I am a sucker for a good SketchUp exploded view (although I think the screws shown in this one just have to be wrong). This plan comes from the usbddr project. From the readme file:
usbddr is firmware for the Atmel ATMega8 which implements a usb controller (“dance pad”) for games such as stepmania. In contrast to other homebrew dance pads, it uses capacitive sensing, eliminating mechanical wear and allowing adjustable sensitivity.
There’s some good discussion of the project over at Hack a Day,