Step 1: The Principle
The principle is very simple. There is a powerful light that is projected through an LCD into a lens so the image is projected.
Some things you want to take into consideration:
-The better the quality on the LCD, the better the projected image. (I'm using a LCD with 640X480 pixels)
-The LCD can't take more than 40C, so if you decide to use some other type of light source, pay good attention to the cooling system.
-Without a good fresnel lens will be really difficult to light the LCD evenly. (I'm using the always faithful fresnel lens from an old OHP)
-The fresnel from the OHP is really a twin fresnel, the one that faces the light is expanding the light to the full area of the lens, the other one is focusing all the light to one point. If you keep the two of them together (like this projector) it will be a lot easier and less danger of damaging or scratching them. Any big scratch will be reflected in the projected image. The down bit is that you won't be able to do keystoning adjustment. So the projector will have to be placed in a perpendicular position with the screen.
-The use of mirrors allows you to make the box smaller as if there are no mirrors, you will have to place the LED farther from the fresnel lens, and also the LCD from the main lens.
-You need to have everything inside some sort of enclosure, or the light will be coming out and it will light the room making the projector a lot less efficient.