Darlington Pair Relay Driver

Circuit Diagram

How to Build It

1. Place two BC337 transistors on the breadboard.
2. Connect the first transistor's emitter to the second transistor's base (this is the Darlington connection).
3. Connect both transistors' collectors together, then to one relay terminal.
4. Connect the relay's other terminal to the positive rail (12V).
5. Connect a 1N4007 diode across the relay coil (cathode toward positive) — this protects the transistors from the voltage spike at turn-off.
6. Connect a base resistor from the control signal to the first transistor's base.
7. Connect the second transistor's emitter to GND.
8. Apply a small control signal — the relay should click audibly.

Bill of Materials