I'm interested in making a 2-phone intercom

Discussion in 'The Projects Forum' started by Videogamer555, Jul 7, 2013.

  1. Videogamer555

    Thread Starter New Member

    Mar 27, 2013
    4
    0
    But the only place with the simplest circuit has me confused. http://electronics.howstuffworks.com/telephone4.htm

    It says I need a 300 ohm resistor, but I don't know why. Phones are simply appliances that use the DC voltage on the phone line as their power source, and since they are appliances (loads) they have their own resistance (and the required resistance is usually just achieved by having 2 phones in series, I think). Right?

    Further more it shows a battery's positive terminal connected to the red of one phone, but the battery's negative terminal connected to the red of the other phone. From my understanding of other sources I've read that talk about Tip and Ring, the Ring is the negative connector and has a red wire, while the Tip is the positive connector and has a green wire. Because of this, I'd think the correct wiring is the red wire of phone-1 should go to the negative terminal of the battery, the red wire of phone-2 should go to the green wire of phone-1, and the green wire of phone-2 should go to the positive terminal of the battery. If you don't do it this way, one of the phones will have proper polarity, and the other will have reverse polarity. This may not be a problem if you are lucky enough to get your hands on an antique telephone that simply had a moving-coil speaker, and a carbon granual microphone, but in modern phones the reverse polarity will fry many circuits. Even the microphone in a modern phone is an "electret" mic, with its own builtin semiconductor pre-amp, and the buttons on many phones are illuminated with LEDs. These semiconductor devices will be fried with reverse polarity. Many modern phones have all kinds of circuits that depend on the DC voltage of the phone line for power. If I follow the diagram on HowItWorks, one phone will work, but the other will go up in smoke (literally).

    Now maybe I'm wrong about this, and if so, please explain why.
     
  2. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Your argument about Polarity is correct. But, in all electronic instruments, the first unit at the telephone terminal is a Bridge Rectifier to take care of the Polarity. So No Smoke; No Fire. :D And the Exchange feed is 48 Volts, Not 9 :eek:

    Ramesh
     
  3. Paul Kerry

    Member

    Jan 9, 2012
    36
    4
    Exchange feed is 48V until the line is in use when it drops to around 6V
     
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
    758
    57
    For modern telephones which have polarity steering built-in; use simply a 12 V battery in series to only one of the interconnecting wires, any polarity, no resistors added :

    ------------------------------(+)12V(-)----------------------------------Tel 2
    Tel 1
    ----------------------------------------------------------------------------

    This will provide the intercom function, not telephone operation with dialing, dialing tone nor ringing.
     
  5. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    you will not get the phones to ring unless you use a 20hz 90V ac source (in the US anyway).
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    It's very difficult to build a "private exchange" just to run two phones.

    A much easier option would be to buy a couple of cheap household intercoms, and pull out the guts and install it in the telephone.
     
  7. Videogamer555

    Thread Starter New Member

    Mar 27, 2013
    4
    0
    It's 48 for onhook. The moment you pick up a phone, so it goes offhook, it drops to about 9volts. Since a phone in its onhook state it is pretty much useless, I really only care about simulating its off hook state with 9volts.

    As for the bridge rectifier, do standard computer "56k" modems also have a bridge rectifier in them? I hope so, because I eventually want to be able to connect 2 computers together via their modem and be able to send data between them. My use as an "intercom" with 2 telephones would just be to test out my setup.

    Also, do modems require the presence of an actual dialtone audio signal to work? Do they require the presence of a ringing signal to work too? What about the presence of an onhook 48v DC, is that also required for the proper working of a computer modem? Please let me know. Thanks in advance?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Really? That's been obsolete for what, over 25 years?
     
  9. Ramussons

    Active Member

    May 3, 2013
    557
    92
    ..........

    Ramesh
     
  10. whatsthatsmell

    Active Member

    Oct 9, 2009
    102
    4
Loading...