Using Old Phones As An Intercom In Your VW Bus (or Anywhere Else)

In case you haven’t noticed from my many comments on the subject, I drive a VW bus. It is a 1976 Westfalia camper with sage green paint and green plaid upholstery. I absolutely love it and so does the rest of my family. We go for drives in the country as well as camping regularly. We have found that the kids have a hard time communicating with us while we’re going higher speeds. These things aren’t the quietest automobiles in the world. Pushing this bread loaf shaped hunk of steel down the road with an engine that might top out at 75hp results in wind noise, engine noise, and of course, vibration.

I decided to employ a really old hack to put two functional telephones in the bus so my kids can talk to my wife (or whoever the passenger is) without screaming quite so loud. This hack is extremely easy, fairly cheap, and can be done in just a few minutes. The result is a functional intercom that you could use pretty much anywhere!

The circuit to make two phones work as an intercom is extremely simple. You can see in the picture below, all you really need is a little bit of spare phone cable (or a spare junction box), a resistor, and a power source. A single 680 ohm resistor should let you use the 12V directly from your car or even a standard 9V battery if you’re wanting to make this portable (in a treehouse for example).

This circuit allows both ends to be picked up and audio to be transmitted either way. If you want it to ring, you’re going to have to come up with a slightly different circuit. However, since you’re not using the black and yellow wires, you could theoretically rig up a circuit with a buzzer that would run off the normal 12V.

While doing this project, I expected it to be mildly amusing. Possibly a unique use of the old hack. I was pleasantly surprised by my children’s reactions. I just installed this today and they’ve already played with it more than the Pinball Stomp that I built last time! This may have to become a permanent addition to my bus, if not, maybe it will be moved to their bedrooms.

For those who are interested in the mural on the ceiling, here you go.

68 thoughts on “Using Old Phones As An Intercom In Your VW Bus (or Anywhere Else)

      1. @Saul, note the circuit is using the current in the loop for the transfer. Very likely, if you add more devices to the loop, you may want to change the value of the resistor (very likely you can use a pot instead to control the volume).

      2. You’d need a different circuit for more than two sets; the power is in series with the hook switch.
        Unless everyone had their handset up for every call, no juice flows, so no voice.

      3. You could build a PBX for that. Circuits abound.
        Or you could go old school with operator plug boards.

        Or even older school by hooking them up in series and using a higher voltage. You’ll have no privacy, though. This method was still used in rural parts of the USA until the late 1980’s when the last private phone exchanges sold out to the LECs.

        We called it “the party line”, and tried very hard not to breathe or make noise while we listened to the neighbor’s phone calls.

        I believe you could have up to three or four phones on the lines, which is why ringer equivalence numbers used to be important – if you had too many phones off the hook, the line voltage would drop and could no longer reliably signal voice data.

        PS – st2000 is right.

        They’ll work off 6vdc without problem.

        The -48vdc is because the old central relay stations/switching offices ran with battery banks that were grounded at the positive terminal. They were awful vats of acid in tanks that looked like aquariums.

        You can still find old 6V brown telco dry cell batteries used to boost phone lines in ancient office buildings, slowly leaking into eternity.
        These were often used for remote ring boosting and intercom functions.

        1. They still had party-lines up in Northeastern Vermont (Northeast Kingdom) up until the late 80s.

          I have a couple of old Bell Telephone rotary phones I got from my mom’s estate. Instead of selling them, and since my teenagers think they are cool, I had wondered if they could be used as intercoms…I guess they can!

          I know they still work up in Vermont on the regular network as long as you specify to the phone company (Fairpoint) that you need the pulse dial service.

      4. Oliver thanks for the info on the little building batteries. I have seen a couple in building demos but since the party line stuff was a bit before my time, I didn’t know what they were for. The state that the batteries were in, I just assumed (incorrectly) they were old cardboard wrapped fuses (that look like BB tubes hehe). You are also VERY correct on the ringer voltage spike as you caution below. Folks can get a nice tickle if they aren’t paying attention. :) Thanks again for sharing your knowledge!

