Having had—and seldom worn—an Apple watch since 2019 or so, I often found myself liking the idea of a smarter-than-typical watch but never quite aligned with the notification-heavy and tethered nature of the modern smart watch.

A few months ago I came across this blog post by John Graham-Cumming in which he performs a literal engine swap of his Casio F-91W with a mainboard from Sensor Watch (purchased from their Crowd Supply page).

This sweet little board is a drop-in replacement featuring:

• An ARM Cortex M0+ microprocessor
• About a year's worth of battery life on a single CR2016 cell
• A very accurate 32.768 KHz crystal
• Red-ish, green-ish, and yellow-ish LED backlighting
• Access to the original piezo buzzer with some light soldering
• A 9-pin connector to access the I²C bus and five GPIO for additional sensing

So yes, I ordered an F-91W and a Sensor Watch mainboard with a thermistor sensor board.

Surgery

What's nice is that all you need to perform this transplant is a small Phillips screwdriver, some tweezers (a nice-to-have), and a soldering iron/solder to move over the piezo buzzer clip.

This video from the creator, Jose Castillo, is more than adequate to get things moving:

Firmware

By building and flashing the community Movement firmware onto the board, you can leverage their toolchain to configure general functionality, and even design your own watch faces.

If all you're interested in is choosing and ordering the set of existing watch faces, you can do so via movement/movement_config.h and re-compile/re-flash the firmware.

My current watch face array:

const watch_face_t watch_faces[] = {
    simple_clock_face,
    countdown_face,
    stopwatch_face,
    thermistor_readout_face, /* Requires the thermistor sensor board */
    thermistor_logging_face, /* ^^^ */
    pulsometer_face,
    day_one_face,
    sunrise_sunset_face,
    moon_phase_face,
    preferences_face,
    set_time_face,
};

The Sensor Watch board has exposed traces such that you can plug it directly into a USB Micro B cable and flash new firmware via the build-in UF2 bootloader. Easy.

Docs are here.

Enjoyment

I highly recommend this project if you're losing interest in smart watches, but still like the idea of a tiny ARM computer on your wrist.

My Crowd Supply order shipped within a week—so this project is seemingly no longer in the pre-order phase!