Community & DIY
Mods
Products that replace or upgrade the F-91W's internals or case, and common customisations you can do yourself.
Products
Ollee Watch
A drop-in circuit board replacement for any Module 593 watch that adds Bluetooth, step tracking, an RGB backlight, world time, five programmable alarms, and a few watch faces including some games. No soldering needed and it takes about ten minutes. Available as a DIY kit or already installed in a new F-91W.
olleewatch.comSensor Watch Pro
An open-source circuit board replacement by Joey Castillo, built around an ARM Cortex-M0+ microcontroller and programmable over USB. The community firmware, Movement, includes dozens of watch faces covering things like moon phase, sunrise/sunset, and interval timers. Battery life stretches past two years partly because there's no Bluetooth. The Pro variant adds an RGB LED, an infrared sensor, and a modular sensor connector, all without soldering.
sensorwatch.netHeeler Watch Co Ti-1
A machined Grade 5 titanium case that takes the stock F-91W module, buttons, and caseback. It comes in raw, blasted, brushed, and polished finishes with a sapphire crystal and a choice of dial prints. Available fully assembled or as a case kit. Made in Chicago.
heelerwatchcompany.comCustomisations
NATO/Zulu strap swap
The integrated resin strap is 18mm and can be swapped out with a pin tool for any compatible strap. It's the most common mod by a wide margin and has its own subreddit (r/F91Ws_on_NATOs).
Negative display
The LCD polarising film is removed and replaced with a rotated piece of linear polarising film, which inverts the display so the digits appear light on a dark background. No soldering involved. Replacement film is cheap on Etsy or can be cut from old 3D cinema glasses.
Backlight spreader
A thin plastic diffuser sits behind the LCD in place of the stock backing, spreading the single LED across the whole display rather than just one corner. No soldering and no effect on battery life.
LED swap
The stock green LED is replaced with an SMD LED in another colour. Red, blue, white, and warm white are the common choices. Brighter than stock but requires soldering and draws slightly more battery.
Colour display filters
Thin adhesive gel filters applied over the crystal tint the display. They work well alongside the negative display mod. Etsy sellers offer packs covering most colours.