Whenever the intercom button is pressed the switch closes and the cap starts charging. At about 4.1V charge it triggers the op amp which closes the relay, operating the door.
Will this work in real life? I created a similar circuit with two stages of npn and while it worked for a few years, it started to degrade and the delay was reduced from 3s to about 1s.
I hope that by using a smaller cap (maybe even ceramic?) and removing the BJTs the circuit will last longer without deviations from whatever time constant I set.
Also, I've seen similar circuit with a BJT at the output of the op amp. Is there a reason a simple diode will not work? Will the BJT help protect the diode?