GND to GND That’s the only other thing an mBuino or similar should need.
It’s possible the GNDs on your power supply are isolated from each other, so 3v may still be only 2v when referenced to the other GND. If you connect both GNDs on your power supply and then use the 3v to trigger it, it should operate.
Agreed, although I haven’t RE’d the circuit to see what the opto is really doing . My actual desire was for 3v3 relay operation so I didn’t need to worry about multiple power supply issues (in fact, I was most worried about ground reference issues in my scenario because I didn’t have control of one side of the device) and the opto wasn’t a feature I cared about.
It’s really only doing the same as the BC517 in the circuit that njbuch is using except in this case you are lighting an LED to switch the transistor. Because the GROUND and the VCC are common, it serves no isolation purpose whatsoever.
In fact, with use of dual supplies, the opto-isolator is a good idea as then you don’t need to have a common ground. You can keep them completely isolated and it will work fine.