LDR Dark-Activated Relay Switch

Circuit Diagram

How to Build It

1. Place the BC547 transistor on the breadboard.
2. Build a voltage divider: the photoresistor from the positive rail to the transistor's base, then a fixed resistor from the transistor's base to GND.
3. Connect the transistor's collector to one relay coil terminal, the other terminal to the positive rail (12V).
4. Connect the transistor's emitter to GND.
5. Connect a 1N4007 diode across the relay coil (cathode toward positive) — mandatory, protects the transistor.
6. Cover the photoresistor with your hand — the relay should click on; uncover it — the relay should release.

Bill of Materials