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 ...

This was a pretty simple build – four 10 Amp relays and a few terminal blocks. The PCB was designed in Eagle as a single-side board for ease of manufacturing.