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Interfacing with old phone mic/speaker.

Discussion in 'General Electronics Discussion' started by Mitch Patenaude, Jul 27, 2016.

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  1. Mitch Patenaude

    Mitch Patenaude

    Jul 27, 2016
    For a project I'm working on, I need to interface a small embedded computer to an old phone. The problem is the USB sound dongle I have expects low impedance speakers (4-8 ohms), whereas the old phone has a 500 ohm speaker. Should I just use a matching transformer, or is there something more subtle that I can do?

    I have no idea what might be necessary to interface with the old microphone on the sound card. I'm tempted to just hook it up and see what happens.
  2. poor mystic

    poor mystic

    Apr 8, 2011
    Hi Mitch
    I'd "suck and see" with the speaker question, and if there wasn't enough oomph in the result I'd think about amplification.
    I'd be very careful with bringing the microphone into the sound card though.There could easily be an inconvenient DC voltage in a telephone. I hope you can inspect the mic signal using an oscilloscope.
  3. Mitch Patenaude

    Mitch Patenaude

    Jul 27, 2016
    Oh, I should have explained... the phone won't be hooked up to a land line so no inconvenient DC or AC voltages. I've gutted the phone (so no other circuits to worry about), and all that's left is the rotary dial (which I've interfaced with before, so that's not a problem), the ringer (which I think will require 48v AC to make ring, so I may remove that as well), and the handset, which has two wires for the speaker/earpiece and two wires for the microphone/mouthpiece. I think the microphone is an older style electret (sp?) mic, not the newer style piezo mic.
  4. Chemelec


    Jul 12, 2016
    Most of those OLD Telephone use a Carbon microphone element
    It contains carbon particles that get compressed with sounds, and than expand without sounds.
    This Compression and Expansion cause resistance changes to the applied DC Voltage.
  5. Externet


    Aug 24, 2009
    The 8Ω sound card audio output should drive well any telephone earset with no need to match any impedance,.
    The telephone microphone should be connected to the sound card 'mic in' ; with good chances that if polarity is proper, it will work with electret, magnetic and perhaps carbon capsule.
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