Introduction: Modded Casio F-91W

About: Innovation engineer. Electronic and DIY enthusiast, with personal interests in retro tech, board games and old cars.

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


Required tools :

Optional tools :

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

Step 1: Disassemble the Watch

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.

Step 2: Negative Screen

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

Step 3: LEDs Modifications

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)

Step 4: Clean & Reassemble Everything

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

Step 5: Enjoy & Wear Your Modded Casio F-91W

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 !

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