I had an old and broken landline phone, and a desire to build my own device to interface with my computer to play Minecraft and act as a headset. This was one of my first fairly involved Arduino projects where I had to learn about some design choices in consumer products instead of just blinking an LED.
investigation
Taking apart the phone, I discovered that the buttons were connected to the board via a transparent film, with (what seemed like) carbon pads on the buttons to complete the circuit. Some research led me to believe that this was simply a button matrix, which allows many inputs/outputs to be shared among fewer pins. More information is available in the YouTube video at 1:24.
I mapped out the pins by hand using my eyes and a multimeter. Below is a diagram showing which pins are connected to each button in case you want to make your own for some reason ????.
The Arduino Pro Micro was chosen because it features and integrated USB interface, allowing native Human Interface Device support and easy reprogramming/debugging.
build
Materials
- Telecom T200 Mk 2
- Arduino Pro Micro
- USB sound card
- Perfboard
- Soldering equipment
Knowing the pinout for the button matrix, the connector was placed onto a piece of perfboard and hooked up to the corresponding 0-12 pins on the Pro Micro (shown below).
The handset contains a speaker and what I suspect to be a carbon microphone. This connects to the outside of the phone, so the jack was connected to a cheap USB audio card I bought off of eBay. All the parts were hot glued inside the case and USB cables were routed out from the Pro Micro and the sound card.
The code simply polls all the keys to check which buttons are being pressed, rather than an interrupt based process. This was done mostly for simplicity and I didn’t require fantastic latency characteristics.
The phone works great as a programmable keyboard. I currently have it set up to map the WASD area and some other hotkeys for playing Minecraft. The microphone has a very old-timey sound, which isn’t surprising since there was basically zero consideration into preamp quality or matching the microphone.
Upcycling the phone was a great project to learn some electronics fundamentals and apply some skills I had learnt at university.
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