Step 1: Stuff You Need
So, you need:
Two LEDs (Color of your choice)
Fine Sandpaper (400 - 500 grit)
Tri-Wing Nintendo Screwdriver or small flathead screwdriver
Transparent Purple Gameboy Color
Helping hands (Optional, but really helpful)
Hot glue gun and glue (Optional)
Step 2: Take It Apart
Lift the bottom half of the case off the top one, then unscrew the three screws near the lower half of the PCB. Make sure you save these.
Once you have that done, you need to disconnect the screen. Lift both black plastic pegs toward the orange ribbon cable at the same time, then lift the cable from the connector.
Step 3: Cut the Switch Hole
Start by scoring the case of the Gameboy in the spot you want to put the switch. I found there is a lot of room just underneath the battery compartment. Cut the hole a little smaller than needed. You can always cut it bigger, but you can't cut it smaller. Keep cutting until you cut all the way through. It takes a long time, but it's worth it for a clean cut.
After you have the hole cut out and the switch fits, drill the holes for the screws. I like to put the switch in first, then drill through the hole for the screw.
Step 4: Diffuse the LEDs
Why diffuse it? Diffusing the LED makes it not as bright, but makes the light more spread out, which is ideal for this project. Photos three and four show you the difference between a diffused LED and a stock one.
Step 5: Buncha' Soldering
Take your switch and solder two wires to the switch. I didn't use red and black, but you can if you like. IMPORTANT: Put the switch in its hole before you solder the wires to anything, or you will have troubles later.
After you put the switch in, solder one of the wires to the negative battery terminal labeled BT- on top of the white part of the PCB. Next cut the leads shorter on your LEDs, then solder red and black wires to one of them. Solder the other ends of the wires to your other LED, making sure you pay attention to the polarity. You can tell on the LED which side is positive because the negative side has a flat spot at the base.
Solder the other wire from the switch to the negative side of one of your LEDs. You should have something like in the sixth picture.
Finally, solder a short wire from the positive side of your LED to the positive battery terminal, labeled BT+. My LEDs are rated for 3.1v, but if your LEDs are rated for a lower voltage, you may want to add a small resistor (100 Ohms) to extend the life of them.
You should now have everything soldered together! Flip the switch and see if everything lights up okay. If not, check all your connections, and make sure you have the polarity right on your LEDs.
You may want to insulate the connections to the LEDs, but I didn't find a problem with it.
Step 6: Get It Back in One Piece
Finally, screw the back cover on again, and you're finished!
Sorry the first picture is out of place, I just wanted a good "main" image. :P
Step 7: Admire
Thanks for reading, and enjoy your neat mod!