RotaryPhoneDataEntry

Code and instructions for converting a rotary phone to a USB numeric data entry

Rotary phone "Numeric keypad"

by Duane Benson, September, February 2024

This configuration uses:

Unexpected Maker TinyS3, ESP32 based microcontroller board, CircuitPython V 8.X

1950's vintage Bell System model F1 rotary desk phone

Written in CircuitPython

Read more in the accompanying article in Design News, by Duane Benson

Rotary phone dials (yes, this is where the term “dial” a phone comes from) transmit numbers as a series of

electric pulses. When you pick up the handset, a current loop is created between the phone and the local

phone exchange. If I recall correctly, the two active conductors, labled tip and ring carried 48 volts at

low current when off hook (the phone handset was picked up). Each number in the dial would have a momentary

switch to break the loop and the dial is spring loaded. The break is approximately 40 milliseconds long with

a 60 ms pause between breaks. The timing is regulated by the mechanical clock spring that spins the dial

back to its home position.

If you turn it to the number 3, for example, it will spin back to its home position and break the current

loop three times, once for each number it passes. Zero (0) is in the last spot and gets ten pulses. That

means that in an analog rotary phone system, numbers one through ten are represented, but the MSB for ten

is essentially stripped off to turn it into a zero.

Note that my naming convention uses a leading capital for hardware pins and a leading lower case for constants and variables

While Python doesn't really have a constant, I use a leading lower case "c" to indicate that it is supposed to be a constant

Diagnostic "print" functions will only show if you connect a terminal application in your PC to

the USB/Com port from the TinyS3.