0 Light Activated Switch

During night time, LDR (light dependent resistor) resistance is high, making the circuit off. On the other hand, during day time, the relay is energized. Potentiometer POT1 controls the sensitivity of the circuit to light intensity. Increasing the resistance of POT1 helps the circuit trigger even in less lighted environment, while decreasing the potentiometer resistance decreases the sensitivity.

light controlled switch schematic

Part List:
R1, R2, R3 - 10 kilo ohms 1/4W
R4, R5 - 1.2 kilo ohms 1/4W
POT1 - 100 kilo ohms potentiometer
LDR - any LDR
C1 - 470uF electrolytic capacitor rated 16V
C2 - 10nF Mylar capacitor
D1 - 1N4148, 1N4001 or similar diode
Q1 - 2N2222, 9013, or similar NPN transistor
IC - LM311 or similar comparator IC

2N2222 or 9013 pinout

lm311pins

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Light activated Relay