There are multiple ways to set up your computer and document camera to projector through your LCD projector, but most teachers will want to avoid having to unhook one device in order to connect another. Since this question comes up a lot, especially at the beginning of the school year, we figured it might be nice to have a visual aid for how to hook up your equipment in the optimal way.
Ideally, your document camera should act as the middle man, and the switching between inputs can be done with the press of a button. This button is labeled "Source" on some document cameras and as "Camera/PC" on others.
In the image included with this article, you will see that the computer (represented by the "dongle" VGA adapter) is hooked to the input of the document camera with the black cable. The blue cable goes from the document camera's output to the LCD projectors input (usually labeled "Computer"). Of course, the VGA cables can both be blue, black, or beige - it doesn't matter. The two different colors are for illustration clarity only.