I'd like to connect the entry phone of the office I'm working to a PC to let everybody hear who is ringing, reply and open the door without moving from the desk.
The buzzer/door relay part is controlled by Arduino of course, using optocouplers.
The big question is, how to connect the audio lines to the PC sound card without frying it?
I know the buzzer works at 12V AC, but don't know if it's the same on the audio signal. What kind of measurement should I make? Screaming to the phone and take the highest voltage?