LEDs and Light Switches

OK! OK! Enough people have written in about the light-bulb puzzle at the beginning of my column on LEDs that I have to put this in. [chuckle and roll of eyes]

Here’s the puzzle for those who missed it:

You’re standing outside of a room with a tightly closed door. There are three light switches in front of you and three lamps in the room. You want to know which switch turns on which light in the room, but you’re only allowed to open the door once. How do you do it?

The answer I gave is: “Turn on the first switch for a few minutes. Turn it off. Turn on the second light. Open the door and go into the room. The first switch controls the bulb that’s warm, the second switch controls the bulb that’s lit, and the third switch controls the bulb that’s off.”

What everyone is pointing out is that the “rules” say you can only open the door once, so why not stand in the doorway and flick switches? Good point! The puzzle was meant to be rhetorical, so I didn’t put enough time into it.

See, another version has you up in the attic with the three switches, but the bulbs are in the basement. It was a longer explanation so I used the ‘doorway’ version. Silly me!

Thanks to everyone who wrote in!

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Follow-Ups
On a different note, thanks to Doc LeDuc who pointed me to a site by a company that makes high-power LED-based lights for theatre-type work. Cool stuff. Check out Pixelpar!

And Tommy Huynh, who has worked in developing white LED lighting systems, writes in:

They make sense in mobile applications but to replace incandescent lights in most applications, [compact fluorescent lamps] make more sense. White LEDs still lose intensity over time (just like fluorescents, they rely on phosphor also) and have lower efficiency, not to mention costs. Sure this will change as economies of scale weigh in but those bulbs aren’t going to the museum anytime soon.