How Audio and Video Intercom Systems Work | Falcon Security Systems

What is actually happening when the bell rings

An audio intercom is a two-wire circuit: pressing the button closes a contact, the indoor unit rings, and lifting the handset connects a voice path. It is simple, it rarely fails, and it tells you nothing about who is standing there.

A video system adds a camera and a screen, and that changes the wiring requirement because video needs either a dedicated coaxial pair, a four-wire bus, or a network connection. This is why you cannot simply swap an audio handset for a video screen on existing two-wire cabling — the cable is the limit, not the handset.

Wi-Fi door stations sidestep the wiring problem entirely. The door station joins your network, and the "indoor unit" is an app. The trade-off is that the system now depends on your internet and your router: if the connection drops while you are out, you cannot answer. For that reason we usually keep a physical indoor screen as well in family homes, and treat the app as the convenience layer rather than the only way in.

The door release is a separate circuit from the intercom itself. An electric strike needs its own low-voltage supply, and its power draw is what wears out cheap intercom power units — which is why a strike fitted later by a different technician so often causes the intercom to start behaving oddly.