I’ve been trying to understand how my door entry system works.
Between the monitor/base station and the bell/camera/speaker there’s only 4 wires:
- 12V DC
- Voice (2 way)
- Video (1 way)
I opened it up to find a few ICs. One in the base station seemed to be an ASIC specifically for this kind of device, but the datasheet didn’t say anything about ringing/sending unlock command.
Another IC turned out to be a “voice-switched speakerphone”: https://www.profusionplc.com/parts/mc34118g-s28-r
I’m guessing the ‘dial tone’ detector may be something to do with how the base station activates when someone presses the bell button?
I have no idea though how the relay at the remote end can be controlled by the base? There’s a simple two pin header that completes a circuit with the strike, but it’s not clear to me how that gets activated.
So far as I can tell the video line is just composite - though I’ve read some data can be stuffed in a not visible part. I doubt that would be in the base -> remote direction though.