@Robin thank you so much for your response.
I completely agree that using a transitor would be the right way to go. It's also what @allObjects is suggesting (thanks!). If I understand correctly I would connect G to the ground, S to the connector on intercom I want to ground and D to one of GPIO of MDBT42Q. Than I would control it with a code like
analogWrite(TRANSITOR_D, 1) to close circuit and
analogWrite(TRANSITOR_D, 0) to brake it again. Is my understanding correct?
The only problem I have with this approach that I don't have a transitor at hand :) and if I order one online I will have to put my project to a shelf for a week or two until it's delivered.
I measured the voltage between the circuit I need to short using universal meter. They show 0.6V of difference. And it really looks like unlocking is just triggered by grounding this wire. If I understand correctly this means "pull-down" signal, right?
I reviewed the tutorial with the loop and detecting when it gets grounded. I guess it's slightly different than what I need. The tutorial detects that the voltage has dropped. However in my case I need to cause the voltage to drop and the voltage is supplied by the external source (transformer coil of the intercom). Can you please elaborate on how I could cause a voltage drop on a wire connected to GPIO?
Do you have resistors in the 5K to 10K to 50K range to use as a jumper to test?
I don't. But I do have a universal meter. What should I test for?
Btw, you've probably already noticed I have very little knowledge about microelectronics. I write software for living. This is my toy project to learn something outside my field of expertise.
Thank you again for helping me out.