Does anyone know how an old apartment intercom with door unlocker works? When I open it I see 2 wire connected to it.

The way it works is very simple: if someone at the building entrance pushes my name on the button I can hear a buzzer. When I pick up the phone I can talk and I can push a button to release the door.

It seems everything is going through those 2 wires.

Using a voltmeter I can read 13v. And when I push the button it go to 0v.

Any one know more about how this works?

  • That sounds about right. If you're a tenant, it's not for you to alter the building. – Harper - Reinstate Monica Oct 3 '19 at 23:52

There are a lot of varieties of these systems. The more modern ones work over phone lines, and a phone line (POTS) is just two wires.

  • Intercom is actually a small computer with a directory of names
  • Select a name and the intercom dials the phone number associated with that name, hopefully ringing a phone in the correct apartment
  • When you answer the phone and press a button on your phone, the intercom detects that button via DTMF and triggers the latch to open the door (typically an electromagnet).

However, older systems may work differently - e.g., using separate buttons and wires to trigger the door opening mechanism instead of DTMF signaling.

  • I think the one at my apartment is less recent than the one you describe. I think each flat intercom has his own line. Anyway it's seems, as you describe, that signal is sent through phone line since there is only 2 wire. But which is strange, is that, current go from 12v DC to to 0v DC when someone ring or when I unlock the door. So I'm able to reproduce the door entrance by linking 2 wire with a relay, but I'm not able to detect when someone ring. I guess the ring as some kind of signal even if current seems to go to 0v. But what is this signal ? How I can catch it ? – Dragouf Oct 8 '19 at 10:15

At the entrance, there are two components: an electronic strike which can be unlocked remotely by push-button, and a two-way intercom. Each of these needs power and ground to work. When a visitor pushes your button, it activates your intercom. When any resident pushes their remote, it releases the electronic strike to allow a visitor to enter.

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