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Turning the door intercom into a smart intercom (I)

Keeping on with the hard task of turning my home into a smart home, I want to build a VoIP gateway for the door intercom. Using it, if the postman brings an Amazon parcel and rings the intercom I won’t have to rush down two floors of stairs (as my office is in the attic) to answer the intercom before they think there’s nobody at home.

The door intercom is owned and managed by the owners’ community, and uses the classical system of 4+N wires. With this project I want to connect a Raspberry Pi to the intercom so, when receiving a door ring, it redirects it to the DECT phones in the house, allowing us to answer back, and maybe open the door from them.

So far, I’ve bought:

  • One Raspberry Pi 3B+
  • One Automation HAT (I’ll make use of two of its relays and its analog input).
  • One X350 USB sound card for the Pi, in order to receive and send sound from and to the intercom (the Pi has an analog sound output, but no audio inputs)

I plan to install an Asterisk virtual PBX in this Pi and manage everything from there. At this moment, all the telephone systems in the house already use a VoIP gateway, so this Asterisk would be an extra intermediate step for the analog-VoIP gateway I already own, which I’m currently using to connect my DECT wireless phones to my phone-line provider.

So, basically, the system would work this way:

  1. Someone rings the intercom doorbell.
  2. The system detects the call as a voltage increase in the analog input.
  3. The system launches a VoIP phone call to all the telephones in the house.
  4. When someone picks up the phone, the system activates the relay that “picks up” the intercom, and the voice communication is established..
  5. The sound from the VoIP call is transmitted to the intercom through the audio output. The audio input from the intercom is sent through the VoIP call.
  6. If the sharp key (#) is pressed in the phone, the corresponding DTMF is detected and the relay that activates the signal to open the door is closed.

My only current doubt is how to interconnect the audio wires. I suppose I’ll need some extra circuits to connect the input and output audio signals from the sound card with the speaker and microphone connections of the intercom…

Any help about this would be more than welcome.

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