Last Sunday I set this light switch puzzle, which I can now reveal I found in the magazine section of that weekend's dnoc (but it's not available online). I'm about to give their solution, so if you don't want to see it, look away now:
Elect one of your number to a special status. Suppose it's you. Everyone else is to put the switch down if they find it up on the first occasion that they visit the room. Otherwise they do nothing. You must restore the switch to the up position, counting 1 every time this happens. When you've counted 5, then you've all been in at least once.Matt K, Squander Two, Andy W, Bernard F, the son of Ilana W, and Michael G had variants of this solution. Well done them.