HOWTO make an intercom out of obsolete corded phones

Here's a great, simple Make project that teaches you how to make an intercom from a pair of old, corded phones, a 9V battery and a resistor. I loved walkie-talkies and intercoms when I was a kid -- the idea of setting up your own house-wide wireline intercom is super-cool, and the project is dead-simple.

* At its most basic level, a telephone network is just two microphones, two speakers and a power source. In this project we are reducing the phone to these basic elements. The handset of the phone contains the speaker, the microphone and any necessary processing circuitry. All we need to add is the power source.

* A regular corded telephone doesn't require much electricity to operate. It just needs about 9 volts and less than 30mA. It normally gets this from the phone line itself. This is why many phones can still work even during a blackout. However in this project, we are using a single 9 volt battery to power our phones.

* The battery is wired in series with a 300 ohm resistor and connected to either the red wire or the green wire in a phone cord. The phone cord is then plugged into both phones. The battery is able to supply enough electricity to power the speaker and microphone circuits of both phones. This allows you to use them to talk back and forth.

Simple Intercom From a Pair of Old Corded Phones [Jason Poel Smith/Make]

Notable Replies

  1. thekaz says:

    Wait, corded phones are obsolete?

    And intercoms aren't?

  2. This project doesn't seem to have a method of making the remote phone ring, making it pretty useless as an intercom with the phones they used. I guess if they're both speakerphones and you leave them off hook constantly it might work, but you'll want to have them on mute.

    The good thing is that if you've ditched your landline, then you probably have phone cord wired through the house already, so this project would be super simple. If you haven't ditched your land line, then you can just dial that number that makes all of your phones ring and use that as an intercom instead.

  3. cdinvb says:

    Dunno. If there's wiring in place, it's probably four wires. Might be six (3 pair) or eight (4 pair). In any which case working up a signaling system should be a fairly straight shot. I don't think I'm going to run right out and do this. But I might look at what's in thrift-store junk bins next trip that way. A wall wart should make a decent power supply with very minimal modification.

  4. Most**US home telephone wiring is 2-pair, with RJ11 connectors. The primary line is on the red/green pair, and if you have a second line it'll be on the yellow/black pair. You can get two-line phones that use both, or two-line connector boxes that break out both lines into two jacks (using the red/green pair on both, so regular phones work.)

    So if you've got one land-line, you can still build this intercom on the second pair without it interfering.

    * *Re: "most" - Your mileage may vary, especially in older houses. Maybe they've got two-wire, and I've lived in a place that had 3-wire, and maybe the wires have old discolored cloth insulation instead of colored vinyl, so good luck telling which is which. But anything since the 60s or so should be useable.

  5. Just yell "HEY PICK UP THE INTERCOM PHONE THINGYTHING". Repeat until they pick it up and then have your conversation.


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