Thanks a lot, Dennis. I’ve been trying this, unfortunately this proved to be a dead end. I made the same conlusion, that 4 volt should be enough for Opener to recognize a ring signal on the yellow wire. But not. Of course I tried many other wiring combinations, to no avail.
The problem might be that the 4 volt spike is quite short, actualy not even easily read on multimeter. Might be closer to 3,5 volt.
This has lead me to the idea of the amplification of the signal, e.g. by including a relay in the setup. Thus my question here Elcom BFT-210 Nuki Opener working with hardware mod - #5 by martin.dolan. I’m really not a pro, so the question might sound dumb. It was based on an assumption that relay is a relatively simple, purely “analog” component. But a “relay module” is not the relay itself, I see.
I’m feeling I’m close. But the last step to the success I may never reach…