Circuit Diagram
How to Build It
- Place the BC547 transistor on the breadboard.
- 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.
- Connect the transistor's collector to one relay coil terminal, the other terminal to the positive rail (12V).
- Connect the transistor's emitter to GND.
- Connect a 1N4007 diode across the relay coil (cathode toward positive) — mandatory, protects the transistor.
- Cover the photoresistor with your hand — the relay should click on; uncover it — the relay should release.
Bill of Materials
| Product | Code | Qty |
|---|---|---|
| Diode/bridge rectifier | 1N4007-DIO | 1 |
| Relay (12/24VDC/auto) | — | 1 |
| Resistor | DR47K-8/9 | 1 |
| Resistor | DR100R-418 | 1 |
| Transistor (incl. IGBT) | — | 1 |