Casio F91W Mod scene
The Casio F91W, the most sold watch in the world, is sold in many colour variants.
Perhaps unsurprisingly, to customise the F91W beyond the officially-sold variants, many mods exist for the F91W, such as strap swaps, backlight mods, and even boardswap mods (the latter of which being particularly exciting).
Perhaps the best-known mod is the negative display, which inverts the default black-on-white display to a dark mode display.
There are many sellers on various marketplaces selling negative display Casios (as pictured), however these generally have a significant markup. Doing this mod yourself is really rather cheap, and in my experience, is very easily done.
Prerequisites
Required for this mod:
- Casio F91W
- Surgical scalpel
- Microfibre cloth & IPA
- Adhesive linear polarising film
- A screwdriver (of course..)
These are all easily purchased on any marketplace, such as eBay.
How it works
An LCD screen uses liquid crystals between two polarising layers. When a voltage is applied to the liquid crystals, they change how light passes through them.
A liquid crystal display works as follows:
By removing the front polarising film, and replacing it with one rotated by 90 degrees, this will reverse which parts of the display transmit light.
The modding process
Firstly, disassemble the backplate of the watch by undoing the four screws.
Then, remove the board from the watch. There is a battery encasing attached to the board via 4 clips; you can use either your screwdriver or the scalpel to undo each clip.
Next, lift the board from the white plastic part.
To remove the display from the white plastic, use your thumbnail to pull down the clip labelled 3 in the image. This will allow it to easily be removed.
For those curious, the pins labelled 2 on the board are the COM and SEG lines, which provide voltage to the segments that correspond to each COM-SEG combination.
The voltage is applied to the display itself through a zebra connector, labelled 1.
I feel the need to comment on how adorably tiny the zebra connector in the F91W is.
Now, the crux of the mod: to replace the polarising film!
Visually find a corner of the polarising film, then gently peel this corner with the scalpel.
I say gently, but I have been pretty rough when doing this, without scratching or damaging the screen, so don’t be too worried.
Once you have a lifted, simply peel the layer off with your fingers.
Now to apply your own polarising film.
First, know that most polarising film comes with a protective layer on both sides.
Find which side of the film is adhesive by cutting off a tiny section, peeling off both protective layers, and feeling which side is sticky.
Then, take the film sheet, and with the adhesive side face-down and the display face-up, place the film over the display and rotate it until you find which angle makes the display appear the blackest.
In my case, this happened to be at a 45 degree angle, but it will differ based on your polarising film.
Then simply cut out a display-shaped section of polarising film, remove the adhesive protector, and apply the film to the display.
Apply it like you would a screen protector to a phone; start at once side and press it down across the length, to remove air bubbles.
Afterwards, remove the front protective layer.
I would highly recommend spraying a tiny amount of IPA onto a microfibre cloth, and wipe down both the display, and the transparent part of the watch shell, to remove smudges before reassembling.
The hard part is now done, so just reassemble the watch.
Make sure the battery is inserted the correct way, with the negative terminal making contact with the board.
Also ensure that when putting the reassembled board unit back into the watch shell, that the metal cylinders comprising the buttons are fully pressed out. The image shows them pressed in: when they are like this, use your screwdriver to fully push them outwards.
Put the backplate back on, and your negative LCD F91W is complete!
To conclude…
The negative mod is a staple of F91W modding, and is an easy place to start.
It’s not perfect, however: the negative display has somewhat poorer viewing angles, and is less visible in dark lighting conditions. So it’s really a tradeoff between aesthetics and practicality.
Still though, it is much cheaper (and more enjoyable) to do-it-yourself than to pay an Etsy seller a 4x markup.
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