How to detect that my apartment door buzzer is being pressed
Connect a relay with a 24vAC coil in parallel with the buzzer.  Use a NO (Normally Open) relay and when the buzzer button is depressed, you will have a closed circuit.  If you run a 5v rail from your Arduino into the relay, then you can use an arduino input to read the 5v pulses that will occur when the relay is triggered from the buzzer button.







simulate this circuit &ndash; Schematic created using CircuitLab