Mute switch for wired telephone

Discussion in 'The Projects Forum' started by mjscott, Jun 24, 2016.

  1. mjscott

    Thread Starter New Member

    May 21, 2014
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    I wish to build a mute switch for my wired telephone. The cable into the handset has 4 wires; I assume two for the earpiece and two for the mic. I would be installing this mute switch in-line to the handset cable.

    I would guess the simplest solution is to install a spst switch onto one of the wires leading to the mic. What I don't know is if this will negatively affect the phone operation with the mic circuit being open. Also, opening then closing the mic circuit might create an undesirable 'pop' on the line, which I don't want. If so, perhaps this could be eliminated with a capacitor??

    To further improve this mute switch, it would be awesome if I could have a LED which indicates that the mute is active! Is there enough current in either the mic circuit or the earpiece circuit to drive a LED? I could use a dpst switch (or similar) to light the LED if so.

    I have searched online to buy such a device, and was surprised to find virtually nothing.
    With thanks,
    Mike
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    745
    I would just put a switch across the microphone and short it out for mute, iw wont illuminate the led.
     
  3. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Shorting the microphone pair will disable it; opening the earphone pair will disable that.
    Unplugging the cord out of the handset will disable both.
     
  4. mjscott

    Thread Starter New Member

    May 21, 2014
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    0
    Thanks folks. I have 2 votes for 'shorting the mic pair'. I have to admit, this surprises me as my limited electronics experience would guess that this could be bad for the overall circuit. I will try it.

    As to the second half of the question: does anyone know if there is enough current in a telephone handset to drive a LED?
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Most modern phones have some form of micro, and the Vdd rail is usually stabilised by a Zener - you can get an idea of the power allowed for by the rating of the Zener.
     
  6. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Not in the bare handset, unless there is a weird telephony integrated circuit that I do not know of. There is enough current in the telco pair and probably at some node in the integrated circuit that does the line interfacing.
    Hook a led directly to the telco pair, no resistor. If does not turn on, reverse its polarity.

    Now to complete the chance: If the handset is actually the whole telephone* (contains the whole circuitry) Yes. There is enough power to turn on a LED in both operation, on hook and while ringing.
    *sort of like this style (no cradle) ----> https://images-na.ssl-images-amazon...roduct_content/swissvoice/ePure_handset_L.jpg
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    If its got buttons - its got a chip.

    The specification for off hook current is basically just to allow for leakage. Were talking about a few M Ohms..........

    You can flash a LED while off hook by charging a capacitor via a large resistance - the absolute simplest flasher is adding a diac to dump the charge into the LED when the capacitor gets up to 32V.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Per post #1 it is a wired telephone with a base and a handset. 4-wire handset cable almost certainly is mic and spkr only. The question is, is it an electret, dynamic, or carbon mic?

    ak
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    I've never seen a dynamic mic in a phone - the original standard was always carbon, there are/were carbon replacement capsules with an electret and a few transistors. AFAIK: the standard circuitry in an electret mic handset is very similar to that in the upgrade/replacement capsules. The original system was designed to accommodate a carbon mic - most implementations of electrets more or less mimic that.
     
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