Modded Casio F-91W

Hi everyone,

On this Instructables, I will explain how I manage to modde one of the most iconic watch ever made, the Casio F-91W.

I made 4 easy modifications on my F-91W :

• Change the plastic strap for a black nato
• Modify the color backlight, green light becomes blue
• Add a backlight LED
• Convert display with negative screen

• 1x F-91W watch
• 1x 18mm Nato strap
• 1x LCD polarizer film
• 2x Blue 1206 LED
• 10cm of flying wire for PCB

Required tools :

• A cutter
• Some glue
• Watch repair tool kit
• A set of tweezers

• A soldering iron

Optional tools :

• A multimeter
• A soldering 3rd hand
• An old toothbrush and some Isopropylic Alcohol to clean the PCB

This step is probably the most easy one :

• Remove the strap with the corresponding tool
• Unscrew the 4 bottom screws
• Remove the metal part and the seal
• Turn the watch case to remove the inner part
• Disassemble the battery holder and the screen

Now it's time to start modding.

Follow the steps below to transform a white screen to a black screen :

• Disassemble the screen and the connector from the plastic support
• Use the cutter to remove the initial polarizer film (Do not scratch the screen)
• Take your polarizer film and turn it in front of the screen to find the correct angle
• With your cutter, cut the film to the right dimension
• Stick your film to the screen (Try to avoid bubbles)
• Reassemble the screen and the connector in the plastic support

This step will convert your F-91W which has only one pale green led into a watch with two bright blue LEDs.

I know that the watch autonomy will decrease, but brights blues leds it's much more fun.

So let's start :

• With a soldering iron, remove the resistor closest to the LED
• Replace it with a piece of wire
• Remove the green LED and replace it with a blue one (Pay attention to the orientation of the LED)
• For the 2nd LED, carefully cut two PCB track with the cutter (See picture 4)
• Glue the LED
• Use the PCB pad to solder one side of the LED
• Use two flying wires to connect the two LEDs in parallel (Attention the bare copper must not touch any other component)

The last steps before wearing your watch.

• Clean the PCB with isopropylic alcohol with a Q-tip or an old toothbrush
• Assemble the white screen support with the PCB
• Put back the battery holder
• Replace the seal
• Close the watch case with the corresponding 4 screws
• Replace the strap

To easily read the time, you have to get into the habit of orienting at the right angle your watch.

Despite this it's so cool to have a modded iconic watch on his wrist.

Have a nice day !