Connect with us

Hooking 2 POTS phones togeather as intercom

Discussion in 'Electronic Repair' started by Michael Kennedy, Oct 3, 2005.

Scroll to continue with content
  1. I was wondering if it would be possible to hook 2 POTS phones togeather
    witout the telephone network. What voltage is required to run a phone and
    what voltage is required to make it ring? I think it is somewhere around
    18vac and 55vac to ring but I was a kid playing with a voltmeter when I
    took those voltages so I'm not sure what the exact voltages were. I guess I
    could just find out with my voltmeter but I was wondering if there was
    anything else needed other than approx 18vac and a wire between the two.

  2. Guest

    You can use two telephones to talk, but you will have to shout to the
    other end to get the person to pick up the phone. It is 90VAC at 20 Hz
    for ringing, and 24-48 volts DC to power the electronics in the phone.
    Some cheap personal radio service walkie-talkies would be better unless
    your house is made from shielding wall and floor and ceiling materials.
    Foil-covered sheet rock is normally used only for Kitchen walls, so
    you should be aboe to use the psesonal radios anywhere. In addition to
    24-48Vdc to run the telephones, you have to have wires, etc that are a

    If you do go the telephone route, some phones are polarity sensitive to
    operate the touchtone dialing signals, but you don't neet them so it is
    not a big deal. I would take any power supply 24 - 48 VDC and put the
    power supply in series with the two telelpohones that are also
    connected in series, Then bypass the power supply with a 10 uF or
    larger capacitor of 50VDC or higher rating. This makes the power
    supply a low impedance to the talking frequencies.

    I designed telco central office circuits for Bell Labs/AT&T 40+ years
    ago and did what you are proposing doing for a home novelty. But, the
    radios are far easier, have internal signalling, don't tie you down to
    one location, can be used for car caravanning, skiing trips, and all
    sorts of other things and are not very $$. Available at sportuing
    goods and Radio Shack stores.

    H. R.(Bob) Hofmann
  3. Ken Weitzel

    Ken Weitzel Guest


    Or see if he can't find a couple of the old mag sets :)

  4. Well I have an out building with cat 5 running to it for a network. I'm
    using only 4 of the 8 wires and I figured that I'd use 2 for a telephone
    line and 2 for an intercom since I have a 2 line phone. I have radios but
    keeping batteries in them is a pain and this phone has speakerphone and a

    I can't imagine any easy way to produce a 90v 20hz signal. Hmm...
  5. Ken Weitzel

    Ken Weitzel Guest

    Hi Michael...

    Just for the heck of it, in case you're a younger fellow, a mag set is
    a self contained single telephone set. Doing remote radio broadcasts
    for instance we'd rent two pairs from the local telco. One audio
    quality for the actual broadcast, and one DC pair. Just a pair of
    wires from the remote site back to the station control room. To each
    end we'd connect a "mag set". Crank the handle on the set, and ac
    would be generated that "rang" the other end. :)

    However, enough old man tales of the olden days, how about an
    alternative suggestion, if I may?

    Take a look at the new'ish two set cordless phones. Cheap now.
    Plug the first one into the house line, use one set in the house.
    Plug the second set base into ac in your outbuilding. You then have
    a telephone set to use there, and an intercom between the outbuilding
    and house. Transfer calls back and forth between handsets, do three way
    calls. They're great! (I use a pair at the lake, one in the cottage,
    the other in the boathouse)

    The only downside I can think of is that a power failure disables them.

    Just a suggestion.

  6. Hey I know where to get a magset I just didn't know that was what it was
    called! Anyway I live 30 minutes away from Skycraft. They are a big surplus
    dealer which has anything you need to build about anything... Well almost. I
    have tallked with people all around the US and quite a few know of it so I
    guess that it is sort of unique. Anyhow that sounds like a good way to
    generate the 90v so long that it is 20hz
  7. b

    b Guest

    Michael Kennedy ha escrito:
    try this:

    regards, Ben
  8. jakdedert

    jakdedert Guest

    I don't know if I'd run 90vac down the same cable as my network, but it
    actually 'should' work.

    In my case, I took a couple of 'power line' intercoms and modified them
    to use the phone line instead. I see them all the time at thrift stores
    for less than $5 apiece. Disconect the FR output to the AC cord, and
    solder on a phone plug (or jack, if you want to get fancy). Route that
    signal (RF) down a pair on your Cat 5.

    Works for me....

  9. Well if that is the voltage that the phone uses I've done it many times.
    without any problems : )
    - Mike

  10. Michael, I have some power supplies from the old 1A2 Key telephone
    systems that supply the DC talk voltage, and also have a subcycle 20 Hz
    ring generator if you want one. I'm not far from you, just north of
    Belleview on Hwy 441.
  11. Chris Jones

    Chris Jones Guest

    If I understand correctly, you suggest connecting a 48V power supply across
    the series combination of the two phones. I believe that in addition to
    the power supply, you probably need a current limiting resistor. The phone
    system must get shorted quite often and they wouldn't want the wires to
    melt when this happens. I think the phones might rely on the current being
    limited. I think that the phone loads the line so that it has an off-hook
    voltage of about 6V or so, on a normal phone line, but you could check this
    and you could even measure the off-hook current in a phone too.
  12. Guest

    Chris - You are correct. There is impedance built into the central
    office supply system to limit the current to about 100 ma as I
    remember. I did this work 40 yhears ago when I first joined Bell LAbs,
    been doing EMC work since 1970 or so. You can put a 1000 ohm resistor
    in series with the two phones and the power supply, and then bypass the
    resistor with a 10 uf electrolytic polarized correctly so there are no
    audio losses across the resistor.

    Bob Hofmann
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day