First, a little science. Anyone who has taken a basic photo course knows at least the basics behind film processing. The light reacts with the paper, and then chemicals alter the products of these reactions so that an image can be seen. Now throw that idea out the window. Funny thing about RC photo paper is that it reacts noticeably to light regardless of any chemistry you may add after the exposure. Its something to do with the silver ions and electrons and other things that bald guys who carry around calculators for fun would be able to explain. But what does that mean for you if you're not bald and don't carry around a calculator for fun? It's the key to your long exposures. If the paper is exposed much much much longer than any exposure time that would work with darkroom chemistry, a discernible image will fry itself into the paper. You can capture entire days this way: the sun can be seen in a trail across the sky and only objects that remained stationary for long periods of time will appear. This image can then be taken to a flatbed scanner and scanned onto the computer.
So all you gotta do now is build your own camera to put the paper in. Your camera can range from a basic shoebox with a pinhole to something with shutters and SLR lenses and all kinds of crazy things. For this instructable, I'm going to show you how to go crazy with it. Cool things can be captured by just opening up a lens and continuously letting light in, but what if you want an Arduino- controlled shutter that allows you to control when the paper is exposing and when it isn't? That's what I'm going to show here.