Apr 012015
 

Light activated switch circuit

This light activated switch circuit with LDR and an Operational Amplifier has many applications. It could act as a photocell and 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 volts.

Light activated switch circuit

– When the LDR is not illuminated, its resistance is high and causes the voltage at the non-inverting terminal of the operational amplifier drops below 6 volts. The output voltage of the operational amplifier is high, the transistor Q1 is off and the relay is not active.

– When the LDR is illuminated, the resistance and voltage across its terminals decreases. This causes the voltage at the inverting terminal, of the operational amplifier, increases to more than 6 volts.

The potentiometer P is adjusted to have, under normal conditions, the same ohmic value than the LDR. The battery can be 12 or 9 volt.

List of circuit components

  • 1  741 operational amplifier (IC1)
  • 1  2N3702 PNP transistor or equivalent (Q1)
  • 2  10K resistors (R1, R2)
  • 2  1.2K 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)

Notes:

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