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This brand of door entry system is probably the most popular in the UK. It's very simple, but is often installed badly by security companies, so it's useful to understand how it works to fix the legacy problems they create.
I'll cover the main door voice unit and button panel in a separate video. In summary, it's basically a simple power supply and a dual amplifier to amplify the local microphone to the common phone speaker buss and also to amplify the incoming common microphone buss for the local speaker.
This comes across as a well evolved legacy design that is refreshingly simple, but with lots of clever features. The use of a microcontroller to create the ring-tone was a surprise, but a nice one. The PIC microcontroller literally has just three connections. 0V, 5V and audio signal out.
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15 thoughts on “Inside a door entry phone”
  1. Avataaar/Circle Created with python_avatars Paul Ste. Marie says:

    R and T stand for ring and tip, which are the two wires in a standard (at least in the US) phone line. They carry a current loop which is the sum of both signals, and traditionally there are specialized signal transformers at each end to split the signals apart. On a normal phone line, standby has 48VDC across the lines, ring superimposes a 90(?)V square wave on the pair, and in use the voltage drops to about 5–6V if I recall correctly.

    As a trivia point, the names refer to the contacts on a phone plug as used on an old fashioned switchboard.

  2. Avataaar/Circle Created with python_avatars Andy BoBandy says:

    So, if you pick up the handset in your apartment, can you be actively listening in to what's going on in the entryway downstairs?

  3. Avataaar/Circle Created with python_avatars Doug Browning says:

    Here in Canada, these door phones have mostly been replaced by a system that rings your home phone. It requires each tenant to register their phone number (landline or mobile) with building management. Unlocking the door is accomplished by pressing a number (usually 9) while the call is live. Much more secure than the old door phone, but you can't participate without a phone.

  4. Avataaar/Circle Created with python_avatars Napalm Holocaust says:

    I know this isn't a car diagnostic channel, but its just a comment;- so my ecu is no comm and the shop (needed them when I hit a diagnostic brickwall) claims a broken second o2 sensor wire shorted out without leaving a burn or popping any fuses and that roasted the ecu (ecm). I don't buy it.

  5. Avataaar/Circle Created with python_avatars jlucasound says:

    You Dude-lude-lude-lude-loot that so well!! ☎📞Make sure it's not the Land Shark!! 🦈 😜

  6. Avataaar/Circle Created with python_avatars Shaun Stephens says:

    Cheers Clive.

  7. Avataaar/Circle Created with python_avatars Steadfast The Renowned says:

    Sorry I missed that. I was eatting a chip. How does the phone go again?

  8. Avataaar/Circle Created with python_avatars rpavlik1 says:

    It blew my mind when somebody finally pointed out to me that "electret" is to static electricity, as "magnet" is to magnetism: a permanently charged thing.

  9. Avataaar/Circle Created with python_avatars d. t. says:

    Did you steal that example set from the lavatory door? Good luck. 👍

  10. Avataaar/Circle Created with python_avatars FvckYoutube'sCensorshipAndTheirAdBuyers UseAdblock says:

    My building's front door works even simpler: you simply call who you are trying to visit and they meet you at the front door to let you in. Pretty much everyone has a cell phone these days so when the old door entry phones eventually die landlords don't bother fixing them. My building has one but it hasn't worked in 15+ years.

  11. Avataaar/Circle Created with python_avatars Jørn Birkeland says:

    It uses a bus 🚌 ?

  12. Avataaar/Circle Created with python_avatars Phonotical says:

    Should have seen mine utterly stupid inside, for a long time I was able to steal power from it, until security noticed and shut mine off, worst system in the world, I think it was made by cometti, crackly as hell, way too quiet, and a out as secure as a spider on the handle

  13. Avataaar/Circle Created with python_avatars Craig Jensen says:

    I used to have an asshole neighbor who would just tape the button down in his unit and leave the buzzer going so all of his drug customers buddies could come and go as they please. Then the solenoid burned out and the door was locked shut. So the manager just unlocked it with a key and left it open for 3 months until they fixed the buzzer. Also I discovered that if you held the neighbor's button down after they answered, they couldn't disconnect the call so a couple of times I held his button down and blasted an air horn into it until he called the cops and ended up getting busted when they saw drugs. And then the place was livable for a while.

  14. Avataaar/Circle Created with python_avatars twocvbloke says:

    Interesting they call themselves "Bell System" as that's what the US' main telephone monopoly was called (along with American Telephone and Telegraph, aka AT&T) up until they were broken up in the 80s into the various Bell branded franchises…

  15. Avataaar/Circle Created with python_avatars Sulev-Madis Silber says:

    with all those tulululu's he could start recording ringtones for intercomms

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