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If a room or an area has an electric eye which causes a light to turn on when someone passes by, and the light is off, may someone turn off the light-switch on Shabbos so that people can walk by without causing the light to go on?

Even if the light-switch is muktzah I would assume it is not more than a כלי שמלאכתו לאיסור, kli shemalachto L'issuur, which is permitted to be moved for לצורך גופו להיתר - - for a permitted use of itself. so if the act of turning the light-switch off is permitted, there should be no muktzah problem either.

Note: Some reasons why this may be different than the issue of adjusting a mechanical timeclock on Shabbos (to stop a light from going on):

  1. The time-clock will definitely make the light go on - The Electric-eye will only make the light go on if someone walks by. so the change is not definite.

  2. The time-clock will make the light go on without any other action - The Electric-eye will only make the light go on if someone walks by. Thus there is still another action needed to make the light go on.

  3. On the other hand, maybe the mechanism of an electric-eye system works differently than a time-clock. Maybe the system is considered MORE ON and ready to go on as soon as someone passes. I don't know how it works so I can't compare the 2.

  • This is largely a matter of Shitos. – Al Berko Jun 27 '18 at 19:55
  • @AlBerko would you care to elucidate? – RibbisRabbiAndMore Jun 27 '18 at 20:02
  • Electrically both work the same. Relays – Draizy-Levi Pine Jun 27 '18 at 21:27
  • @Draizy-LeviPine ok but still, maybe there is a halachic difference between a relay which is more "ready to go on". – RibbisRabbiAndMore Jun 27 '18 at 21:29
  • Can you define that if they both use the exact same technology principles? What makes it more ready? Timer, electric eye, light sensor, motion, water - all the same past the event trigger. Doesn’t answer the q - just point 3 – Draizy-Levi Pine Jun 27 '18 at 21:31
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The following addresses the case where the switch controls only the power for the light. That is to say, the switch does not also turn off something in the electric eye circuit.

The Chosen Ish holds that opening and closing an electric switch might be binyan and stira. Rav Auerbach says that it is like opening and closing a door, it was made to open and close, and therefore permitted.

There is a related discussion concerning changing a timer to delay when something will happen (providing it is not done by removing and inserting pieces). Rav Feinstein holds that timers are muktzah. Rav Auerbach argues that they are not and most authorities permit them to be adjusted in this way.

It would seem that if changing the timer to delay an action is permitted, then surely turning off the wall mounted switch so that the light will not come on, where that does effect anything else, should also be permitted.

We note again that we are not addressing the case where the switch effects the electric eye circuit and no inference should be drawn to that case.

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