raschemmel:
That's messed up. I know it works electronically but they could have at least included a terminal for the relay supply.
Do you mean that had you or I designed it, we would have provided terminals on one side for the isolated control signals from the Arduino, and on the other side for the relay supply?
One suspects that the designers of these things were not really engineers, but someone tasked to turn a circuit diagram (from somewhere else) into a PCB pattern, so that is what they did, not understanding the concepts involved.
Considering all the "terminals" are simply pins, it makes little difference. The "Sainsmart" 2-channel relay module
is arranged in a slightly better fashion but has the misleading superfluous "GND"on the right hand header.