When [rbaron] started a new job, he got a goodie bag. The contents included a cheap fitness tracker bracelet that used Bluetooth LE. Since this is Hackaday, you can probably guess what happened next: hacking ensued.
For something cheap enough to give away, [rbaron] claims it cost $10, the device has quite a bit in it. In the very tiny package, there is an OLED display, a battery, a vibration motor, and a Nordic 32-bit ARM with BLE. The FCC ID was key to identifying the device. Opening the case, which was glued down, was pretty difficult, but doable with a hair dryer and a knife.
Obligingly, the PC board had pads for the serial wire debug (SWD) protocol. This is probably for programming and testing in production, but [rbaron] likes to think it was some unknown engineer’s gift to us hackers.
Once you have the debugger working, the rest is just sweat work. However, the post details a lot about how to manipulate the hardware, including driving the OLED and using Arduino code thanks to the arduino-nRF5 project. He even adapted the Adafruit OLED library to work with the wrist band’s quirky display.
If you have a similar fitness tracker, this post takes care of a lot of the legwork. If you have something else, the process is illustrative of how you can start with something as simple as an FCC ID and wind up with total control of the device. Of course, that isn’t always possible depending on what’s inside and how it is locked up or obscured, but — especially these days — your chances of finding some commodity part inside that you can access is higher than its ever been.