Payphone_Mk_2

I have re-deployed the payphone, just in time for Cyberdelia!

The most notable upgrade is the display on top: the old one was just a gimmicky name badge glued to the front and displaying the same scrolling text all the time, but this one is a MAX7219 32x8 LED display that's controlled by the Pi, so it is reactive to what you type. Challenges included: finding any kind of enclosure that it would reasonably fit inside; getting a friend with a machine shop to cut said enclosure down to size; and drilling holes into the top of the phone to attach it and route the cables. Seriously, that took hours. Payphones are made of stern stuff, and I don't have a machine shop.

Instead of using that nightmarish Cirrus audio card that I used last time, I just got a tiny $8 USB audio interface with 3.5mm TRRS. That all went much more sanely.

Also I assembled a little 5v 6w audio amp, so the handset is now loud, possibly even loud enough to be heard in a nightclub.

That necessitated replacing the speaker in the handset, since the old one was clipping with the new amp turned all the way up. And that necessitated getting a new handset, since I couldn't unscrew the endcaps from the old one, even with a giant pipe wrench. Either they were glued on, or they had fused after who-knows-how-many decades melting in the Sun.

And since the amp is stereo, I used the spare channel to attach a second speaker to the inside of the phone case. I never worked out how to drive the solenoids on the physical bell clapper, but now I can play an MP3 that sounds kind of close, and so I have an "attract mode": every now and then the phone rings and the display says something to the effect of "IT'S FOR YOU".

Previously, previously, previously, previously.

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23 Responses:

  1. Ángela Stella says:

    "Hello, Case."

  2. Waider says:

    Now I need to go listen to Fluke for a while. Atom Bomb is good, but I really like Absurd.

  3. nooj says:

    Attract Mode has so many possibilities!
    ring!
    "Hello?"
    "Your mission, if you choose to accept it..."

    It can be multiple stops in a complex scavenger hunt, too!

  4. thielges says:

    “I never worked out how to drive the solenoids on the physical bell clapper”

    It requires a 75 volt 20Hz sine wave. I found out one day when I had my hands on a phone line and a call came in. 20Hz creates quite a weird shock.

    • James C. says:

      I discovered what that felt like once as a young dumb teenager. I was stripping the wires of a live line with my teeth and it rang. I jumped across the room in surprise! Fortunately I wasn’t hurt. I learned two lessons: (i) never strip wires with your teeth, and (ii) even though they’re not high voltage, telephone lines are still dangerous when they’re live.

      • thielges says:

        Prior to the shock I had thought those little phone lines carried only low voltage audio signals. The zap was shocking enough felt through hands. I can’t imagine what you felt having the same 75V signal through your mouth. Yikes!

        That phone line incident happened during the “dumb 20s” phase of my life and the same year I ran over myself with my own car, an accident also caused by messing with live wires.

        • Nick Lamb says:

          Presumably there would be similar stories about how thermostats work except that since they're mains voltage (here 240 volts) with enough power behind them to run the boiler and pumps - any kids who find this out by taking a shock directly to the mouth aren't likely to be left in a fit state to joke about it.

          They could have used just a signal voltage, but the people who invented them aren't inclined towards modern sorcery like electronic circuitry, so just driving the load directly probably seemed like the correct approach to them. This is ironic when today many thermostats are WiFi devices that connect to the Clown - the kid who understands how to properly secure your network can't be trusted with gas burners and the engineer with the scars to prove they respect the elemental forces (in the case of a typical heating system - fire, water and electricity) hasn't the faintest clue what a PAKE is...

          • jwz says:

            Reminds me of that histogram [citation needed] of returned WWII planes' bullet holes, where someone said "these are where they get shot, reinforce these areas" until someone else said, "no, these are where they survived and came back, reinforce the other areas."

            • Nick Lamb says:

              Abraham Wald. The reality is as usual more subtle than the Just So story you've likely heard in a presentation or whatever. But at least the basic story isn't pure fantasy (like most evolutionary psychology or a lot of Biblical parables somehow still being used in presentations in the 21st century).

              https://en.wikipedia.org/wiki/Abraham_Wald links (eventually) a PDF of a reproduction of Wald's hairy statistical model "A Method of Estimating Plane Vulnerability Based On Damage of Survivors" - which was indeed really used to make this sort of decision in WWII. I thought my grasp of stats was pretty good but I don't follow it entirely, however even the bits I do understand are a lot more than "reinforce the other areas".

              A key insight is that in fact there's just no way the enemy is like "Hey, let's shoot the lower part of the rudder on this aicraft". They're shooting at the whole plane, and so a reasonable approximation is that any part of the plane they actually hit must be statistically related to how big that part is. If the big bits of the plane are disproportionately undamaged on returning aircraft, it can't realistically be because no planes were hit in those parts, so those must make up more of the lost aircraft, and vice versa.

          • thielges says:

            I was shocked to discover that a thermostat I’m installing directly switches the raw 20A 240V power to the new baseboard heater. Most thermostats switch a safe and sane 24VAC that is stepped up to full power within the heater chassis.

            Even stranger is the old built-in gravity gas heater that seemed to work on no power. When it stopped working the first thing I tested was for the 24V power expected to drive the thermostat. My meter read “nothing” so I assumed that the 24 V transformer burnt out or otherwise lost power. But there was no transformer to be found. The circuit was a simple two wires between the thermostat and the solenoid valve on the heater. How the heck does that work?

            Turns out that it is an old fashioned millivolt system. The reason my voltmeter read “nothing “ is that I had it in the range expecting 24V when the actual signal was more like 450mV. Power is generated by a little thermopile that continuously roasts in the heat of the pilot flame. Amazing that thing can generate enough power to drive a big clunky solenoid valve. As a bonus that heater continues to work even when the power goes out.

      • jwz says:

        Me too, age 8-to-12 or so, building a phone from scratch, with a telegraph tapper cut from a literal tin can as a dialer, based on vague and dangerous instructions from a book published in 1915 that I bought at a flea market, that I still own because I never throw anything away:

        I also learned how to construct a telegraph between the house and the barn using only the barbed wire that already existed! It was very practical! (* Note, did not have a barn.)

  5. Lloyd says:

    In this era of love in the age of coronavirus, how do we know that the payhone is sanitised for our protection?

  6. Richard says:

    I love these sort of books, thanks for sharing. I'm particularly fond of the articles that start "The accompanying sketch illustrates a two-cylinder, single-action poppet valve steam engine of simple construction that can be made at home by the average boy having a few tools:"

    Trivially straightforward. Next item: Removing Ink Stains using Ordinary Milk !

  7. K J says:

    If you decide that you want to drive the ringer, I would suggest a ring generator (also sometimes called a ringing generator) that you run the output of through a relay. That will isolate you from most of the EMF weirdness that can fry your electronics, particularly if you use a relay on the other side of an optoisolator. I've done similar to drive ringers as part of art installations, and the only issues I've had have been with bad connections between the transformer and the ringer. The electronics and relays have survived far longer than I have needed.

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