Here's a guest blog post to The NXT STEP by Joshua "NXTesla" who's done some interesting stuff with illuminating the NXT brick:
After I did all of the wiring and put the white cover back on, the LEDs were shining through the plastic and it looked horrible. So I spray painted the inside of the white cover black so that the light would shine only though the input and output ports. I also put a piece of paper between the LEDs and the ports to diffuse the light so that, rather than shining through the ports, the LEDs would glow through them.
If anyone would like to do something similar, I suggest using LEDs no larger than 3mm.
Please note, also, that doing this will, in fact, void your warranty.
LEDs make everything cooler, right? Even LEGO MINDSTORMS? LEDs look pretty cool on the new EV3, so why not put them on an NXT? I had some nice, bright LEDs lying around, so I thought I’d try it. Here’s what the finished product looks like.
If you would like to see more pictures go take a look at my Imgur album: http://imgur.com/a/
I uploaded the schematic so everyone can see how it works. My original plan was to solder the two power wires to the battery terminals located next to ports 1 and A. It was a good plan, but let me just say that the plastic case melts a lot easier than the solder does. Yes, the one corner of the case right next to the negative battery terminal is melted. Luckily, the white part of the case covers that part. Proceeding with caution, I found that it was nearly impossible to melt the solder, and I was forced to find a different place where I could solder the ground wire to the ground plane. The wire that connects to the positive battery terminal could not be moved as easily, so I had to do a really sloppy job of soldering that connection. In the end though, it turned out to be somewhat acceptable.
The Bluetooth power supply is apparently turned on only when the NXT is turned on, so I used it as a power switch for the LEDs. It would also be possible to, instead, use the signal that is used to control whether Bluetooth is turned on, which would make it so that the LEDs turn on only when Bluetooth is turned on. I decided against the latter because of the Bluetooth battery usage.
After I did all of the wiring and put the white cover back on, the LEDs were shining through the plastic and it looked horrible. So I spray painted the inside of the white cover black so that the light would shine only though the input and output ports. I also put a piece of paper between the LEDs and the ports to diffuse the light so that, rather than shining through the ports, the LEDs would glow through them.
If anyone would like to do something similar, I suggest using LEDs no larger than 3mm.
Please note, also, that doing this will, in fact, void your warranty.