3 Bulbs, 1 Trip

This one requires some out-of-the-box thinking, but still, there is no "trick". Solving this one will give you insight into an approach for a large "class" of logic puzzles - questioning assumptions and using residual data. Good luck.

You have three light switches by the front door downstairs. These are tied to three bulbs down in the basement. You have no clue which switch controls which bulb. You can't see the bulbs from the front door. Thankfully, you do know that toggling each switch "up" turns one of the three bulb.

You are lazy, or maybe just self-challenging. Regardless of your motivation, you think there has got to be a way to figure out which switch controls which bulb with a single trip to the basement.

Again, there aren't any cute tricks... You can't count on the switches being ordered similar to the bulbs. You can't see or access the wires. You have no helpers or remote cameras or anything like that.

But, taking one trip down to the basement (while you're still down there) you can tell which switches control which bulbs! How?

If you give up, here is the solution:
http://blogs.sun.com/roller/resources/dcb/SOLUTION_3_Bulbs.html

Comments:

Simple!

Assuming the bulbs have been off for a while:

1. Turn on switch one and two.
2. Wait 5 minutes.
3. Turn off switch one.
4. Go down to basement.
5. The bulb that is on is controlled by switch two.
6. The bulb that is off, but still warm is controlled by switch one.
7. The bulb that is off and cold is controlled by switch three.

What do I win? Mary is giving away t-shirts! :)

Posted by Jason Santos on February 25, 2005 at 06:19 AM EST #

Very good! The key is that the bulbs are tri-state, not binary (on, off, off-warm).

Mary is in marketing :-) I'm in delivery.

Posted by Dave Brillhart on February 25, 2005 at 07:13 AM EST #

in your solution ( http://blogs.sun.com/roller/resources/dcb/SOLUTION_3_Bulbs.html ), #'s 1 and 3 have opposite info. it's switch number 1 that matches the cold bulb that's off.

Posted by arvin gatmaitan on November 17, 2005 at 10:41 PM EST #

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