Help interfacing DTMF 8870 decoder with an audio source

Discussion in 'General Electronics Chat' started by ymg200, Aug 26, 2017.

  1. ymg200

    Thread Starter New Member

    Oct 2, 2015
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    Hi all,
    I'm using DTMF audio decoder based on 8870 chip. It's a breakout board that I bought from Ebay like one of these:
    https://www.ebay.com/sch/i.html?_od...er.TRS1&_nkw=8870+dtmf+audio+decoder&_sacat=0

    The decoder works fine if I connect it instead of a speaker of a desktop phone's receiver. However, the decoder doesn't work if I connect it instead of (or in parallel to) an answering machine's speaker.
    I don't have an oscilloscope and can't see the difference in audio signals between a phone receiver and an answering machine's speaker.
    Can somebody point out why one works and another doesn't?

    Thank you.
     
  2. ymg200

    Thread Starter New Member

    Oct 2, 2015
    17
    0
    I'll try to explain again since there were no answers to my question.....

    DTMF decoder works if I take the audio from desktop phone's handset (remove speaker from the handset and use those leads to take audio).
    However, DTMF decoder doesn't work if I use answering machine instead (remove speaker from the answering machine and use those leads to take audio).
    Is it a difference in voltages between headset speaker voltage and answering machine speaker voltage that fails the DTMF decoder?
    I don't have oscilloscope, only multimeter, and can't really see what's going on with those two audio sources.
    Thanks all.
     
  3. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    There is a big difference in microphone level and speaker level. IF I understand you correctly.

    Have your got a print on the decoder board?
     
  4. ymg200

    Thread Starter New Member

    Oct 2, 2015
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  5. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    Did you get any paperwork at all with the board? A lot will depend on the board input network/circuit.
     
  6. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Read its data sheet and check the permissible audio input level. A phone earpiece should be around -9dBu (0.25V), the answering machine can be 100 times louder at its speaker level, deafening the 8870.

    Try a much lower volume or resistors to attenuate the speaker level; or pick the signal at its volume control wiper instead of at its speaker.
     
  7. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    Did you get some kind of paper like this?


    dtmf-decoder-schematic.gif
     
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