Old Phone Intercom Wiring (w/ 5-phones)

Discussion in 'The Projects Forum' started by brass86, Jan 22, 2013.

  1. brass86

    Thread Starter New Member

    Jan 22, 2013
    I am trying to connect 5 phones together for an internal (non-phone provider) intercom system. I am trying to get 5 phones to all pick up and be dedicated mic and speaker to the rest of the circuit at the same time.
    I have been successful with connecting 2 phones together by connecting the Line-in/out jack to the other phone jack and applying 12VDC between one of the wires (green), and having the other (red) wire be directly connected with no connection to the power (it gets it through the phones).

    My thought process and confusion:
    Issue I am running into is the theory behind the circuit once more phones are introduced. All the red wires need to be connected to the other red wires; however, the green wires need to have a connection to the rest of the green wires- and still be on the same circuit, to me it is not logical. I cant connect power the red as it will short out.

    How would I connect 5 phones together to all be hot-linked-direct-wired to the other phones. I don’t need tones, ring, etc… just pick up and talk for room to room configuration where the user can see the other end, yet separated by a piece of sound proof glass.
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    What brand are the phones?

    Are they meant to be standalone systems, or work through a PBX?

    Without a PBX to handle intra-office dialing/routing, attempting to wire the possible connections would be rather expensive. Locating a used PBX without line cards may be your best bet to get it working, if they are typical office phones meant to work with a PBX. If they are standalone 2 line phones or VoIP, then things are simpler by a bit.
  3. brass86

    Thread Starter New Member

    Jan 22, 2013

    They are basic phones, model Cortelco ITT 2500, I just got done troubleshooting and was able to get the circuit to work by introducing 18VDC, and 4.9mA in a series configuration, but alternating the Red and Green leads at each connection/phone. I know most phones work in a parallel configuration (think legal wire tapping) and am pondering to wire my circuit next.

    My system is meant to be a standalone system and the best way to describe it is as a conference call between 5 local-network phones, all hardwired together. I do not need advance circuits for ringing or key pad operations. Simply pick up and talk. (the other end will be aware by other means of the need to also pick up. The only down side of my series configuration is all phones need to be off the hook to allow a closed circuit.

    Thank you for your help, I am far from finished.
  4. dsk

    New Member

    Feb 24, 2013
    Try to put a a coil, e.g. a relay in series with your power-supply. Then connect all the phones to this circuit in parallel, red and green doesn't matter.

    Last edited: Feb 24, 2013