Overhead projectors are not used in schools anymore. So you can usually get them for just a few bucks at the online school auctions.
If the projector works, it can be a good way to enlarge objects and trace them on pieces of plywood. But they’re even more fun to disassemble and scavenge parts for future projects. My grandson and I took two projectors apart this afternoon (we have more).
One of the best finds is the fresnel lens located just below the glass platen. These lenses are about 12″ square and have a focal length of just a few inches. Just what you need for solar energy projects. Be careful, it’s easy to start fires with these.
Other parts such as power cords, power switches, limit switches, super bright lamp holders, fans, metal pieces, and optics can be found. The projection head almost always contains a high quality first-surface mirror.
They usually come up in auction in groups of a half-dozen or more. So if you’re not careful, you can end up with a garage full. Check auctions at Publicsurplus.com and onlinepros.com