A discussion came up with some guests over shabbos. One asked if one could use an overhead projector on Shabbos if it was turned on from before, i.e. could one put a new plastic sheet on it or not? In theory, it should be the same with old school slides. Any ideas?
Contemporary Poskim are divided on this matter:
- Rav Nissim Karelitz (Chut Shani 1:20, 1) : Rabbinically prohibited because this is considered impermanent writing.
- Rav Shmuel Wosner (Shut Shevet Halevi 10:60) : It is not writing at all but is prohibited because of zilzul (slighting) in the honor of Shabbos
- Rav Shlomo Zalman Auerbach (ibid) permits it.
Regarding making finger shadows all permit it since it is transient and doesn't last more than typically a few moments.
Source: Dirshu Mishne Berurah 340: note 45
As I understand it, the projector would be sitting on top of a cart that moves back and forth so that the projected image (unto the screen) would be focussed. That of course would entail a Melacha (at least rabbinically), if the cart were to move. Even if the projector would be sitting on a stationery table, there would be no guarantee of not tampering with the unit to ensure clarity of the image.
Another point to consider would be that the internal temperature of the unit might reach a point (remember that it would be turned on for several hours) where the plastic film begins to melt, a 'form' of cooking.
Finally, projecting an image onto a screen could be on some level Molid.
As far as I understand, projectors are muctse. They are electrical objects of no vital use, so you couldnt touch it. The projectors I know usually overheat after a long period of use so, if you let it turned on, you would may be worried about it and tempted to turn it off, wich is a way to transgress Shabbos, G-d forbid.
Moreover, Shabbos activities are rest and spiritual growth, for that it's best to distance ourselves from things that may decrease our enjoyment of the special spiritual light that comes down only at Shabbos. This is a concept that applies also to why we shouldnt watch a TV thats already been turned on before Shabbos.
Ps: Ask a rabbi for his answer too! Pss: Shabat kasher ve sameach!