raschemmel:
What's missing is the Common Ground connection from the 5v ps to the 12V ps to the relay bd GND.
All three should have a COMMON GND.
if the relay board has a power supply and an opto, then I believe a common ground is not required.
the relay coil can be isolated from the control signal by the opto and it definitely isolated from the contacts.
I think that the most of the cheap e-bay relay boards are already tied to share a common ground between the power supply of the signal and the power supply of the coil. simply by the fact that most of the e-bay specials only have one input for voltage and one input for ground and one pin for each signal. ergo, ground is tied together.
the contacts side of the relay has no need to share any electrical connections with either the coil or the signal.
the OP said he has a 5v 2 amp power supply, I am not sure of the 50mA for the relay coil. I would really like to see the data sheet. the typical relay is the Songle and they draw about 100mA and about 0.45w.