Exchange your old slide projector's massive bulb for a energy-efficient ("green") LED
Leitz was a German company that built optical devices such as microscopes and slide projectors. These slide projectors were quite expensive at the time they were built (a worker had to work one month to buy one in the 1950s), but they are worth it: there are still many of them around on Ebay for a few bucks, most of them working.
We want to keep the optical system but want to scrap the electrical components. So you can buy a Leitz Prado without a bulb or generally faulty, but you need all optical components. To make sure you have everything, you can take a look at the Link from Pradoseum, a virtual museum dedicated to Leitz Prado.
Leitz Prado 250 and 500 only differ by the bulbs that were used, but the Leitz Prado 250/500 were changed a little over the years. If you can, try to grab one with a ventilation channel at the front, it will make things easier. Do not buy a Leitz Prado 150 or something, it's completely different. The Leitz Prado 750 is quite rare, but you can use it, too.
Do not buy any "special edition", we need the very basic dia slider.
Do not switch the projector on! Most electronic parts are rotten and pose the risk of electric shock. Just leave as it is, we will replace the system.
Open the projector at the top and remove the bulb including the holder, the condensor lense and the heat filter (the other thing inside).
Try to unmount the objective (the large thing that looks like a cannon), so working is easier.
Turn over the projector and clear out the "cage" on the bottom. Unscrew the bottom (keep all screws, of course) and remove all electronic parts. Please note that you only need to unfasten two screws in the front. The rear is a sliding mechanism.
This is the first time you have to let go, you need to cut the AC cable. You have to do it to remove the cooling system. There is a lot of space for your ideas like a Raspberry Pi or something in the cage.
The LED produces a lot of heat, so you need a cooler for that. The best fit are rather small CPU cooler with a 60mmx60mm fan. The height of the heatspreading element should be around 30mm. Socket-A or Socket-462 boxed cooler kits are best fit for our project. Avoid anything special or round. Unfasten the screws of the fan and keep the parts.
Now we need to mark the position of the LED onto the heat spreader. Take the bulb including the holder and the heat filter and place it next to each other with the top aligned. Put the heat spreader onto the heat filter centered and in a way that the glass is completely covered and the spikes are vertical (thus not blocking the airstream). Make a vertical marker exactly in the middle of the heat spreader.
Take a water proof marker and mark the position of the bulb's coiled filament on the heat spreader. Be as accurate as possible with this, a very small misalignment will cause a significant decrease of brightness.
Do not mount the LED by now to avoid damage to it (I had to try some things, so I have it mounted in the pictures, but you should not do it).