Telephone Line Monitor (Plans)

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Get yourself a low-voltage DC relay, like a 3v relay… Set it up as follows:

              Audio Isolation 
      To      <--)||(---------------------+ 
                 )||(                     | <==== Relay Contacts 
      Speaker <--)||(---+    +--------o/  o 
               600ohm   |    |        mmmmm DC 3v Relay Coil 
                        |    |       |     | 
       RED   -----------|----+-------+     +----------> To Dispatcher's Phone 
       GREEN -----------+-----------------------------> To Dispatcher's Phone 
       |                               | 
      -+- indicates a connection,     ---  is not connected. 
       |                               | 

You may have to use a Diode or two to make this telephone-line FCC clean… I’m not saying this is a clean circuit at all. It’s cheap and dirty! You may have to use a Op-Amp (Use an LM386, they’re good for speakers) on the speaker. Depends. Experiment!

Circuit Theory

When the Dispatcher picks up the phone (in a standard circuit, I have NO clue what your PBX does.. this will work on standard home phones, and I used to use it for a tape-recording controller) Hey, there’s an Idea - spend $25 on a telephone recording device, and hitch it to a nice loud amp and speaker combo, instead of a tape deck. It’ll save you loads of time…

Anyhow.. the voltage will turn into DC, approx 6-10VDC when the phone is picked up, (which is why you’ve gotta put it before the dispatcher’s phone) and click the relay. The relay will connect the transformer, and feed the speaker. it might be towards your advantage to use a SPDT relay, and connect BOTH ends of the transformer, and not just switch one end in and out. That might prevent some line noise…


  • jna[at]



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