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Couple DTMF signal to phone lines

Discussion in 'Electronic Design' started by eeh, Oct 27, 2005.

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  1. eeh

    eeh Guest

    Hi,

    Could anyone give me a schematic example how to couple the DTMF signal
    to the telelphone lines so that I can dial out?
     
  2. Get a cheap phone, and use that. It gives you the touch-tone generator,
    and it's already coupled to the phone line.

    Failing that, you look at the datasheet for the DTMF generator, and
    likely that includes how to connect it to the phone line.

    Michael
     
  3. DaveM

    DaveM Guest

    The simplest (but not recommended) method is to connect the DTMF tones
    through a 600:600 ohm transformer connected directly across the phone lines.
    The transformer will provide a DC path so you can draw a dial tone.
    But that doesn't allow for the switching and audio Tx/Rx if you want to talk
    and listen on it too.

    But, you didn't ask about that ... . you're only interested in the DTMF
    tones.

    Cheers!!!!!!!!!!
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
     
  4. eeh

    eeh Guest

    If I do not want to use transformer, is it OK to just use capacitor?
     
  5. Tim Wescott

    Tim Wescott Guest

    What are you trying to accomplish by sending DTMF? Is this part of a
    larger system that you're trying to make work, or do you just want to
    make noise on the line?
     
  6. Pooh Bear

    Pooh Bear Guest

    Not really. It may upset the line's performance.

    You're not really meant to couple ground referenced signals into phone
    lines. Consider also what happens if the line connection gets reversed.

    Graham
     
  7. Spajky

    Spajky Guest

    IMHO no if only using that, because a telefon line needs a DC load
    (resistort o transformer of certain DC resistance) to switch to
    outgoing line (from 48 drops to 8Vdc) before it can accept DTFM tones
    for dialing ..
     
  8. Rich Grise

    Rich Grise Guest

    Maybe he wants to put it alongside some phone that doesn't have
    buttons or something, albeit I can't imagine where you'd find
    something like that these days. ;-)

    Anyway, if that's the case, couldn't he use a pair of, say,
    ..1 or 1 uF caps from something with a differential output?
    (the phone will be providing the DC path - it'd be a little
    loud in the earpice, though. :) )

    Cheers!
    Rich
     
  9. If you want that sort of thing, then you feed the touch-tone generator
    into a small amplifier, and then feed a speaker, which can be held
    up to the phone's microphone for dealing with the menus.

    Michael
     
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