DTMF IC Test_failure

Discussion in 'General Electronics Chat' started by vinodquilon, Jan 14, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    0
    Pls find the attachment.
    After setting the circuit (using mt8870) , when I switch on the circuit LEDs 1,2,3, & 4 are all suddenly ON without detecting ring volt and key press. Does this mean the IC is out-of-work. I have only one IC, otherwise I would have gone for IC replacement. ​
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The MT8870 output will be unknown on power up.

    Would pressing any keys on the phone pad changes the LEDs?
     
  3. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    No, no state change for ON LEDs after pressing keys.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    What is the audio level of the dtmf tones?
    The datasheet claims it must be round -25dBm.
    See the datasheet for more details.

    Greetings,
    Bertus
     
  5. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    But the audio level is determined by the phone from which we generate key tones, I think all over the world it follows a typical level value and it can be easily identified by the DTMF decoder.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Is the circuit connected to the phone line?
    The phone itself get the power from this line.
    Without connecting the circuit to the phone line no dtmf tones can be generatted.

    Greetings,
    Bertus
     
  7. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    Telephone line directly connect to the circuit and a parallel local telephone connection is also taken as in figure.
     
  8. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    well done!!!!!!!
    Ya, I have replaced with another vendors' IC (MT8870CE). Now it is working fine.But on

    power-up all states are HIGH, then on pressing keys it obeys the actual Truth table. I think

    it wouldn't be a bottleneck for my project as I feed the DTMF IC outputs to a decoder. I can

    utilize the decoder outs in any way as I desired.
    Thanks for all support by
    Siong Boon
    http://www.siongboon.com/projects/2005-07-18_dtmf_circuits/
     
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