RC Timer / Delay-On Relay

Circuit Diagram

How to Build It

1. Connect a large electrolytic capacitor in series with a resistor, from the positive rail to GND — this is the RC timer.
2. Connect the junction between the resistor and capacitor to a BC337 transistor's base, through a base resistor.
3. Connect the transistor's collector to one relay terminal, the other terminal to the positive rail.
4. Connect a 1N4007 diode across the relay coil.
5. Connect the transistor's emitter to GND.
6. Power the circuit — the relay should activate only after a few seconds, not instantly.

Bill of Materials