This instructable is based on the classic Casio F-91W. Many other cheap digital watches have a similar problem with their backlight and the same remedy can be applied to them. This instructable will show you how to change the color and increase the brightness of your LED backlight. This mod will make a WORLD OF DIFFERENCE. Check out the 2nd picture to see the difference. This project will cost you less than a few dollars if you already have a soldering iron.

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Tools required:

1) Small screwdriver

2) Soldering Iron

3) Small tweezers (cheap hair-plucking ones work well)

Materials required

1) 0805 SMD LED (easily salvaged from old electronics or available on ebay in MANY COLORS)

2) Glue (superglue, contact adhesive, any glue will do except paper glue)

Warning: The removal of the LED requires some dexterity and near vision. You should be fairly experienced with a soldering iron before attempting this instructable.

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The Casio f-91w is a classic digital watch launched in 1991 that remains largely unchanged since then. Many of us would probably have owned this cheap trusty watch in our lives. The build quality of these cheap little water-resistant Casios are incredible for its price.

During my short stint of military training, mine survived being dunked in muddy trenchwater for 2 nights without a single complain. Every 5am in the shellscrape the alarm would still beep while i cock my eye and try to see the time with its weak led backlight.

(In comparison, my SAR21 assault rifle was dripping rust-gunk from its muzzle and the charging handle would not budge. The magazine was stuck and couldnt be removed. The integrated "weatherproof" laser was flooded and non-functional.)

My only wish for the F91-W is a better, brighter led backlight. The sickly green LED is so dim its as if Casio was not even trying at all. I know the f105w is a clone with EL backlight, but I'm a miser and forking out twice the price for it seemed decadent. Plus, my F91W wasnt something I would just leave behind.

I felt that there could be an easy way to improve the backlight of the f91w - it turns out, the solution was simpler than i'd imagine.

Step 1: Problem With the Original

The original LED on the 0805 Surface Mount chip that is soldered beside the circuit board. As you can see in the diagram the chip is designed to project light upwards. Only some reflected rays light up the screen and what you get is a sickly dim backlight.

The plan is to remove the original LED and solder another one at 90 degrees to its side so that the LED shines light directly at the side of the LCD screen.

Step 2: Disassembly

Disassembly of the F91W is easy - There are only 4 screws holding the backplate to the watch body. The rest is snapped together and can be pry-ed apart with a small screwdriver fairly easily. See pictures for instructions.

Step 3: Closer Look at the Circuit Board

Identify the LED on the circuit board. The board is sparsely populated as most of the magic happens in the microprocessor encase in black resin. The LED is located near the side. The other components are the led resistor, quartz oscillator, an inductor, and pcb tracks to communicate to the LCD screen.

Step 4: Desoldering

As you can tell, the SMD LED is FREAKING TINY. The 0806 measures 2.00 x 1.25mm (about 0.08 x 0.05 inches). However, the techniques in the following steps in going to help us in this soldering feat.

First, stabilize the board using whatever you have: Helping hands are the best, but I'm a Barbarian and I made do with a heavy spanner and a wooden clothes peg.

Desoldering the LED should be easy if you are not planning to save it for future use. I recommend against trying to salvage the LED because these SMD LEDs are cheap and abundant. Trying to salvage it might cause excessive heat to the board and destroy other components.

Solder Glob method: A glob of solder is used to melt both anode and cathode at the same time, then the entire LED is removed.

Melt a glob of soldering on your soldering iron tip - large enough to encase the entire LED. Lower the Solder Glob onto the LED making sure the glob is in contact with both the anode and cathode of the SMD LED. When you are certain that the solder joint is molten(about 2 seconds), flick the solder glob away with the tweezers. The LED should be encased in the solder glob(see pictures)

Step 5: Obtain New LED

Obtain and Prepare a new 0805 LED. Purchasing it online is much easier. I bought a 100-pack of 0805 LEDs in an assortment of colors for less than $2 USD.

Most importantly, identify the anode and cathode of the new LED. (LEDs are diodes that only work if voltage is applied in the correct direction)

The diagram should illustrate how the markings on the LED indicate the polarity.

Step 6: Glue LED to Board

Soldering such a tiny component is a challenge for even the steadiest of hands. I found that using some glue to attach the component to the board first will allow us to properly align the components and then solder the parts with ease.

Determine which side of the LED is going to be in contact with the board first - respect the polarity of the LED!

In the picture, I am using contact adhesive, but superglue will probably work better. Apply the glue sparingly using a toothpick. Try not to get any glue on the metal contacts - but do not worry if you accidentally get some glue on the contacts. Most glues are deactivated by the heat of molten solder and a little glue on part of the contacts is not going to ruin your solder joint.

Step 7: Solder Your New LED

After the LED has been carefully seated on the circuit board (trusty tweezers are helpful), you may proceed to solder the joints. Please check again that the polarity of the LED is correct(see picture).

I found that melting the solder on the tip of the soldering iron first and then bringing the molten solder to the joint is much easier than heating the joint first then adding solder.

ie.

1) Melt some solder on the tip of soldering iron

2) Bring the tip to the joint and wait for the solder from your iron to "seep" onto the joint surfaces

3) Withdraw soldering iron carefully

(However, for larger components, you should observe correct soldering techniques by preheating the joint then melting the solder at the joint itself)

Apologies for lack of photos of the soldering process because the soldering required all of my two hands.