Ollee Watch Turns A $20 Casio F-91W Into A Smartwatch

The Casio F-91W is one of the most popular watches on the planet, mostly because it’s cheap, plentiful, and just simply a classic anyone can afford.

Ollee Watch, a small Canadian outfit/maker, took that hackability and turned it into a packaged product: a replacement mainboard that drops into an F-91W or A158W and turns it into a Bluetooth-connected smartwatch without changing how the watch looks or feels on your wrist.

What They Built

Ollee Watch is a custom PCB, built around a Cortex-M4/M0+ processor with Bluetooth Low Energy, sized to fit inside the F-91W’s existing resin case in place of the stock Module 593 board. Once installed, the watch keeps its classic LCD and buttons but gains phone-synced timekeeping, world time, a step counter with daily goals, pulsometer-based heart rate readings, hourly temperature logging, an RGB backlight, and a handful of built-in games including Blackjack and a five-card draw poker mode. Battery life runs up to 10 months with daily tracking, or several years with motion sensing turned off.

How They Did It

The install itself is squarely in DIY-mod territory - opening the case, swapping the movement board, and reassembling the watch — which is exactly the kind of teardown F-91W owners have been doing for decades, just with a more capable board on the other end of it. Worth being precise about one thing: despite some coverage describing the project as open-source, Ollee Watch ships as a commercial kit with its own companion app, and no public firmware or hardware source has surfaced alongside it. When the project hit Hacker News, several commenters noted its resemblance to the genuinely open-source Sensor Watch board and asked whether credit was due — a fair question for anyone deciding which F-91W mod fits what they actually want from the project.

Go See It

Ollee Watch’s full spec sheet and ordering options are on the official site, Yanko Design covered the launch, and the Hacker News discussion is worth a read for the community’s take, including pointers to fully open-source alternatives like Sensor Watch.