      1. The ringers are considerably higher than that, depending on vintage. The ringers work on AC, and you might be able to fake it on more modern phones with a dpdt relay wired to reverse polarity of the DC line and then drive the relay from a 555 timer set at ~20 hz.

        The ringers were originally signaled by the use of a magneto that generated anywhere from 45v to 120v. You can couple a ringer AC source to the circuit to inject the ringing signal, but it makes it more likely that someone will get shocked.

        Generally, people that did this for intercom use disconnected the bells from the comm circuit and then stuffed a circuit inside to ring the bell when a tone was imposed on the lines or the voltage dropped.

        You can detect the voltage drop with an op-amp used as a comparator and then ring the bell until the second handset is picked up.

        Get a multimeter and watch the voltage difference between the phones. Pick one side up and then pick up the other side, and note the voltages.

        Phones should actually have a 48v source by design, but anything made after 1986 will be digital and run off almost anything.

        You will get a ticket if a cop sees you using this.

        I recommend that you purchase a brain bucket, goggles, a silk scarf and a couple of throat mikes…. “Flying Officer Biggles to Backseat Aerodrome… Come in, Aerodrome… Mom smells something back there and wants to know if you guys need to use the bathroom…. Guys???”

        If they lay down on the floor while the heat is blasting, and look out the windows while you drive down bad roads at night while the stars are out, your kids can pretend it’s a space capsule simulator.

        The floor of a VW bus with the heater on sounds and feels about the same as riding on on a Gemini space capsule. You’ll need to play Bowie on 8-track to get the full effect.

        1. Good point, the old Bell phone is still a handheld device. I’d like to see the cop-cam video of that traffic stop! Down here in NC you can still talk on the phone without hands-free, so this would be OK for all the old and young hippies down in Asheville.

          1. These days it would be far simpler to buy a couple of ‘phones from Radio Shack that plug into a power outlet. They interconnect via the household electrical mains power supply.

      1. Many more recent Subaru engines are a boxer design ripped off directly from the porsche 1600 engine. They are easy to bolt up, especially on the later camper and euro vans. Scads of HP, decent mileage and they’ll run for a very long time.

        But you can’t fix them with a pair of vice-grips and a hammer.

  1. 1600cc fuel injected beast! All joking aside, nice van, and great idea. If only something like that would work in my dune buggy. Its near impossible to hear someone at highway speeds. Maybe I could use the parts from one of those to make a headset lol

  2. To make it ring – and to have more phones, you might install a small PABX with an inverter powered by 12V from the car *g* – but most current PABX won’t support rotary dial but will support touch tone
    Another idea is to built a mobile phone device into each phone and use “normal” GSM connection :-)

    1. Same way, if it’s one of the old carbon-mic handsets (from a phone with a real bell and a dial!). Connect the mics and ears in series in each set, then connect the pair together like the diagram above. That’s essentially how all phones used to work.

  3. Getting two phones to work with each other is pretty simple. Now, for ring you need to send 90VAC at 20Hz. Easy way to do this would be to detect the off-hook on either and have a secondary circuit send 90VAC down the line until it’s picked up.

  4. If you want the phones to ring, you will need to build or find something called a “ringdown” device. You can convert an unlocked Linksys PAP2T (found for less than $30 on ebay) into such a thing. Here is a good youtube video showing how such a conversion works:

    I can confirm the PAP2T works great with old phones… just don’t expect the rotary dial to do anything useful. As soon as you pick up one phone, the other rings automatically, which is exactly what I wanted.

  5. There’s a very cheap and easy way of making the phones ring:

    – connect all the phones in parallel.

    – connect a ballast (yes, a fluorescent tube ballast coil) in series with the DC power supply (12V should work) and power all the phones from there.

