The four-channel relay module contains four 5V relays and the associated switching and isolating components, which makes interfacing with a microcontroller easy . There are two terminal blocks with six terminals each, and each block is shared by two relays. The terminals are screw type, which makes connections to mains wiring easy and changeable.
 
The driver circuit for this relay module is slightly different compared to traditional relay driving circuits since there is an optional additional layer of isolation. When the jumper is shorted, the relay and the input share the same VCC, and when it is open, a separate power supply must be provided to the JD-VCC jumper to power the relay coil and optocoupler output.
 
The inputs for this module are active low, meaning that the relay is activated when the signal on the input header is low. This is because the indicator LED and the input of the optocoupler are connected in series to the VCC pin on one end, so the other end must be connected to the ground to enable the current flow. The optocouplers used here are the PCF817, which is a common optocoupler and can also be found in through-hole packaging.