Troubleshooting CM8870 DTMF Decoder Circuit

Discussion in 'The Projects Forum' started by techadiya, Jan 11, 2013.

  1. techadiya

    Thread Starter New Member

    Jan 4, 2013
    3
    0
    I'm trying to build a DTMF decoder using the CM8870 DTMF decoder IC. I made the project on a breadboard. But the decoding never works!! On powerup, it displays a random BCD output on it's 5 output pins. And then that Outputs stays high till I turn the power off. I used the exact circuit here + a few more decoupling capacitors too.

    [​IMG]

    I'm dead sure about the connections of the circuit. And the clock ( 3.579545 MHz ) is working fine too.. What maybe the problem here? Anybody could help?
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The circuit looks fine, as it is the same one taken straight from the data sheet.

    You can do the following checks:

    a. check that pin 4 is approximately half the supply voltage
    b. increase the resistor between pin#2 and pin#3(not the one connects to 100nF) from 100K to 220k, 330K or 470K. This increases the input gain from 1 to 2.2, 3.3 or 4.7 to suit your input signal strength.
     
  3. techadiya

    Thread Starter New Member

    Jan 4, 2013
    3
    0
    Thank You! Increasing the value of GS resistor solved the problem.. :) Now it's a 300K resistor.. :) :)
     
  4. suranisahil786

    New Member

    Mar 11, 2013
    1
    0
    techadiya can you please send me encode circuit of dtmf at <SNIP>
     
    Last edited by a moderator: Aug 23, 2013
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    Why not share it with everyone else!
     
    Last edited by a moderator: Aug 23, 2013
  6. sagarjagyasi

    New Member

    Aug 23, 2013
    1
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    techadiya can you please send me encode circuit of dtmf at <SNIP>
     
    Last edited by a moderator: Aug 23, 2013
  7. rogs

    Active Member

    Aug 28, 2009
    279
    37
  8. techadiya

    Thread Starter New Member

    Jan 4, 2013
    3
    0
  9. gauravpwr8

    New Member

    Feb 13, 2014
    1
    0
    techadiya can you please send me the working circuit for dtmf decoding with all the issues resolved.
     
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