The Sensor Watch

Some people like to wear a watch to make a statement. What statement does someone wearing a Casio F-91W make? It's ambiguous. Perhaps no statement at all. A product of the late 20th century, it's robust, cheap, and almost invisible.


I'm very fond of mine. So when over on Twitter Joey Castillo showed his prototype update for the F-91W I was definitely interested. There's something very appealing about an everyday object that's been secretly enhanced.

And to my delight he offered to send me a hand-made early prototype, and it arrived last week.


It comes with a very clear set of instructions on how to install it in your F-91W, replacing the original 90s-design electronics. This is clever. By reusing the case, battery clip and display, the Sensor Watch avoids many of the tricky mechanical aspects of open source watches.



What's the advantage over the stock F-91W? Well, quite a lot:

• a 32-bit ARM Cortex-0 microcontroller with 256K flash and 32K RAM
• 2-color LED, a big upgrade from the famously puny green backlight
• Micro-USB port, just plug it in and it appears as a flash drive
• A complete SDK, documentation here
• All open-source, of course. I pulled his repo, typed 'make' and it built

So why is it called the Sensor Watch? Well, it's much more than a watch that you can program. Amazingly, there is room on the board for a flex connector, and room in the case for small peripherals. It's actually a tiny sensing platform. The 9-pin connector carries power, I2C and some GPIOs. There are a few reference designs provided, like this temperature/pressure sensor:


It's pretty early days for the Sensor Watch project, and I'm really looking forward to just having it around as part of everyday life.  As a small contribution I have written a software emulator. It runs in the browser, using Emscripten to run the C application that drives the watch. There's some Emscripten and JS glue to connect the running emulation to the web page.

You can run the demo here: https://excamera.com/files/sensor-watch/segmap.html
And see this application's code in the Sensor Watch repo here.

The Sensor Watch github repository is here.

Thanks for reading.