[Glitch] got his hands on a slew of relays which are meant for use in industrial equipment. They are designed to operate at 24V. He wanted to use these with common microcontrollers and instead of buying a driver he designed and built his own.
There’s a few things to consider with a project like this. You need a power source, a way to level convert the driver pins, and some protection in case something goes wrong with the circuit. Looking at the board above should give you some idea of what’s going on. There’s a big transformer taking up half of the footprint. This steps down mains voltage to something a 7824 regulator can handle. That’s a 24V linear regulator which is fed by a bridge rectifier along with some smoothing capacitors. With the source taken care of [Glitch] uses an optoisolator for both protection and level conversion. After working the bugs out of the design he was able to control the relay using 3.3V, 5V, or 12V.