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Telephone Intercom with 'Proper' Ring

Discussion in 'Electronic Design' started by [email protected], Sep 21, 2005.

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  1. Guest


    I've been looking for a schematic of a telephone intercom that will use
    a pair of phones, and deliver a proper ring voltage to the phone being
    called (If I understand correctly, somewhere around 100V, 20Hz AC. I
    would prefer if it also gives a dial/ring tone to the phone making the

    So far, what I've found are simple schematics that use a buzzer instead
    of a ring signal, or line simulator schematics (such as this: ), which
    seem to have too many features, and are too complex.

    At the same time, I've found circuits to simply ring a phone, like this
    pair (

    Here are my questions:

    1) Does anyone have a circuit that will already do what I am looking
    2) Could (and how would) I combine one of those ringer circuits with a
    simple intercom circuit?
    3) If I were to put the 20Hz AC signal into both the lifted and
    unlifted phones at the same time, would it ring on the unlifted phone,
    and give a dial tone on the other?

    I should mention that I am trying to get this to work with 'old-style'
    bell phones, so I would need to use a 20Hz signal, not 60 Hz, which I
    have read works with new phones.


  2. Ken Taylor

    Ken Taylor Guest

    If you want dial tone as well, that is completely separate from the ring
    tone generating. Unless you're dead keen on building your own, buy a cheap
    PABX on Ebay or at a surplus store.

  3. Ori

    Ori Guest

    Hi Ken,

    Dial tone would be cool, but it's not absolutely necessary... and from
    what I've found on EBay, a PABX is a bit on the pricey side (the ones
    that have a low price come with huge shipping fees).

    What if I were to take a simple phone-intercom schematic ( ),
    the kind with a buzzer, and replace the buzzer with a relay that
    connects the phone to a ringer circuit (perhaps just have one, with
    it's outputs running along a third and fourth conductor, making use of
    the other lines in a phone cord) in parallel. Would that work? When the
    ringer circuit output is connected to the phone, could it cause the
    phone to appear "off the hook" prematurely?


  4. Rich Grise

    Rich Grise Guest

    Here - read this:

    Good Luck!
  5. Ken Taylor

    Ken Taylor Guest

    I'd be surprised if you couldn't find a 2 line by 8 extension PABX surplus
    for less than US$25, but as I have no idea where you are I'll take your word
    on it. :)

    Your idea about running the ringing volts around separately are valid,
    particularly if you are using an old style phone which you can hack into to
    separate out the bell wiring. If you're using a modern electronic phone it's
    not as easy though. There is a fair amount of telephone related circuits at
    QSL ( circuits/telephone_circuits.htm )
    which may be handy.


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