    – set the phone for pulse dialing. take a phone off the hook and use the ‘0’ to ring all the other phones.

    the reason this works is that the pulse dialing shorts and opens the ballast, which will produce enough high voltage to ring the bells.

    I had a system years ago that I wired to connect several old phones in the house,garage,etc and it worked like a charm.

  6. 0. Car manufactuer of today go to great lengths to keep thing from flying inside of a car in case of an accident. I would not use such a heavy device. But since you are, do fasten them down really really well. I might switch out the heavy hand sets for light platic ones as well.

    1. Like most have said, the actual ring voltage is about 90 A.C. at about 20Hz. Not really something you want especially w/kids.

    2. The off hook voltage is -48V D.C.

    3. Looks like you already took out the cross over transformer from the looks of your scematic. So I would just forget about all the real POTS votages above and run a separate line to some 12V annouciators. Surly there are enough “leaf switch contacts” on the on/off hook switch to create a signal when one of the phones are off hook while the opposit phone is on hook. If you need help, look up what a 3 way switch configuration looks like.

    1. 2. The “ON” (not off) hook voltage is -48V D.C.

      3. The trasformer in old phones (I think) is actually part of or is the “hybrid” circuit (not the cross over transformer). It changes the 4 wires from the mic and speaker to a 2 wire interface. I think you just hooked everything up in series from the looks of your scematic.

  7. If you don’t mind gutting the phone you could remove the ringer (the bells are huge in those things) and put a cheap wireless doorbell inside and stick the ringer button next to the phone (front ringer to back bell and visa versa), hate to see a nice old phone gutted though. And if the cops catch you you’ll get a ticket and they’ll probably find your stash (hey its a VW bus, there has to be a stash in there somewhere. :) )

  8. im wondering if Caleb is from British Columbia Canada? looks like an old friend of mine and i used to have the same vw bus. those things never die even if the engine catches fire while driving it. (mine did)

  9. Cambridge Electronics makes a small AC ring generator. It’s a 1″x1″ chunk of plastic with a hipot encapsulated circuit inside. 4 pins, 5VDC or 12VDC in one side, 86VAC ring signal out the other side. I’ve used them on several projects, very simple to use. Single quantity price was in the $15-$20 range if I remember correctly.


  10. nobody suggested adding the capacitor for MUCH louder AND clearer sound?

    i cant remember if it goes in parallel with the wires to the phones or in parallel with the resistor, but i have done this before and one way will take away most of the sound and the other will make it MUCH louder

    but it will probably make the microphone so sensitive it will be useless on a 60’s VW bus due to road and vehicle noises.
    so add a mode switch! lol

  11. I have a type 2 first gen, so I should use a candlestick phone or mobile handset vintage just post war. Oh! The Stromberg Carlson desk phone OK!
    Since my internet just got flaky, I didn’t read the source but I assume this is for a wired set of POTS for campground use.
    What about hacking the old 46-49mHz or higher CORDLESS phones for MOBILE convoy use. Handset in B base in A and current loop to classic deskset on dash. Most sets run on 12v or less, some support intercom multi unit handsets.

  12. So before the “intercom” hack, did he hack the length of his VW Bus?

    Other wise, it’s what, 8 feet front to back inside?

    So the kid in the back (which means a whopping 4-5 feet away from the front seats max), yells up to the front “Hey pick up the Intercom”, and the Adult in the front yells back “what?”, the kid replies “PICK….UP…THE…INTERCOM”. The adult picks up the intercom, which is also transmitting all the noise in the VW BUS and the kid say “Are we there yet?”.

    With no ring/light/clapping monkey to indicate the Intercom is being used – seems like a waste of time. Even with a ringer, come on, it’s only a few feet front to back.

    1. Circa 1971 VW bus was pretty loud from the factory.

      I suspect his has leaking seals, missing hi-temp sound insulation (oh god, that stuff really helps) and road noise… lots and lots of road noise. Now add in all the squeaks and rattles, and it’s a high noise environment. You could go deaf from a few hours of this in 1971 – imagine what it must be like now.

