Таксофон

A Soviet payphone out of Kazakhstan, rebuilt to take quarters and play you a recording when you dial the right number, so it can hang on the wall of a bar and raise money for charity.

The phone: AMT-69, steel and enamel, rotary dial
Takes: US quarters. It was built for kopecks.
Brain: Raspberry Pi Zero, about forty lines of Python
Status: Active build

active build · still in the workshop, not on the wall yet

My brother got gifted a payphone.

He owns a bar, and he wanted to turn it into some interactive amusement he could hang up there. Just something unique and interesting, with the proceeds going to a local charity.

What makes it unique is that it’s a Soviet model payphone out of Kazakhstan, brought over by a friend of his who used to live there. Steel body, enamel finish, rotary dial. Everything you’d imagine an early 70s Soviet payphone would look like.

He asked me how hard it would be to fix it up and convert it into something cool. Put some electronics inside, make it a curiosity. Put a coin in, dial a number, hear something interesting.

The front of the AMT-69: pebbled grey enamel, a rotary dial with Cyrillic-era numbering, a black handset hanging on the left, and a metal plate listing the free emergency numbers — fire 01, police 02, ambulance 03, gas service 04. — As it arrived. The plate lists the calls that were always free: 01 fire, 02 police, 03 ambulance, 04 gas. Inside, the terminal block is labelled for a telephone exchange that doesn’t exist any more.

The open back of the payphone, showing the original coin mechanism bolted in on the left and a porcelain terminal block on the right, its terminals labelled in Cyrillic and wired with cloth-jacketed cable. — As it arrived. The plate lists the calls that were always free: 01 fire, 02 police, 03 ambulance, 04 gas. Inside, the terminal block is labelled for a telephone exchange that doesn’t exist any more.

The wrong problem

The coin mechanism was the main problem, and I spent the first few days working on the wrong one.

The phone was built for kopecks. We wanted it to take quarters, which are bigger. My first instinct was to modify the existing mechanism to accept the larger coin, so I started sketching metal parts and figuring out which original components I could shim or alter without destroying them.

After a few days I realized I was asking the wrong question.

The original mechanism exists to satisfy an electromechanical telephone exchange. It handled billing and signaling to a central office. That infrastructure is gone, so none of that matters anymore. Strip it all away and the phone only has to report three things.

A coin went in. The handset is up or down. And the numbers being dialed.

Once I looked at it from that angle, it stopped being a restoration problem. I wasn’t modifying the old mechanism. I was building a new one with a much simpler job.

Five worn coins laid out on dark cloth: a brass South African 20 cent, a Soviet 10 kopeck dated 1982, the eagle reverse of a US quarter dollar, a Soviet coin showing the СССР state emblem, and the 1989 obverse of a US quarter. The quarters are visibly wider than the kopecks. — A few coins that turned up inside the payphone, next to the quarters it has to take now. Notice the size difference. That’s the whole problem.

Six tries

It took six printed versions before the coin path worked.

The hard part is measurement. You put calipers on a sixty-year-old steel casting, move the numbers into 3D modeling software, and then have to hold a fraction of a millimeter on a part that drops into a space you can’t redesign. Every version was close. Close wasn’t good enough. A coin either falls through cleanly or it jams up the phone.

So: measure, print, doesn’t fit. Measure again, find where I was half a millimeter off, print again, doesn’t fit for a different reason. The sixth one worked. Coin in, coin through, switch clicks, coin lands in the bucket. I stood there feeding the same quarter through over and over just to watch it happen.

Six 3D-printed versions of the coin mechanism fanned out on a printer bed, five of them hand-labelled in marker V1 through V5, and the sixth carrying the embossed word Таксофон instead of a number. — Versions one through five got a number in marker. The one that worked got its name embossed on it.

Then the coin had to actually get into the phone. The slot on the front is cut for a kopeck, and a quarter won’t physically fit. That turned out to be more precarious than I’d imagined, because there was only one of that part and it’s on the outside. Print a new one and the phone looks inauthentic.

So I filed it.

There’s now a hand-filed slot on the front of a Soviet payphone. Call it vandalism or call it restoration, but nobody was going to feed it kopecks at a bar in Saint Paul.

Close-up of the coin slot on the face of the phone. The cast plate is stamped 15 КОП and 2 КОП, and the slot itself has been filed wider, leaving bright bare metal against the weathered surface. — Left, the slot filed out for a quarter; the bright metal is the new width. Right, version six installed with the microswitch that reports a coin. The original bracket above it still says 2 КОП.

