Recreation of John Giorno’s Dial-A-Poem
https://www.moma.org/magazine/articles/884

Made at Recurse Center with @joshestein

Person dials a number, a corresponding poem is selected based on the number dialed and that's played through the speaker in the phone handset.

We used a combination of this tutorial video and this wiring diagram to figure out how to connect the phone to the pi.
Orginally we had the button connected to GPIO 18 as per the video but we later changed it to 17 because the amplifier wiring also uses GPIO 18 and it's not possible to use one pin for both input and output.

To determine which number was dialed we measured the time from the release of the dial until it had returned to it's original position.

• We knew when it was released because at that point we could see on the oscilloscope the signal on one of the pins (count_trigger) dropped to zero.
• We knew it was back to the original position because at that point the signal on the other pin (button) dropped to zero.
• We figured out the time range for each dial just by dialing each number a bunch of times and logging the count.

We had to use the lite version of the raspberry pi OS because the amplifier doesn't work on the full version.
(See 2nd warning message here - https://learn.adafruit.com/adafruit-max98357-i2s-class-d-mono-amp/raspberry-pi-usage)

Docs etc.

• amplifier wiring info
  https://learn.adafruit.com/adafruit-max98357-i2s-class-d-mono-amp/raspberry-pi-wiring
• diagram of raspberry pi gpio pens
  https://www.raspberrypi-spy.co.uk/2014/07/raspberry-pi-b-gpio-header-details-and-pinout/
• rotary phone wiring diagram
  https://www.telephonecollectors.info/index.php/browse/wiring-diagrams/western-electric/500-series-wall-telephones/2186-500-series-wall-telephones-554a-tl/file
  (full library of US rotary wiring diagrams - https://www.telephonecollectors.info/index.php/browse/wiring-diagrams)
• video on how to connect a rotary dial to a raspberry pi
  https://www.youtube.com/watch?v=JChn7tV_qBk
• orignal audio recording used
  https://www.youtube.com/watch?v=_21D3TE7YQk

Parts

Rotary dialler

The dialler has 4 pins. One white and the green go to ground. The other two pins are blue and white—the blue connects to pin 23 and the white to 17.

There is one signal that goes high when you start rotating the dial (start dialling). The problem with this is the time taken to move a number to the end of rotation is quit variable - you could rotate very slowly or vary quickly.

There is another signal that is always high but goes low when you release the dialler after reaching the end of rotation. This is better, because the time taken to spin back to the start position is mechanical. When you release we call start_counting which increments a counter - the total count will then correspond to some number dialled.

Audio

We are using a MAX98357 audio amplifier to drive the speaker. The guide from adafruit is great!. To play the actual mp3 files we are using mpg123. We open/close a subprocess that calls this command with the correct mp3 file.

Hook

The hook is a normally open switch. When you take the phone off the hook the circuit completes (calls when_pressed). Until the phone is off the hook we block other processing. When the phone is replaced on the hook we terminate any open audio playing.