Entry-level Nikon DSLRs don’t have many built in features that the higher-end cameras do, like an interval timer, for instance. It’s an exceedingly useful feature. If you want to take star trails, time lapse, repetitive self-timer shooting, or whatever, an intervalometer is really important.
Another feature that Nikon seems to have forgotten about for us lower-class photographers, is a built in, physical, connected cable release. If that existed, this project wouldn’t be that spectacular. What they do have, though, is an infrared sensor for taking photos with a remote. So, logically, I took advantage of that for the project.
He has a nice write-up about the project, including schematic, Gerber files, source code, and even a list of things to change for version 2.