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 ...

[Ben Jones] just started a new site called Maker-Guide, where he makes some very informative and well produced DIY videos on anything from homemade photography hacks, to controlling an outlet using… ...

Even the starter motor, at hundreds of amps, has a solenoid (and usually a fusible link these days). The average auto relay is rated at 30 or 40A, ones for large loads (heated screen etc.) are ~70A.