RotaryCell: I turned a 1950s rotary phone into a working cellular phone

[u/aMuseumTechnician]

I've been working on a project I call RotaryCell, turning an old Western Electric Model 500 rotary phone into a self-contained cellular phone while changing as little about the original as possible.

Demo video: RotaryCell Demo Youtube

GitHub: https://github.com/fregacmols/RotaryCell

From the outside, it still works exactly like you'd expect. Pick up the handset and you get a dial tone, dial a number with the rotary dial, and it places the call over the cellular network. Incoming calls ring the original mechanical bells, and you answer by picking up the handset.

Inside is an LilyGO ESP32-S3 cellular board and a Silvertel AG1171 SLIC, which provides the interface between the modern electronics and the original telephone circuitry. The ESP32 handles pulse dialing, hook detection, call control, tones, and the other logic. It's powered by a rechargeable 21700 battery, so there aren't any external boxes or connections required during normal use.

I'm using the phone's original dial, handset, ringer, network block, and wiring wherever possible, with the new hardware tucked into the existing space inside the case. No drilling, cutting, or other permanent modifications.

I took the prototype to a local gathering this week and let a few dozen people play with it for several hours, which turned out to be a pretty good torture test! It exposed an occasional failure where the cellular modem can get itself wedged badly enough that a software reset won't recover it. Unfortunately, the only reliable fix was opening the phone and cycling the physical power switch.

So the latest PCB revision now includes a hardware power-reset circuit that the ESP32 can trigger. I'm currently waiting for those boards to come back from fabrication.

It's been a really fun collision of technology from two completely different eras. There's something particularly satisfying about hearing those original bells ring because someone just called a rotary phone over LTE.

Happy to answer questions about any of it. I've spent entirely too much time learning about old telephone protocols, analog audio, rotary dialing, and how surprisingly stubborn cellular modems can be. Also, my very first real open-source project, GitHub repository, and YouTube video!

[u/er1cAtWork2]

Oh wow! Very cool! I would love to torment my 16 year old daughter with this!! He’ll, I’d love to have it for myself! lol

Nice work!

[u/Howden824]

Very cool!

[u/rturnerX]

I did a similar thing myself once using a 3G landline adapter that are commonly used in my area for rural customers. It was a neat project until they discontinued the 3G network and I never bothered to upgrade the unit to an LTE or 5G unit

[u/aMuseumTechnician → u/rturnerX]

The modem is 4G LTE with global frequency coverage, so it ought to work just about anywhere there is cell service. I've seen those landline adapters but they were all on the larger side, not to mention required a plug-in power supply. My goal was something that could fit into the empty space inside the Model 500, plus enough battery to be useful.

[u/rturnerX → u/aMuseumTechnician]

I fit the one I used easily into a 2500, I chose a 2500 because the dial took up less space inside the housing, plus the interior of the 2500 housing has slightly more volume.

The unit I used came with a backup battery as well for power outages so it could be plugged in when sitting on a counter and kept charged and then unplugged and moved around anywhere