The finished 3D-printed coin mechanism installed inside the phone, a flat grey panel with the word Таксофон embossed on its face and a microswitch wired to it. The original 2-kopeck coin bracket is visible above. — Left, the slot filed out for a quarter; the bright metal is the new width. Right, version six installed with the microswitch that reports a coin. The original bracket above it still says 2 КОП.

Teaching a Pi to read a rotary dial

With the physical work out of the way, I got to what I knew would be my favorite part of the build: turning an analog rotary dial into something a Raspberry Pi can read.

A rotary dial has no electronics in it. None. It’s all mechanical, so there’s nothing to plug into.

What it has is a spring and a cam. You pull the finger hole around to a number and let go. The spring drags the dial back, and on the way back a cam taps a switch open and shut once per number. Dial a 3 and the switch clicks three times. Dial a 7, seven clicks. Dial 0 and it clicks ten times, which is why 0 sits at the end of the dial instead of the start.

So the Pi just counts pulses on a wire. The dial switch goes to a GPIO pin, the Pi holds that line high, and every time the switch breaks the connection an interrupt fires, bumps a counter and notes the time.

Two little details make that reliable. First, a mechanical switch chatters for a few milliseconds on every click, so a naive counter hears one click as several. Anything within 5 ms of the last click gets ignored.

The second one is the fun one. The Pi has no idea when a digit ends. The pulses in a 7 are identical to the pulses in a 3 followed by a 4. Same wire, same switch, same seven clicks. The only difference is timing. Inside a digit the clicks come about ten a second, because that’s how fast the spring returns the dial. Between digits there’s a pause while your finger goes back for the next one. So the rule is: once 250 ms goes by with no new clicks, whatever’s been counted is the digit.

Try it yourself.

pause between bursts

Seven pulses on the wire. Drag the pause: longer than 250 ms and the Pi hears a 3 then a 4; shorter and it hears a single 7.

Seven pulses is a 7. Ten or more is a 0. Digits stack up in a buffer, and once there are enough of them the code compares them against the numbers it knows. A match plays the recording. A miss plays an error tone and starts over.

The other two wires are easier. One is the hook switch, up or down, nothing to count. The other is the coin switch, where one click is one quarter.

And that’s the whole thing. Count pulses, and use a pause to decide where one digit ends and the next begins. It’s the same logic a telephone exchange ran in 1950, except that took a room full of relays and this is about forty lines of Python.

The payphone open on a workbench, both halves laid flat. The left half holds the porcelain terminal block and the original wiring; the right half carries the rotary dial mechanism and the AMT-69 schematic plate stamped 1979. Below them sits the printed plate with the Pi Zero, wired up into both halves.

The brain

It’s a Raspberry Pi Zero, on a mounting plate I printed to fit the cavity the original electronics left behind.

Sound goes out through a MAX98357, a little I2S amplifier, and into the original handset earpiece. That choice matters more than it sounds like it should. The voice comes through a 1969 receiver, with every bandwidth limit and bit of coloration that implies. A clean modern speaker would’ve been easier to install and would’ve sounded worse. That narrow, boxy handset sound is the effect.

I printed a coin bucket too, since the coins have to land somewhere.

Top-down view of the printed mounting plate: a Raspberry Pi Zero and a MAX98357 amplifier board sit on grey printed standoffs, wired through a strip of green perfboard to a brass terminal strip along one edge, with red, white and green hookup wire running off the plate. — The Pi Zero and the amplifier on the plate that drops into the old cavity, and the printed bucket where the donations land.

The payphone standing upright with a white 3D-printed coin bucket sliding out of its base. — The Pi Zero and the amplifier on the plate that drops into the old cavity, and the printed bucket where the donations land.

A small blue I2C OLED module held up in front of the open phone, its screen reading Таксофон above the address 10.10.10.139. — A little maintenance display inside the case. It tells me the phone’s name and where to find it on the network.

What it does

You pick up the handset, put in a quarter, and dial a number. Someone talks to you.

The recordings are messages from celebrities and notable people, each one behind a number you have to know, guess, or hear about from someone else at the bar. The only interface is the dial and word of mouth.

The money goes to a local charity. Which is why the coin mechanism had to actually work, instead of just clicking convincingly.

The idea I can’t put down

Letting people leave messages. Dial a number, hear a tone, record something, and it sits there for whoever dials that number next.

That’s a very different machine from the one I’m building. It turns a jukebox into a place where strangers talk to each other across time, unsupervised, in a bar. It might be the best idea I’ve had this year, or a moderation problem with a rotary dial attached.

Probably both. I’m not deciding until the first version is on the wall and I’ve watched people use it.

Finish the wiring and chase down a few mechanical issues so the dial is solid and reliable.

Then it ships from my workshop in Texas back up to Minnesota, and goes on the wall at the bar.