What is it? Well, it's a controller for the backlight's brightness as well as a battery monitor.
Why did you make it? After I upgraded my old friend with a backlight I noticed it would be helpful being able to adjust its brightness, but didn't want to drill a hole in the enclosure. Instead, I wanted to use the buttons of the GameBoy. Also the battery indicator LED didn't tell much about the state of the battery so I decided to measure the voltage, and when below a certain threshold toggle the LED which should draw some attention.
What makes it special? The modified GameBoy still looks like an original one from the outside
Please visit also my blog www.tinker7one.at for more details. It's in German, but you may find the pictures/videos helpful. Just copy+paste the documentation URL into https://translate.google.com to translate it. At least English seems to work fine.