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):
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.
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.
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.