I had an old Nokia 1100 phone laying around and wanted to find some way to reuse it, so I decided to see if I could turn it into a smartwatch. Taking it apart, it had a nice, hackable screen as well as a little vibrating motor for notifications. With help of a few resistors, I was able to get the screen working using an Arduino. Then with a bluetooth module and a little bit of code, I was able to send it phone and text notifications from my phone (as well as time and date, of course). A 3.7v rechargeable battery and a charging board was used for power. Adding a 3d printed case and a watch strap made it a fully viable (if not thick) smartwatch!
While there are many different ways to make DIY smartwatches, this one was intended to revolve solely around recycled parts from the Nokia Smartphone. The biggest challenge was making the old school LCD work with the Arduino, while still keeping it in a reasonably small watch form factor. This was my first project that used an Android phone to communicate with an Arduino, so it was fascinating seeing the nearly unlimited potential and possibilities combining those two devices. If I were to do this project over again, I would change two things. First I’d opt for a thinner battery. Second, I’d make a full fledged circuit board with surface mount components. Between those two changes, it should be a much thinner smartwatch.