The camera obscura is one of the oldest concepts in optics. It's been in use since at least the 4th century BC, and the simple method of a small hole in a darkened room or box may even have been one of the key techniques to the artistic breakthroughs of the Renaissance. And since it's a relatively straightforward piece of optical engineering, it's something you can do to this day — as Photojojo demonstrates in its new DIY.
Turning a room into a camera obscura isn't hard, though it does require some diligence. You have to block of all the light in the room, barring a small amount coming from a single, small opening. Photojojo recommends blocking off the windows with black poster board, and then hunting for light leaks, and making sure to block them off. You also need a lens for the light to come through, this DIY suggest either using the lens from a flashlight, but you can use a piece of cardboard with a hole cut in it too.
As with a standard camera, you can also adjust the aperture to increase the sharpness at the cost of light (or vice versa). All you have to do is vary the size of the light hole.
So if you're looking for a fun weekend project, why not try converting a room into an enormous camera? All it takes is some basic supplies from a craft store, and a bit of time, and you'll have the world projected (and upside down) on your wall.