What would your ideal smartwatch look like if you owned every sensor reading and every line of firmware inside it?
That is the question GitHub maker Spectrewolf777 set out to answer with the Stm32l4-Watch. Unimpressed by the closed nature of the Apple Watch and Galaxy Watch, he designed a wrist wearable from a blank PCB up. It will not out-polish a commercial watch, but it hands the wearer something those devices never will: full ownership of the hardware and the code.
What is actually inside it?
The watch runs on an ultra-low-power STM32L4P5 Cortex-M4 microcontroller paired with an nRF52805 Bluetooth Low Energy chip for wireless links. A 1.28-inch round touchscreen at 240 x 240 handles the watch face. The custom PCB also carries a dense sensor suite: a six-axis IMU, a separate accelerometer, a magnetometer, a barometric pressure sensor, a humidity and temperature sensor, a digital microphone, a high-precision real-time clock, a battery fuel gauge, and onboard NAND flash for logging.
How do you talk to the hardware?
Using TinyUSB, the watch enumerates over USB-C as both a virtual serial port and a USB mass-storage device, so recorded audio and config files copy off it like any thumb drive. The firmware already covers step counting, timekeeping, a stopwatch, tilt detection for the UI, and serial command handling. It can also emulate a USB keyboard or mouse, or run as a BadUSB platform for hardware-security testing. Over Bluetooth it pairs with a phone for weather sync, or acts as a controller for Home Assistant and ESP32 gadgets.
Try it yourself
The hardware design is fully open source, so you can read the schematic, tweak the GPIO wiring, or fold in your own sensor. The next revision shrinks the board from 44 mm to 40 mm and moves to an STM32U5 processor, so now is a good time to fork it. Start with the build notes and photos on the Hackster write-up, grab an STM32 dev board and a round IPS display to prototype the face, then breadboard the IMU over I2C before you commit to soldering a full PCB. You can source your STM32, IMU, and display parts at circuit.rocks to get started.
Frequently Asked Questions
What microcontroller powers the Stm32l4-Watch?
It is built around an ultra-low-power STM32L4P5 Cortex-M4, paired with an nRF52805 chip for Bluetooth Low Energy connectivity.
What sensors does the DIY smartwatch include?
A six-axis IMU, accelerometer, magnetometer, barometric pressure sensor, humidity and temperature sensor, digital microphone, real-time clock, battery fuel gauge, and onboard NAND flash.
What will I learn if I build this?
You will practice custom PCB design, low-power MCU firmware, integrating I2C and SPI sensors, and building USB device stacks with TinyUSB, all skills that transfer straight into an ECE thesis or capstone project.
