Wrist Sensor Disc
This was my first attempt at building something for personal health tracking. I wanted a way to measure heart rate from the wrist, like a fitness tracker, but without having to give up wearing a normal analog watch. So I put together a small disc-shaped module that could stick under any watch using a thin silicone adhesive layer, letting it sit against the skin and collect PPG data quietly in the background.
The first version was simple: a coin-cell battery, a xiao nRF52840 board for BLE, and a MAX30102 optical sensor. It wasn’t elegant and the adhesive wasn’t perfect, but it worked well enough to stream basic heart-rate data to a phone.
After playing with it for a while, I redesigned the system with my own PCB using a smaller nRF module and upgraded the sensor to the MAX86141, which offered better signal quality and more control over LED drive. The later version was slimmer, cleaner, and more reliable, but the core idea stayed the same, add health tracking to a normal watch without changing how it looked.
This was not my best work, but gave me my first exposure to pcb design, BLE streaming, and analog sensing, and eventually led me deeper into wearable health devices.