Reverse Engineering A Two-Wire Intercom

There was a time when an intercom was simply a pair of boxes with speakers joined by a couple of wires, with an audio amplifier somewhere in the mix. But intercoms have like everything else joined the digital age, so those two wires now carry a load of other functionality as digital signalling. [Aaron Christophel] installs these devices for a living, and has posted a fascinating reverse engineering video that we’ve also placed below the break.

Power for the system is present as a constant 24V DC, and the audio is still an old-fashioned analogue signal that we’ll all be familiar with. On that 24V DC though are imposed a series of pulse trains to trigger the different alarms and other functions, and he describes extracting these with an oscilloscope before showing us the circuitry he’s used to send and receive pulses with an Arduino. The bulk of the video is then devoted to the software on the Arduino, which you can also find in a GitHub repository.

The result is an interesting primer for anyone who fancies a bit of serial detective work, even if they don’t have a intercom to hand.

6 thoughts on “Reverse Engineering A Two-Wire Intercom

  1. What about 4 wire intercoms? Mine has a power pin for +22V, a 5V “ALARM” pin that cuts on and off, a separate “DATA” pin and a ground. Complicated but with a simple board, probably to make the units cheap. II’ pretty sure that that DATA pin carries both voice and signals.

    Oh, and the thing also lacks a microphone at my end (I can just listen not talk), cheapo cheapo…

  2. This is interesting.
    I have a longstanding project to extract video and audio from my Panasonic video intercom to feed into my Synology NAS so I can record whenever the button is pressed.

    I found the video appears to be composite video transposed onto the 24V supply, but I haven’t looked deep enough yet to find the audio.

    My end goal is to eventually have some sort of adapter that converts to an ethernet jack so it appears on the local network as an IP camera.

  3. I have a Centurion PoloPhone (https://www.centsys.co.za/polophone/) which operates in a very similar way, that I would love to be able to extract the audio from! Although I don’t trust the 2-wire bus to pass the “gate open” signals, because the bus is exposed outside the gate at the intercom panel! So I have run a separate trigger wire to open the gate itself.

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