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Analogue DTMF (dial tone) Decoder help

Discussion in 'General Electronics Discussion' started by george2525, Jan 11, 2016.

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  1. george2525


    Jan 30, 2015

    I am interested in making an analogue DTMF decoder that will take the dialling signal from a phone cable and then sort the tones into their corresponding phone number(s)

    All the info I can find uses computers to do this however im aware that analogue methods were used previously. Unfortunately I have found nothing whatsoever regarding this!

    my basic guess is that i would need 8 bandpass filters (there are 8 frequencies used to indicate 16 symbols) which trigger some kind of sample and hold thing that when triggered lights up an 8 seg display with the number corresponding to the filters that allow signal through them.

    like i said theres 0 articles on this unless I have missed something (eg - the term for the old decoders)

    if anyone has any ideas/links about this stuff I would be very happy.

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Would you care to use a dedicated DTMF decoder IC?

    Why would you use a sample and hold circuit? DO you need to store the last coded number? Tell us a bit more about your application. This will help us help you.
  3. AnalogKid


    Jun 10, 2015
    I think he means sampling the decoded output so it is displayed after the tone pulse ends - a data latch.

    An old-school way to do touch-tone is with 8 phase-locked loops. Signetics developed the NE567 for this back in the 70's.

    Page 13, figure 12.

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Sounds plausible - and much simpler than a sample and hold.
  5. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    It is very difficult to read DTMF via a program. It has been done by an absolutely brilliant programmer.
    It is much easier to use a DTMF chip.
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