5V Relay Module - 1 Channel Relay Board for Raspberry Pi with Opto-Isolated High or Low Level Trigger

These relays should be powered by a regular power source, and not by the output of your MCU. The power source can be totally different from the power source for you MCU, but of course the grounds should be connected together. The output of your MCU drives the relay, through an optical isolator, which helps to protect your MCU, because the MCU is only driving an LED. Also, when you disengage a relay, the collapsing magnetic field can create a spike, which can damage your MCU, and you have none of that, since this is optically isolated. If you're working with high voltages, this method will greatly reduce chances of something going wrong, and blowing up your MCU. One nice feature of this relay board, is the fact that you can use an active low, or active high signal to activate the relay.

Ended up stripping a USB cable and using it plugged into an open port in my Raspberry Pi 4. I had the 5v pins taken up with other projects. It worked once i figured out the 3v was not enough to activate the circuit.

These things seems to take more current than expected. Also one of the terminals was nearly unusably tight to screw in/out and I don't think I'll be able to undo it later. They otherwise seem ok.

I use this to switch to a 12V source using a 5V signal in a NO circuit. It’s very small compared to other optocouplers.