The Casio F-91W is a true icon in the world of digital watches. First released in 1989, this classic watch remains widely popular today thanks to its cool retro styling, durability, and affordability. But a growing community of watch modders is taking the customization of the F-91W to new heights through ingenious hacks and add-ons.
It’s been a while since I’ve done any cool mods to the F-91W but after seeing some new projects pop up on the internet I had to place an order on Amazon for a brand-new donor watch to experiment with.
In this article, we’ll explore two must-have mods that can transform your trusty F-91W into a unique, customized timepiece: the Backlight Spreader and the Sensor Watch Lite.
Enhance the Dim LED with the Backlight Spreader
One common complaint about the F-91W is the very dim LED used for the backlight. It barely illuminates a tiny portion of the display.
The Backlight Spreader fixes this issue through a simple yet effective light diffuser. This small plastic rectangle inserts between the LCD and the module housing. It takes the narrow beam from the LED and spreads it evenly across the entire screen. It might not look like much, but trust me, it’s remarkable.
As someone who has gone through the painstaking process of soldering new LEDs into Casio modules, this mod is a breath of fresh air. While you certainly don’t need an engineering degree to solder components, it’s so much easier to do a drop-in-place mod.
Installation only takes a minute. Just pop open the case back, remove the module, slide in the Backlight Spreader, and reassemble. No soldering or mods to the module are needed.
Once installed, pressing the light button reveals a huge improvement. I’m actually blown away by its efficacy. The diffused glow brightens the whole display for easy legibility in any lighting conditions. And it achieves this while maintaining the stock LED’s battery efficiency.
For just $9, the Backlight Spreader is an easy upgrade that makes an immediate impact. No F-91W modder should be without one. Pick it up on Etsy today.
Unlock Total Customization with the Sensor Watch Lite
While the Backlight Spreader enhances the existing module, the Sensor Watch Lite replaces it entirely. This ingenious add-on PCB developed by Oddly Specific Objects turns the humble F-91W into a programmable smartwatch.
The Sensor Watch Lite packs an ARM Cortex M0+ processor with 256KB of storage. After installing it in place of the factory module, you gain complete control over every segment of the LCD through a library of watch faces.
With a built-in bootloader and a clever USB connector built right into the board, you can create and upload your own watch face designs. Or choose from dozens of premade faces offered online, including nostalgic games, moon phases, and sunrise/sunset info (based on your latitude and longitude). It even has a temperature sensor to show current readings.
Despite these smart features, the Sensor Watch Lite retains the F-91W’s outstanding battery life. It runs for over a year on the stock coin cell battery.
Installation does require some soldering to attach the piezo buzzer from the original module. It’s totally optional, but without it, your watch will not beep. However, some modders used a wire or spring to connect it instead. Either way, the process is very straightforward.
Here’s what you get in the latest version of the Sensor Watch Lite:
- ARM Cortex M0+ microcontroller running at up to 32 MHz
- 256 KB of on-chip Flash, with up to 8 KB EEPROM emulation area
- 32 KB of RAM with full retention in low-power standby mode
- 32.768 kHz crystal for real-time clock functionality with alarm support
- Red & green PWM’able LED backlight
- Temperature sensor
- On-board USB Micro B connector
- Reset button with double-tap UF2 bootloader
- Controller for ten-digit segment LCD, plus five indicator segments
- Edge-plated contacts for three interrupt-capable buttons
- Connection pad for piezo buzzer (requires light soldering)
- Fully Open Source
After messing around with the standard, built-in watch faces, I decided to crank out a custom build using a handy online utility. Just choose your watch faces, then build the file, save it to your computer, and drag it over to your watch once you connect it through USB.
Here’s my current daily driver. I built a two-layer set of watch faces (the second layer is accessed by a long-press of the Mode button). The first layer includes:
- Clock – The standard F-91W clock
- World Time – Configurable with selectable two-character codes
- Sunrise/Sunset – Based on your latitude/longitude
- Moon Phase – Moon phase data with an abstract graphic of the moon
- Stopwatch – The stock F-91W stopwatch
- Timer – Nine programmable timer presets
- Alarms – 16 customizable alarms
- Configuration – LED length and color, Timeout, 12/24 clock
- Time Set – Basic time set functionality from the F-91W
…and a cool Zelda or Mario hourly chime.
And the second layer:
- Temperature – A temperature gauge in C or F degrees
- Blinky – An app that shows off the green and red LEDs
- Pulsometer – For counting your heart rate
- Integer – A classic game from Casio calculators
- Coins/Dice – Configurable heads/tails or multi-sided dice roll
- Finetune – Select and configure the precise time
- Nanosec – Adjusts the time based on minuscule crystal temperature
Check out their online documentation to see all the potential watch faces and complications, or just build your own! Pick up the Sensor Watch Lite for about $40 today from Crowd Supply.
Watch Faces – Layer 1
Watch Faces – Layer 2 (Long-press Mode button)
You can view the complete set of watch faces from their website.
Transform Your Casio into a Unique Timepiece
While the F-91W is already a classic and unique timepiece right out of the box, these two mods take personalization to the next level. A must-have, in my opinion.
The Backlight Spreader delivers an instant upgrade to usability, and the Sensor Watch Lite unlocks advanced smartwatch functions. Both are must-have mods for any Casio enthusiast. With these add-ons, you can transform the classic F-91W into a one-of-a-kind modern marvel. They showcase the versatility and potential that keeps this iconic watch relevant over 30 years after its debut.