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autoanswer telephone line

Discussion in 'Electronic Design' started by merco, Jul 12, 2006.

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  1. merco

    merco Guest

    how is it possibile to wait for some rings and then activate the line
    to capture DTMF signals ?
    thank you
  2. OBones

    OBones Guest

  3. merco

    merco Guest

    well, i see...
    but i asked "how" ?

    OBones ha scritto:
  4. OBones

    OBones Guest

    I know, hence why I canceled the message, I missed the "how" part.

    Detecting rings is done with a RC network across the line, like on this
    schematic with R7/C3 (bottom right)

    On this one, you get an optocoupler in the path to interface with low
    level voltage (12V) logic.

    To activate the line, I think it's enough to put a resistor across it
    but can't remember the exact value. And detecting DTMF tones can be done
    with an MT8870 like on this schematic:

    Hope this helps.
  5. merco

    merco Guest

    thanks, the problem i have is how to pick up the line...
    maybe a 600 ohm resistor ?
  6. John Fields

    John Fields Guest

    Yes, that will work.

    The second problem is that you're top posting. Please bottom post.

  7. You need several circuits.

    a. Ring detect circuit

    b. Something to count the rings before answer

    c. something to loop line and draw loop current from line/telephone exchange

    d. something to couple the audio after looping

    e. something to decode the dtmf

    f. something to hangup.

  8. Nico Coesel

    Nico Coesel Guest

    By using a ring detector, a DTMF receiver and a microcontroller.
  9. You mean an "infinity bug"?

  10. Rich Grise

    Rich Grise Guest

    Hack an ordinary answering machine?

    Good Luck!
  11. jasen

    jasen Guest

    a ring detector, monostable, counter, relay, line transfromer,
    DTMF decoder...

    or use a modem. many V90 modems will decode DTMF if you ask them nicely.

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