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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?

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Interesting question; welcome to Mi Yodeya. Might I suggest you change your username? Unless, of course, you're attached to the number 2721. I hope to see you around. –  msh210 Apr 28 '13 at 7:57
Presumably the same question could be asked with regard to shadows -- is it permissible to hold an object in such a way that it casts a shadow in the form of letters or pictures? –  Dave Apr 29 '13 at 17:08
"In theory..." do you know the Halachah in that regard? Could you add it to your question? –  Seth J May 17 '13 at 14:56
In general, temporary writing is Biblically permitted but Rabbinically forbidden. However, the precise definition of "temporary" isn't always clear, and I'm not aware of any sources discussing this issue or a similar one directly. –  Ypnypn Feb 21 at 1:37
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2 Answers

I have consulted with R' Passow of Harvard Hillel, and he says that while there is no direct issue, there may be a problem with reading the writing, as according to Shabbat 149a, one may not read something written on a wall, nor look in a mirror on shabbat.

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what would you need a projector for if you cannot play anything? is this a slide projector, tv projector?

the quick answer is no, you cannot 'use' it, since you are not allowed to play with the tv or computer, and an overhead projector is not slide projector.

hope that helps

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The question is about an old-style overhead projector, where you place clear plastic films with stuff already written/drawn on them in front of a light source. This use does not involve manipulating the projectot itself, only the films. –  Monica Cellio May 14 '13 at 15:28
if it is a slide projector, then there is no problem. you are not creating anything new. It is much like keeping your stove running to keep the food warm, or the light on in the bathroom. –  Ess Kay May 14 '13 at 15:35
That might not be the only issue. You can't necessarily put any food you like into your already-warm oven on Shabbat, for instance; there are restrictions. Perhaps there are here too. And while the answer might be the same, the question isn't asking about a slide proejctor. Finally, while what you say is interesting, none of it is sourced and we don't know what your credentials are. You can edit your answer to improve it. –  Monica Cellio May 14 '13 at 15:49
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