      Also, the buses had a middle seat, which was later adopted by the minivan industry. In fact, the minivan industry was pretty much started by VW, and the GM models simply added a lower center of gravity and creature comforts.

      If his kids are sitting with one in back and one in the middle to prevent the “she’s breathing my air” fights that kids love to pass the time with, an intercom might be pretty useful.

      If the use of old phones helps him get out of the state of misery, it’s a great hack. Now he needs to put in the 17″ LCD display with the answer to “are we there yet” displayed as a count-down timer and moving map.

      Ah, the VW bus. What a great and amazing death trap – my uncle used one as a sled down a ski hill going sideways for a half mile or so and coming to a stop on the road below. Unbelievable.

      1. haha, I’ve owned several of these. Anyone who has ridden one knows that they are quite loud and you have to shout. Even with my more modern insulated one (quieter that my old 1960), you have to yell. I have a camper, so no middle seat. You sit in the very back or the very front.

        We actually used it this weekend. Instead of screaming at us, he picked up the phone and talked normally to my wife. Conclusion: absolute success (a rarity in my exploits).

      2. Oliver Heaviside: Any VW nut would gladly put up with the noise. It’s clear you have never been bitten by “the bug” most VW drivers would rather drive their car with a hole in the floor and the battery dragging the ground than trade it for another vehicle! Caleb: I’m in S.E.MO too, out in the woods near Cape Girardeau. Used to be the ‘bug man’ in Cape years ago. Got five or six buses grazing in my front pasture right now.

  13. The Motorola VT1005 supports pulse dialing and two lines. You could hook up a small PBX using a VT1005 and connect it to a internal LAN with IP connectivity (whatever works best for you) and then you would have two real telephones in your car that could call one another, or call out. It would ring in the old fashioned way, too.


  14. I did something similar years ago using two $5.00 transistor radios. I ran a 4 wire cable between our master bedroom and the kids bedroom and connected the speaker in one radio to the amplifier of the other. I switched the radios on and adjusted the volume to a convenient level, tuned the radios in between stations to find a silent part of the band width and removed the tuning wheels so their position could not be altered. I then connected a tiny telephone headset receiver between the battery bus bar and an active part of the audio amplifier circuit in each radio to act as a microphone. I then wired a non-locking press button in the battery supply lead. The button connected 9 volts of power to the radio when pressed so acted as a ‘Press to Speak’ button. When I spoke into the microphone on the radio in the master bedroom my voice was heard in the speaker located in the radio in the kids bedroom which was great for waking them up in the mornings telling them ‘Wakey wakey rise and shine’ to wake them up to get ready for school. The kids were able to press a button on their phone and reply to let me know they were out of bed. It was a handy intercom built for $10.00 plus a few scrap parts. Because battery power was only consumed when the ‘press to speak’ button was momentarily pressed, the batteries lasted for many years.

  15. Love your bus! And cool idea. I’m restoring a 71, and I’m thinking about building a Raspberry Pi/Arduino computer system for it that’ll allow me to do things such as monitor the engine as well as custom stereo and various other things. The buses electronics are so simple, you can can control everything in it with just a handful of relays.

  16. i have two telephones,both have a display to show numbers and it also have a place to put 3 battery of 1.5 volt each. So, if i put battery on both phones and just connect the red &green wires directly, will it work as intercom?
    if not, what should i do then?
    And what is a “ballast”?

  17. Coming back to this after nearly a decade, but I managed to use this to make an intercom from two vintage magneto telephones (a Leich 901 desk-or-wall set and an Ericsson N2204A wall phone). In doing so, I discovered the Ericsson has a bad microphone, so I’ll need to fix that, but once that’s done, my hope is that with the bypass capacitor across the power supply, you’ll be able to turn one set’s hand-crank to ring the other phone’s bell, thus skipping the need for a ring-down or other alert circuit.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.