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Light Switches

  1. Sep 14, 2004 #1
    You are in a room with 3 light switches. These switches are connected to 3 separate light bulbs in an ajoining room. Access to the ajoining room is via a door that remains closed, there are no windows, other people etc etc.

    You are allowed to enter the ajoining room once and then return. At no time can you view the light bulbs whilst being able to manipulate the light switches.

    Under these conditions, how do you identify which switch is connected to which light bulb?
     
  2. jcsd
  3. Sep 14, 2004 #2

    BobG

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    Need to convert the binary switches to ternary (select to see solution):


    Turn switch 1 on.
    Wait 15 minutes.
    Turn switch 1 off - turn switch 2 on.
    Run into room.
    Lit bulb is controlled by switch 2.
    Warm bulb is controlled by switch 1.
    Cold bulb is controlled by switch 3.
     
  4. Sep 14, 2004 #3

    Gokul43201

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    You did say the bulbs are heat-sunk and water-cooled, didn't you ?
     
  5. Sep 14, 2004 #4
    I know it is right because that was what I was going to post. I was 18 minutes off getting it. Heck. :smile:

    The Bob (2004 ©)
     
  6. Sep 14, 2004 #5

    BobG

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    No, but, unfortunately, he neglected to mention the door was locked and they gave you a key ring with 4096 keys on it. :cry:
     
  7. Sep 14, 2004 #6

    russ_watters

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    My company has a circuit tracer, which can be used to trace wiring through walls...
     
  8. Sep 14, 2004 #7

    Gokul43201

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    Ah BobG, so you whipped out the mini hatchet from your 117-tool Swiss Army Knife (you know, the new one ...that comes with a little slide-rule), and tore down the door ? Or did you go for the micro-battering-ram ?
     
  9. Sep 14, 2004 #8

    Gokul43201

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    Hey russ, I've got one of those too. I call it my multimeter with reeeeaaaalllly long leads.

    No ? Not that one, eh ? :grumpy:
     
  10. Sep 14, 2004 #9
    :cry: Rats, that was either too easy or you saw it before
     
  11. Sep 14, 2004 #10
    You could also flick one on and off for a long long time untill it breaks the filament. Then turn another on and go see which is which. The broken filament one will have a discoloration and ringing when shooken.
     
  12. Sep 14, 2004 #11

    NateTG

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    Need more info.

    So, you've got three switches, and three light bulbs.

    Each switch can be set up to toggle each of the three light bulbs, so that's 8 possible set up's per switch, for a total of at least 512 possible wirings (and that's without getting tricky...). Now, if we go into the room there are at most 8 possible results.
    If you want to get tricky, there are 8!=40320 possible wiring arrangements that insure that all lighting combinations are possible, and for sufficiently creative electricians there are 8^8=16777216 possible wiring combinations.

    Even using BobG's clever ternary technique, there are at most 27 possible states for the light bulbs in the room.
     
  13. Sep 14, 2004 #12
    There maybe 27 different places where they can be but only one (or two ways) to solve the problem.

    The Bob (2004 ©)
     
  14. Sep 20, 2004 #13
    this is utterly easy. pull one light switch completely as in pull out of wall, turn on one light and go look. the one with the busted bulb will be connected to the switch that was yanked out, easy :rofl:
     
  15. Sep 28, 2004 #14
    Solution...but this problem doesn't belong HERE.

    I believe this is an easy one...probably too easy for this forum, but here goes:

    If there are three switches in one room and you cannot directly see the results when you turn a switch, then you must rely on other senses to determine which switches coordinate with which light bulb. To facilitate that, you turn on one switch for about 5 minutes and wait. At the end of 5 minutes, you turn that first switch off and turn another one on. Then you proceed into the room containing the light bulbs. One of the light bulbs will be lit corresponding to the switch that is currently on. Of the two remaining light bulbs that are not lit, you must touch them to determine which is heated from being left on for a 5 minutes. The hot one is the one corresponding to the first switch you operated.

    So, in a nutshell, when one physical sense is neutralized, rely on another for its input.

    "God, in infinite wisdom, made all things simple. Man made them complex."
     
  16. Oct 14, 2004 #15
    What you do is you turn one light on, wait half an hour, turn on another light then go in the room and see witch bulb is hotter. The hotter bulb is the one you turned on first, the less hot lit bulb is the one you turned on second and the unlit bulb it the third switch
     
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