but can only seem to get the 3.3V output from a GPIO pin to pass through to the emitter.
I believe the issue is the transistors I have on-hand (TIP120 NPN) should only switch the ground leg but I'm such an amateur at this I truly don't know
Would a PNP transistor be better?
I am not sure what that Green board is. It has no part number on it and it is not obvious what it is, so more detail needed please. Also what are you trying to do?
Why not?That AC/DC "GND" is different, isolated, from the other.