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-   -   3 switches, one lightbulb (http://www.logic-puzzles.org/forum/showthread.php?t=225)

ephaliren 01-07-2013 09:33 PM

3 switches, one lightbulb
ok, so here's the problem I mentioned earlier:

There is a room, with no windows and one door. There is no keyhole, and you cannot see under or around the edges of the door. The door is closed.

Inside the room there is an exposed incandescent light bulb. The light bulb is off.

Outside the room is a panel with three on/off switches. Only one switch will operate the light, the other two do nothing.

You must figure out which switch operates the light.

You are not allowed to dismantle anything (yes, someone asked about following the wiringÖ)

You may flip the switches on and off as many times as you want before you open the door, and there is no time limit.

You can only open the door once. Once you open it, you can no longer touch the switches and you MUST say which switch is the right one. (AKA no trying a switch, opening the door, trying the next switch, etc.) Once the door is open, you can go inside the room

So, how do you tell which switch operates the light?

pinkgirl25 01-09-2013 04:01 AM

You could turn on the first one for two minutes and turn it off. Then the middle for two minutes and turn it off. Then the last and run inside. If it is on then it is the last is it. If the bulb feels hot then the middle one. If only warm then the first.

Or here is a better method...

Turn on the first one for like two minutes then turn it off. Then turn on the middle one run inside, if the light is on it is the middle one, if it is off feel bulb for warmth. If the bulb is warm then the first one turns on the light, if not then the last one does.

Is this right?

ephaliren 01-09-2013 05:11 PM

You are correct!

pandalover10 01-27-2013 10:16 PM

i would watch under the door and see if there is a dim light under the door. right??

freckles10101 06-06-2013 08:15 PM


Originally Posted by pandalover10 (Post 953)
i would watch under the door and see if there is a dim light under the door. right??

No, you can't see under the door or around it the door is shut.

Red-Herring 04-01-2014 06:25 PM

alternative solution
Well, that solution only works if the ceiling is low enough to reach the light bulb. What if the ceiling is 20 feet?

I think I got a better solution.

Considering that this is a hypothetical scenario and considering that you are allowed to use as much time as you like there is another way to figure out this question.

1.You flip the first switch, and leave it turned on for four years (way beyond the normal lifespan for such a bulb)

2. After 4 years you return and flip the second switch.

3. You then open up the door and walk inside;

a) If the room is dark, you shine a flashlight at the bulb to establish whether it has burn marks or not. If it has such burn marks, then you know for a fact that the first switch operates the light.

b) If the bulb doesnít exhibit such burn marks then you know that itís the third switch that operates the light.

c) If the light is turned on, then you know that itís the second switch that operates the light.

This scenario will only work if it was a brand new light bulb, or sufficiently unused for such burn marks to have materialised.

cr0ck3t 05-31-2015 10:58 AM

if you are allowed a flashlight...
you could also use an infrared camera - that way if the roof is really high, and you cant see if the bulb is burnt out, you can still get a reading... also, you would not have to wait until the bulb burnt out, and the previous method would work.

mrzeno 05-31-2015 11:51 AM

Or you switch each on in turn whilst checking the electricity meter for extra activity.

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