# 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.

– 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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