This toy intercom system is way better than a pair of tin cans and some string


On his blog, [Kenneth Finnegan] recently showed off a replica of a fun toy he used to play with as a kid, a telephone intercom system. The setup is pretty simple, requiring little more than a pair of analog phones, a battery, and a resistor.

The phones are connected to one another using a standard telephone cable, but [Kenneth] uses a 9v battery to introduce a small bias current into the loop, allowing the speakers at either end to hear one another. He also added a small LED into the circuit so that there is a visual indication as to when both handsets are off hook.

The setup is very simple at the moment, though [Kenneth] does have some ideas in mind to enhance his intercom system. He hopes to tweak the remote phone to ring when the local phone is picked up, among other things.

Telephone technology is nothing new, but for just a few dollars (or less) your kids can be entertained for hours as [Kenneth] was way back when.

Continue reading to see a short video overview of the phone system, and be sure to share your ideas for enhancing it in the comments section.


  1. Ha :D finally something at my level ;)
    I did exactly this when I was 9 :P

    Not that I have got better since.

  2. Vince Mulhollon says:

    Technically, ringing voltage is supposed to be current limited, isolation transformered, fused, 90 volts AC at precisely 20 hertz. Generations of theater hands have figured out that plain ole AC outlet power (if you live in a 110 volt area) will make the phones on the line ring. I’m disclaiming all responsibility if you burn your house down and electrocute yourself and family members.

    Note this is also why safety people tell you not to fool around with downed phone lines.. Not just that you can’t tell the difference between phone and AC, not just that the lines may have crossed or might be about to cross, but even if it’s perfectly electrical power safe, someone could ring a phone and ringing voltage is enough to hurt quite a bit when it rings. (Also T-1 repeater amp line power packs quite a wallop)

    • salec says:

      plain ole AC outlet power (if you live in a 110 volt area) will make the phones on the line ring. I’m disclaiming all responsibility if you burn your house down and electrocute yourself and family members.

      ditto for 220 volt areas. But …

      Remember kids: If phone rings continually, DON’T pick up the receiver from the hook! It will get damaged and might as well hurt you too. Plug it out of the wall (and don’t touch the socket terminals).

      Mains is low impedance voltage source and it will fry the phone once its off-hook and low-impedance current path inside it is established.

    • klif says:

      @Vince Mulhollon

      ahaha! long time ago i was repairing the phone cable at my friends house. it snapped and i decided to quickly fix it. not having any tools at hand, i used my teeth (as usual) to strip the insulation. i didn’t unplug the cable.

      of course, somebody made a call at the same moment as i chewed that cable. my head felt like one of those old black telephones when they ring DRIIIIIIIIIIIIING!

      nothing terrible happened, but i felt pretty stupid.

  3. My kids would love this. It just needs a Bat phone or nuclear hot line phone version.

  4. fartface says:

    Welcome to playing with electronics from 1970’s and 1980’s… Been here did that when I was 8.

    What would be impressive, throw in a duino and make the other end ring when you dial a number.

  5. xorpunk says:

    instructions on this is in a lot of old books too..

  6. When I was in high school, I actually did make a circuit that you can plug two phones into… you pick one up and the other rings. I pretty sure it was a circuit published in Electronics Now (Popular Electronics) magazine. My buddy and I both made one and put them into nice little plexiglass enclosures. Drop me a email if you’d like me to dig it out… I know I have it packed away.

  7. Steve says:

    And the amazing thing is that the audio quality will exceed any VOIP or cell phone on the market…

  8. Nuno says:

    I did something similar when I was a kid, but with a small improvement: I connected all the phones in parallel and used a fluorescent lamp ballast (high inductance) in series with a DC voltage to provide power to that bus. This way I had several phones (kitchen, electronics shack in the garage, bedroom, etc ) and the sound quality was pretty good.

    Another advantage is that I could use the pulse dialing to ring the bell, since the (short-circuit) switching would make the inductance create high voltage, enough to make the bells ring.

  9. ferdie says:

    when i whas 10 years old you cam buy this in a toystore it whas pink but it work it is fun as a kid to have a phone a mobile that time for me i never hear of it now all kid,s have a mobile.

  10. Sean says:

    This would be awesome with structured wiring. You could patch two rooms together in the basement as a dedicated circuit with its own power, then use the wall jacks in those rooms just like a “real” phone!

  11. gnomad says:

    I can’t even remember the last time I used a land line…

  12. YaBa says:
  13. razor says:

    I had when I was a kid an article from Pop Electronics of a circuit from Australia that would ring the other line when the first phone was picked up, and stop ringing when both phones were off hook. At that point both phones are connected until one was hung up. It wouldn’t ring again until both phones were hung up, and then one was picked up again. It was mostly straightforward with a couple relays and some simple isolation circuits and transformers. I wish I still had it…

  14. DaveO says:

    I have been planning to install a phone at the end of the garden for my boys. I did this 30 years ago – I think I only used a couple of AA batteries and it lasted for months. I remember the handset needed a smack every now and then to loosen the granules in the carbon mic and restore hi-fidelity. We used to have a hand crank generator too which we used for shocking each other rather than ringing. Happy days…

  15. tantris says:

    for a nice clean setup get a female phone adapter at the dollar store. you can press it open, wire the 9 volt into it and close it up again.
    and you can also use an old wireless phone..

  16. NewCommentor1283 says:

    you want improuvements that dont involve a uC? try doubling the voltage (TWO 9v batts in series) and double the resistor to keep the current the same.

    … it makes the sound louder and clearer.
    only downside is u waste an extra 9v battery for the same runtime.

    PS: yes you CAN do this with dial-up modems and faxes, but it wont work as fast as normal until you add the bypass capacitor. and unregulated wall wort will need humongus filter cap. to eliminate AC-humm/buzz. so use regulated wall wort(20ish volts). (think laptop adaptor)
    PPS: using the internal computer PSU will prolly fry it to hell; youve been warned.

  17. Erik Johnson says:

    I remember when I was 10 or so we didn’t have a printer, but we had a Fax machine that was given to me because it was broken – the scanner lighting leds developed cold solder joints, easy fix. Anyway, our new PC came with a 14.4 modem and WinFax… I knew from playing with a phone my parents gave me as a kid that a battery on the 2 wires made it work, so I got a F-F phone coupler from radio shack, popped it open and connected the battery there. The fax machine had a manual-start button and the PC didn’t require ring tones so it worked really well that I could fax myself on a closed circuit and have a printout! The only downside was the really thin thermal paper

  18. Dan says:

    My dad made one of these for us as kids over 35 years ago. Even had it make a dial tone when picking up and had to dial a three digit number on the rotary phone to connect.

  19. Al says:

    I saw the PopSci ad, too, but haven’t been able to find it on RadioShack’s or PopSci’s website. I wanted to put one of these systems together for my father who is building a big garage in the backyard as a working DIY intercom system connected to a matching phone in the kitchen in the house. I was hoping it would work like the Bat-phone and ring one end when the other is picked up. A few of you have eluded to this being possible. Could someone please provide a link or instructions on how to make this work, preferably with a wall-wart power adapter? My email is costalot at gmail dot com. Thanks!

  20. thomas says:

    Great!!! my neighbours kids got crazy when they could talk to each other, i want to make the prototype line more durable (solder the connections properly). Though i would make it perfect if they (batman and spiderman) could ring each other! Can this really made working like the above comment or somesort? let me know! my email is thomasreneman(at)hotmail(dot)com. Thanks!!!

  21. Thomas says:

    Let me know if you figure out how to make them ring :)


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