# Sensor Watch Pro

14 Hacker News commenters recommended Sensor Watch Pro and 1 criticized it, November 2025 to November 2026. It was recommended in 10 of 13 months. It meets the Hacker Approved badge rule.

**Verdict:** Open-source board that turns a cheap Casio F-91W into a hackable watch with custom firmware and temperature-compensated timekeeping. Critics say precision is not its strength.

## Why people recommend it

- Temperature-compensated, accurate timekeeping (4 comments): "turning a cheap casio into a super-accurate quartz watch with temperature compensation" ([source](https://news.ycombinator.com/item?id=49364256)) "with a few calibrations spread out over a few months, can achieve an accuracy of" ([source](https://news.ycombinator.com/item?id=50114877)) "right up there with the best temperature compensated quartz watches" ([source](https://news.ycombinator.com/item?id=48561091))
- Extra sensors and alarms (4 comments): "I'll take some solace in mine supporting lots more alarms (beep when school recess ends)" ([source](https://news.ycombinator.com/item?id=49363629)) "I really like my F91W case with a Sensor Watch Pro board and accelerometer sensor." ([source](https://news.ycombinator.com/item?id=49366056)) "Use it every day at work, makes counting respiratory frequency painless." ([source](https://news.ycombinator.com/item?id=49371782))
- Custom firmware, open source (3 comments): "An ARM Cortex M0+ board swap for the classic Casio F-91W wristwatch that allows you to write custom firmware" ([source](https://news.ycombinator.com/item?id=48186867)) "It's an awesome project to hack on. Lots of nice people in the community. Highly recommended." ([source](https://news.ycombinator.com/item?id=47422764)) "also OSS as opposed to Ollee" ([source](https://news.ycombinator.com/item?id=49366097))
- Well-known F-91W community project (3 comments): "The incredible Sensor Watch after-market board is really worth a mention on a thread like this" ([source](https://news.ycombinator.com/item?id=45891979)) "no F-91W thread would be complete without a mention of the sensorwatch project" ([source](https://news.ycombinator.com/item?id=46550579)) "Sensor Watch project gets a lot of mileage out of the Casio F-91W display." ([source](https://news.ycombinator.com/item?id=48536783))

## What people warn about

- Limited precision (1 comments): "precision isn't its strength ... it's precise enough for me anyway" ([source](https://news.ycombinator.com/item?id=49366155))

## Where to get it

- [See price on Amazon](https://www.amazon.com/s?k=Oddly+Specific+Objects+Sensor+Watch+Pro+board+for+Casio+F-91W&tag=hackerapprove-20) (affiliate link)
- [Official site](https://www.oddlyspecificobjects.com/)

## Cite as

Hacker Approved, "Sensor Watch Pro", https://hackerapproved.com/p/sensor-watch/ (counts from 1,795,640 Hacker News comments, November 2025 to November 2026).