It's fair to say that very few of us use rotary dial phones on a daily basis — in fact, most of us don't even have landlines — but just because these classic phones are no longer being used for their intended purpose doesn't necessarily mean they're destined for e-waste.
[Scott-28] I was recently sent a particularly well-documented project that turns an antique rotary phone into a digital kitchen timer with an Arduino inside. I'm not sure that “practical” is the word most people would use to describe the finished device, but it's sure to be a conversation starter, and the details of how it's implemented make for interesting reading.
As explained in the README, [Scott-28] I started by using an oscilloscope to analyze the pulses coming from the phone dial. From there it was relatively easy to connect the dial to one of the pins on an Arduino Uno and determine the digits entered by the user. The more difficult part was getting the original bell to work. In North America, a phone ringing requires up to 90 VAC, which is more than a low-power Arduino can handle.
Luckily, he was able to get hold of a Cambridge Electronics Laboratory LS057020 “Black Magic” ring generator: just feed it a 5 volt DC voltage and it produces the ring signal you need. [Scott-28] They point out that the gadget produces a lot of electrical noise during operation, but this seems like a small price to pay given the reduced design complexity.
Once the rotary decoding and ringer were worked out, I just had to write the code to put it all together. [Scott-28] We added a seven-segment LED display to show the current time, two LEDs to indicate whether the numbers are hours, minutes or seconds, and a button to start the whole thing. It rings when the counter hits zero, and of course it stops ringing when you pick up the handset.
We've seen hackers interface with rotary dial phones, turning them into virtual assistants, and who can forget the amazing development of rotary dial mobile phones? Despite being an old technology even to older generations, rotary dial phones seem unwilling or unable to be forgotten, and that's okay.
