Light-Activated Switch

Circuit Diagram

How to Build It

1. Connect the light-dependent resistor (LDR) between +9V and the NPN transistor's base.
2. Connect a 10K resistor from the transistor's base to ground (GND) — this forms a voltage divider with the LDR.
3. Connect the transistor's emitter to ground.
4. Connect the LED (with its current-limiting resistor in series) between the transistor's collector and +9V.
5. Test by covering the LDR with your hand — the LED should turn on when it's dark and off in the light.
6. Adjust the 10K resistor's value if the trigger threshold isn't where you want it.

Bill of Materials