IC dilemma- Please help on DTMF

Discussion in 'General Electronics Chat' started by vinodquilon, Jan 19, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    Pls find the pdf attachment ('CI-2-DTMF').
    After setting the circuit (using mt8870) in differential configuration, when I switch on the circuit LEDs 1,2,3, & 4 are all suddenly ON without detecting ring volt and key press. And remain in the same ON state after pressing * and then decimal keys from the telephone keypad on caller end.
    So, I am tempted to test in single-ended configuration (see second attached file 'dtmf'). I manually short R18 to accept remote calls. But same problem arises. ​
     
  2. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    That is not a very good circuit. You must never connect an earth to a phone line. Earthing one side of the phone line will introduce noise (lots and lots of noise) which is probably why your DTMF detector is switching on even with no signal input.
    The correct way to connect the phone line to your detector is through a 600Ω/600Ω isolation transformer. Take a look at this site http://www.solorb.com/elect/phone/tap/ to see an acceptable way to interface to a phone line.
     
  3. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    Of course, I didn't connect any ground points to Ring or Tip lines.
    But the LEDs are ON without setting up input section of DTMF IC.
     
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