An electronics/software project providing all the information about how to convert an old rotary phone into a modern cell phone.
Who doesn't know it, you are at a party and everyone is checking their phones or swiping through their Instagram. I find it very annoying when people stop communicating and instead dive into the digital world.
Somehow I have always felt jealous of people growing up before the digital age. Don't get me wrong, I really enjoy that tinkering and developing new things is very easy to do nowadays, but sometimes we seem to forget that our smartphone is not our best friend 😅
OK, enough of me rambling on. Let's briefly talk about how it all started and then get into the details.
When my girlfriend's grandma tidied up her attic, she found an old rotary phone they used back in the 80s (it seems like all phones here in Austria were olive green back in the days 🤨). As my girlfriend knows me very well, she saved it from the trash and gave it to me saying, "I know you love this old electronics stuff ..." (she really knows me well 😄). In this moment the idea was born to convert the old rotary phone and bring it's internals to the 21st century. What I always wanted to preserve were the looks and feels of an old rotary phone though. In the beginning this was just an idea but I really did not have a use-case for it.
The whole thing started to take shape when I was at a party where exactly the things described before happened and I thought to myself, 'How cool would it be if there were no smartphones at all and maybe just an old regular phone (a rotary phone) for emergencies or ordering a pizza'. Then immediately the old rotary phone of my girlfriend's grandparents popped into my mind 🙂
As I work in research, the first thing I always do is, well, research 😃 Someone must have had the same idea before or must have thought of at least something similiar. It is always good to use things someone has had already wrapped their head around. And sure enough, I found something. Sparkfun had a product called Bluetooth Portable Rotary Phone in their portfolio a few years ago. Unfortunately, it got retired. Luckily, they still provide all the details about the circuit they used, as well as information about the rotary dial, the ringer circuit, you name it :) A huge thank you to Sparkfun for providing all this information for free.
With research out of the way, I started to think about the components I would need for such a project. As I wanted it to be quick and simple, I mostly used modules instead of developing my own PCB. So really any hobbyist can build it. The hardware components I used are as follows
- Rotary phone
- SIM800(LC) (2G Cellular phone module)
- LN293N (H-bridge)
- Arduino (Pro mini, nano, etc.)
- Voltage boost module (XL6009, SX1308, etc.)
- Li-Po (Li-on) battery
- Li-Po charger module (TP4056)
- USB-Serial Converter (optional)
Those are the essential components. Of course you will need some more passive components like resistors and capacitors, but I assume anyone trying to build this project will have some laying around.
In the schematic you can see that I mostly used PCB modules to keep things simple. The only custom built section is the part where the microcontroller interfaces with the H-bridge. It has all been layed out on a perfboard with connectors for the individual modules.
The firmware has been developed in Arduino C++, again to keep things simple for everyone. The code is by no means optimized or anything and I know C runs a lot quicker on those tiny 8-bit microcontrollers. I personally find the Arduino enviroment very handy for quick prototyping and so on, as a lot of useful libraries exist.
The main firmware structure is oriented towards a finite-state-machine approach. I found a very nice tutorial by a YouTube channel called Playful Technology. He uses an old rotary phone as a prop in an Escape Room game. Definitely worth a watch 🙂!
For an overview I created a state diagram of the main program flow (I tried to use mermaid for the first time, this is why it looks a bit messy 😄).
The main Arduino loop() consists of 4 simple update routines which check the switches, update timers, receive data from the SIM800 module and update the state machine.
void loop()
{
updateSwitches();
updateTickers();
updateSIM800();
updateStateMachine();
}Altough the code is by no means optimized, the main loop runs with an update rate of > 10 kHz, which I think is more than enough. Thanks again to Playful Technology for leading my to a finite-state-machine approach. I also found a nice tutorial regarding FDMs on arduinoplusplus if you are interested.
Many thanks also to
- Michael Contreras for providing a very easy to use arduino-timer library
- Thomas Ouellet Fredericks for providing a very cool debouncing library for switches Bounce2
- Guys at Sparkfun for providing a great deal of information about rotary phones and how a conversion can be done Bluetooth Portable Rotary Phone
- Playful Technology for providing a very detailed tutorial on how to convert an old rotary phone
MIT License
Copyright (c) 2022 Christoph Schmied
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.