analog 2 wire door phone: how does it work? - Page 1
Forum section: electronics / projects

author: manu
author_rank: Regular Contributor
timestamp: February 24, 2016, 02:06:09 pm
content: Hello,I own a house with an analog door phone which probably comes from the 80's.I'm amazed by the simplicty of the thing:from the outside a simple 2-wire interface allows a mic+speaker+call button to ring the headset into the house.How does it possible to mix a two-way communication and remote powering the outdoor unit with only 2 wires?What I've figured out:the indoor unit consists in a headset and is powered from the mains ac.2 wires goes to the outdoor unit (and another pair of cables drives a dry-contact to unlock the door).The outdoor unit consists in a call button, a mic and a speaker with a 2-wire interface.There is not much in it: a diode bridge, a transformer, 2 transistors, a led and a few passives.So it must be the indoor unit that handles the "smart" job, i.e. detect a call when the push-button is pressed, and establish a two-way communication.Any ideas how this thing work?

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 24, 2016, 02:19:56 pm
content: Well, like a regular telephone I presume?

author: manu
author_rank: Regular Contributor
timestamp: February 24, 2016, 04:20:44 pm
content: Quote from: Ice-Tea on February 24, 2016, 02:19:56 pmMore or less... if you have some documentation about the subject (with schematic).I understand the principles, I would like to know a little more about the details (i.e. what is the minimum to make it work, ...) before reverse engineering it if I really have to.

author: helius
author_rank: Super Contributor
timestamp: February 24, 2016, 05:43:05 pm
content: A hybrid transformer adapts between the two-wires in the cable and separate send and receive circuits in the two units. The button can just close a circuit, with larger systems there are oscillators to produce tones for different buttons. Power is supplied by DC component on the cable.

author: manu
author_rank: Regular Contributor
timestamp: February 24, 2016, 09:02:31 pm
content: Quote from: helius on February 24, 2016, 05:43:05 pmThank you, I will check that. As I've dismantled it, it will be easier to probe it and verify if the small transformer is used to mix signal or if it's a power supply combined with the four diodes to make dc from some alternative signal.My first guess was that the outdoor unit was phantom powered by the signal pair but it doesn't seem to make much sense.

author: dom0
author_rank: Super Contributor
timestamp: February 25, 2016, 03:59:59 pm
content: The trick about the differential transformer how it is used in a telephone is, that it subtracts the signal you are "sending" from the signal on the line, so that the signal you are receiving is just what the other end sends. In theory ; but even in practice it works well enough and attenuates your echo by some 20 dB or so. Which is considered a good thing, because people are irritated when there is no echo at all (or something).

author: manu
author_rank: Regular Contributor
timestamp: February 26, 2016, 10:30:40 am
content: Ok. Thanks.

author: Cerebus
author_rank: Super Contributor
timestamp: February 27, 2016, 02:15:54 am
content: Quote from: dom0 on February 25, 2016, 03:59:59 pmWhat you're calling 'echo' is called sidetone in the telephone industry and is quite different from an echo proper. I.e. an echo is delayed, sidetone is not. When people can't hear sidetone it's uncomfortable - ideally sidetone should be at the same physical level as you'd hear of you were just talking without a telephone clamped to your ear. If people can hear their own voice echoing back from the remote end of the line and if the delay is over about 60 ms it becomes almost impossible to talk - the delayed echo of the speaker's own speech just seems to scramble the brain. Thus there is a further class of circuits that you'll find in telephone exchanges - echo cancellers - that actively work to remove any reflected signals from the far end of the line. Obviously that's not necessary on something as short distance as an intercom.

author: helius
author_rank: Super Contributor
timestamp: February 27, 2016, 09:09:40 am
content: Of course today most calls don't happen on traditional telephone handsets; people use flat slabs of glass or in-ear transceivers. The acoustic transfer function of speech back to the speaker's own ear is different in each case due to the different amount of blocking by various equipment. It would be interesting to know whether any smartphone emits sidetone.