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This is a very standard door entry system voice unit that is often found in the main door button panel of door entry systems in the UK. The Bell System units are the most popular in the UK, and when you look at the circuitry in them it becomes clear why. The design is simple, but clever and has good protection against short circuits and installer error.
This module is the main part that deals with amplifying audio from the built in visitors microphone and also the microphone output from each flat/apartment's phone.
There is a separate video showing the circuitry in the version 5 phone here:-
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12 thoughts on “Inside a door entry system voice unit”
  1. Avataaar/Circle Created with python_avatars Okurka says:

    Why are you filming these videos on the floor?

  2. Avataaar/Circle Created with python_avatars Revee Orr says:

    I just watched kinder surprise

  3. Avataaar/Circle Created with python_avatars Weißenschenkel says:

    Clive! Maybe it would be interesting to have a video talking about speaker's impedance at some point in future, why are there 8-4-2 Ohm and things like 6 Ohm (It's a Sony.)

    i didn't realize that intercoms could have such high impedance speakers. Thanks for the video!

  4. Avataaar/Circle Created with python_avatars ChrisGrossmanDOTcom says:

    The resistor across the zener diode keeps the regulator circuit working (with a lower output voltage) when the input power falls below a value that will turn-on the zener. It keeps some voltage drop across the base-collector junction of the pass transistor so it still has headroom to smooth the incoming power. The capacitor to ground at the common node will help remove hum. It becomes a capacitance multiplier instead of a zener reference for low input voltages.

    It will keep the unit working even with a poor power source.

    It is a good design as you stated.

  5. Avataaar/Circle Created with python_avatars davba2 says:

    Very interesting. It’s great to see you cover audio gear. That’s my ‘thing’.

  6. Avataaar/Circle Created with python_avatars Brendan Keyport says:

    T – Transmit, R – Receive, O – Open, C – Common, H – Hot. Z would likely be the return, and I is "Indicate" for the ringers/flasher.. (flasher for the deaf)

  7. Avataaar/Circle Created with python_avatars Tom Cee says:

    The Ford and 70 on resisters to and from the long wire audio bus provide a medium impedance to minimize noise pick up and drive long wires without significant loss. We’re you to drive the speaker directly into the wire it would be an 8 to 16 ohm output and 100 feet of wire would potentially lose signal or gain noise.
    This is very similar to standard wired intercoms with an additional wire to trigger the door solenoid. These haven’t changed significantly since the 1960s or before, except to replace transistor amplifiers with ICs. Prior to that would have used a centralized tube amp and possibly a single speaker switched to act as the microphone.

  8. Avataaar/Circle Created with python_avatars william romine says:

    It reminds me of the phones built by Western Electric for the Bell system. Back then, you got your phone from the phone company. They were hard wired to the system. Bell wanted the phone to last forever, since a service man would have to come to your house to replace the phone. Those phones would probably still work, unlike smart phones that aren't any good in a few years now.

  9. Avataaar/Circle Created with python_avatars twocvbloke says:

    They do inded make a 7809, I converted an old 90s car stereo from a 9v portable CD player power output to a 5v output for regular USB power simply by removing the 7809 and swapping in a 7805, not as common a voltage these days, but it's definitely a part that's out there… 🙂

  10. Avataaar/Circle Created with python_avatars Mark Clark says:

    Tip & Ring

  11. Avataaar/Circle Created with python_avatars coondogtheman1234 says:

    I would so add a line input on this thing. Curious if the circuit can be modded to output stereo audio since the amplifier chip is a stereo amp. I did that once to a gameboy advance light and amp device but the audio circuit was messed up. A couple of modifications and I now have a nice stereo amplifier, just need a case for it.

  12. Avataaar/Circle Created with python_avatars CanizaM says:

    The designers really missed the opportunity to make the terminals say TORCH.

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