In this quick Arduino tutorial I will explain how you can control a relay using the Arduino Board, one 1K and one 10K resistors, 1 BC547 transistor, one 6V or 12V relay, one 1N4007 diode and a 12V fan ...

With these mods, this circuit will last quite a long time, especially if you make things easier for the relay by turning off power to the motor before switching directions. It is simple and effective ...