Sensor Watch, a replacement board for the legendary Casio F-91W wristwatch

Casio's famed F-91W watch, beloved of terrorists, is the frequent subject of ingenious modifications, including a full smartwatch conversion. Now you can purchase a kit board you can swap in yourself to give the zillion-selling timepiece superpowers.

Sensor Watch is a board replacement for the classic Casio F‑91W wristwatch. It is powered by a Microchip SAM L22 microcontroller with built-in segment LCD controller. You can write your own programs for the watch using the provided watch library, program the watch over USB using the built-in UF2 bootloader, and then install the board in your existing watch case to run your own software on your wrist.

I like that it keeps the original display, but now you can write programs for it. HeLlₒ