The Best Casio Mod

Sensor Watch Pro for Casio F-91W Series Watches.

July 27, 2025

There’s a cool project in the classic Casio space that I’ve wanted to check out for a while called Sensor Watch. The project is headed up by Joey Castillo, and aims to add more modern functionality to the Casio F-91W. The latest revision, Sensor Watch Pro, just started shippping a few weeks ago – so I bought one.

So what is it?

Sensor Watch is a drop-in board replacement for the F-91W and A168 series watches. The board is powered by an ARM microcontroller, has support for extra hardware sensors, and has a two year battery life on a single coin cell. The hardware and software are both open source, too. You can choose to run this thing using the original F-91W LCD display, or for $8 bucks more, you can add an upgraded LCD display with more comprehensive output (which I cannot recommend enough).

As you can see, the upgraded LCD can display much more information than the original F-91W LCD, and is far easier to read:

Sensor Watch Pro showing the current temperature in hell.

What does it do?

Sensor Watch allows you to write your own programs (called ‘watch faces’) to use on the watch with Second Movement, the official firmware. Second Movement includes several watch faces by default, but there are many more that you can add. Essentially, you just clone the repo and specify which faces you want to include when you build the firmware, then flash it to the watch with a micro USB cable. I suppose if you really had some gumption, you could write your own firmware from scratch.

The Sensor Watch website has a short list of some of the watch faces that the community has already written:

• The Day/Night Percentage watch face, by Wesley Aptekar-Cassels, shows the current time as a percentage of the way through the day or night.
• The Interval Timer, by Andreas Nebinger, brings nine customizable interval timers to Sensor Watch for high-intensity interval training or time management.
• The Menstrual Cycle watch face, by Joseph Borne Komosa, implements the Calendar Method for period tracking and fertility window estimation.
• The Couch to 5K watch face, by Ekaitz Zarraga, guides you through thrice-weekly training sessions of walking and running, to get you into shape.
• The Wordle watch face, by David Volovskiy, brings the popular word game to Sensor Watch.
• The Tarot watch face, by Jeremy O’Brien, can deliver a three card tarot spread using just the major arcana, or the full deck.

Why I like it

As you can see from the list above, there’s more that can be done with the watch than one might expect, albeit in a very limited capacity – but that’s exactly why I like it. Push notifications are invasive enough without them being on your wrist. I don’t want Apple sending my heartrate and sleep health to my insurance provider. I don’t want to be tempted to glance down at my wrist to read a text in the middle of a conversation, or have another thing that I need to remember to charge.

Joey Castillo sums it up well with, “I wanted a smartwatch that was less smart, and more watch.” So many of the “open-source smartwatch” projects are just trying to emulate modern wearables in a slightly more ethical fashion, but aren’t really presenting anything different. Sensor Watch isn’t really trying to be a “smartwatch,” but a classic Casio that you can control and customize in ways you can’t do with the original. And that’s exactly what I wanted.

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