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 ...

The one benefit the reed relay has over an opto: you needn’t know the polarity or really, the type of signal being switched on the output. Whereas polarity would matter with a transistor.

Four solid-state relay devices are involved (all normally open single-pole, single-throw): ... Arduino Software on Arduino Day 2025. Arduino Day 2025: Celebrating 20 Years.