A while ago, I made a simple yet expansible dual-channel firecracker/rocket igniter using an Arduino ... diagram of one of its channels is shown below (the other channel is identical). A few more ...

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 ...