
  • 1 2N3702 PNP transistor or equivalent (Q1)
  • 2 10 K resistors (R1, R2)
  • 2 1.2 K resistors (R3, R4)
  • 1 potentiometer with a value approximately twice the value of LDR (P)
  • 1 1N4001 semiconductor diode or equivalent (D1)
  • 1 LDR: any value
  • 1 9 volts relay (Bat = 12V) or 6 volts relay (Bat = 9V), with the winding resistance as high as possible, (500 ohms or more)


  • LDR = photoresistor
  • The value of LDR is not critical.

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