Opaque projectors are the ancestor of the more modern overhead projector and essentially a camera obscura used backwards. The basic idea is instead of having a dark chamber where an image from outside can be projected, have a very bright chamber in a dimly lit room so the light reflected off of whatever is under the projector can be directed through a mirror and lens to be projected into the room.
As opaque projectors use reflected light off a piece of paper verses light transmitted through a film as in overhead projectors they are inherently less efficient at the paper absorbs some of the light. They are however optically simpler and able to project the image of any flat surface they are set on. So you can draw something or just pick it up place it under the projector and it will work, no transparencies necessary. We've come up with this bare bones cardboard box model so you can experiment with what is now a somewhat rare and fairly specialized piece of equipment.
•12''x12''x12'' cardboard box (not in photo)
•12'' piece of 2''x2''
• One large (12''x12'') or several small pieces of foam core
• A 6'' x 10'' to 6''x 17'' piece of plex mirror
• 6'' of 3'' dia cardboard tube
• 73mm +3.0 or 3.5 lens with about a 12'' focal length (we got ours All electronics)
• Duct or gaffers tape
• Strong flashlight
• Clip light and 23w compact florescent
• Utility knife
• Hot glue gun
• Small wood saw
Hot glue the lens barrel hole collar over the hole in the box using the lens barrel to align the collar.
To secure the mirror we used a strip of foam core directly above and below the mirror to trap it in place without glueing it. We add 2 small pieces of foam core on the side of the mirror to keep it from slipping from side to side.
We used 3 pieces of foam core spaced about 1.5'' from the end barrel to hold the lens up. To get the spacing even we used a 1.5'' piece of foam core to measure from the top of the tube to the top of the lens holders before glueing them in place.
After the lower lens holders are secured, place the lens on top and then glue three more spacers over it to pin the lens in place.
We cut 2 light ports in the upper front corners using the lens barrel as a template. As the light ports are on the same side of the lens barrel, no light will be able to make it directly into the lens from the light sources placed in the ports so they will not interfere with the image.
Here's the camera with the lens barrel inserted looking from the bottom. Note that the lens is mounted on inside of the box (close to the mirror), not on the outside as in the tracing camera.
Here is the projector, a book and two light sources we tested.