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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 '14 at 1:37

Contemporary Poskim are divided on this matter:

  1. Rav Nissim Karelitz (Chut Shani 1:20, 1) : Rabbinically prohibited because this is considered impermanent writing.
  2. 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
  3. 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

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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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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.

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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!

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Why can't you touch muktza?? – Double AA May 8 '14 at 18:19
To avoid forbidden avodah. Maybe you should revise your Shabat halachot studies because this is a very primary and basic topic. – Tsivia May 8 '14 at 18:45
Tsivia, I think @DoubleAA meant that muktze can be touched, just not moved -- and that's a very relevant distinction for this question. In any event, editing in a source for your claim that an electrical object with no vital use is muktze would also greatly improve your answer. – msh210 May 8 '14 at 19:21
Hammers also have no vital use and are permitted Letzorech Gufo or Mekomo – Shmuel Brin May 8 '14 at 21:41

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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