Mass production is responsible for making consumer electronic devices affordable in a way that simply isn’t achievable in the one-off projects that makers usually focus on. There just isn’t anyway to get components at the same price as a manufacturer that is ordering them in quantities of hundreds of thousands. Luckily, we can take advantage of their pricing and hack their products to fit our needs. That’s exactly what Raphael Baron has done with his incredible work on hacking a cheap fitness tracker bracelet.
These fitness trackers are incredibly inexpensive, and can easily be found for $10 or less at big box retailers. Most of them are identical or very similar, and pack quite a bit of tech for the price. Baron’s had a 96x32 OLED display, a battery that can be charged by USB, an accelerometer, a vibration motor, and BLE connectivity. Most of the fitness tracker bracelets you can find will have a similar set of components.
After pulling apart the device and doing some Googling, Baron found that his model uses a Nordic nRF51822 — a relatively common 32-bit Arm Cortex-M0 microcontroller. That can be programmed via Arm’s SWD (Serial Wire Debug) interface, which allowed Baron to start talking to the board. After wiring an LED to two of the pads, he was able to program a simple “Hello, World!” blinking light.
With access to the microcontroller’s code, he was able to systematically control more of the components. First the digital I/O, then the OLED display, analog I/O, button, and BLE. Finally, he got all of this working with the Arduino IDE, and has even provided code to get you started. If you want to hack your fitness tracker, Baron’s work is absolutely invaluable and very impressive.