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Telephone interface pc-modem

Discussion in 'Electronic Design' started by Number 6, Nov 16, 2004.

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  1. Number 6

    Number 6 Guest

    I managed to get the dialling done acoustically by writing a sine-wave
    synthesis program on the PC which responds to buttons pushed like a real
    phone.
    What I did wrong was that I did not raise the second tone (of the dual
    tones used for dialling) by 3dB as somebody suggested.This compensates for
    the role-off of the line freq response.It refuses to work without this. The
    duration is far less critical.

    Thanks for that - now I can dial just by holding the phone next to the PC
    loudspeaker - works perfect!
    However, I wish to dial directly into the line. I have a telephone 1:1
    isolation transformer for this with zeners to block the ringing voltage and
    a series capacitor to block dc - I can hear somebody on the line fine but I
    cannot seem to 'send' a signal down the line at all.At least the signal is
    very weak indeed.
    So to take things step at a time - how do I get my dial tones to feed
    directly into the phone-line? I tried feeding from the PC speaker via the
    transformer but it won't cut the ice.

    I also tried this the easy way via a modem by sending ASCII commands. This
    works too but again I have no way of sending a signal down the line once the
    phone connects - I can dial and it connects at the other end. I can also put
    the phone on and off the hook.What am I doing wrong? Surely the signal
    strength is not that high - or is it?


    Tom
     
  2. Number 6

    Number 6 Guest

    I forgot to show you teh interface circuit. I use two of these - one for
    input and one for output - is there a better way?
    http://www.solorb.com/elect/phone/tap/


    Tom
     
  3. Dbowey

    Dbowey Guest

    Tom (#6) posted:

    I forgot to show you teh interface circuit. I use two of these - one for
    input and one for output - is there a better way?
    http://www.solorb.com/elect/phone/tap/
    Yes, there's a much better way, but be sure to disconnect it from the line when
    you are not using it.

    Remove the 4.7k resistor, and change the cap from 0.22uF to about 2uF. If you
    don't have a 2uF NP cap you can use two 1uF caps in series but with like
    polarities connected together.

    Although this will fix your transmission problems, it may cause the ring to
    trip if you do leave it connected to the line.

    Don
     
  4. Dbowey

    Dbowey Guest

    Abooboo posted:

    "Remove the 4.7k resistor, and change the cap from 0.22uF to about 2uF. If you
    don't have a 2uF NP cap you can use two 1uF caps in series but with like
    polarities connected together."

    Well, that's what I get for not having coffee yet. Make that two 4uF caps in
    series......

    Don
     
  5. Number 6

    Number 6 Guest

    I assume you are changing the time constant - good thinking. Then I will
    have a residual resistance of about 600 ohms (I think that is the resistance
    of the winding) + whatever gets referred back from the primary I expect.

    Tom
     
  6. Dbowey

    Dbowey Guest

    Tom posted:

    I assume you are changing the time constant - good thinking. Then I will
    have a residual resistance of about 600 ohms (I think that is the resistance
    of the winding) + whatever gets referred back from the primary I expect.
    Changing the capacitor just reduces the 1 kHz. reactance from about 1000 Ohms
    to about 100 Ohms, to improve transmission. If your transformer is terminated
    at about 900 Ohms on the PC side, the match to the line will be as good as you
    need.

    Don
     
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