Received the wrong kind (12V instead of 5v). But it looks like these can still be driven from 5V/3V, it just needs a 12V supply to drive the relay. They use opto couplers to drive a transistor that drives the relay current from the provided voltage JD-VCC voltage. https://www.sunfounder.com/wiki/index.php?title=4_Channel_5V_Relay_Module library://4_channel_relay_schematic.png md5://f1281597e5e8a8b0b721f9bc3808caf5 library://4_channel_relay_PCB.png md5://d1444c443d1b708481cd635566f3b6e6 So it's quite easy to change the driving voltage by connecting VCC to e.g. 3V, and JD-VCC to 5V, 12V, ... This schematic is inverting and corresponds to the 12V boards (marked HW-316). The first one I had (unmarked) is non-inverting. It has IN-R-OPTO-LED-GND on the input side.