Challenge Questions Blog

Challenge Questions

Stop in and exercise your brain. Talk about this month's Challenge from Specs & Techs or similar puzzles.

So do you have a Challenge Question that could stump the community? Then submit the question with the "correct" answer and we'll post it. If it's really good, we may even roll it up to Specs & Techs. You'll be famous!

Answers to Challenge Questions appear by the last Tuesday of the month.

Previous in Blog: Figure Skating: Newsletter Challenge (10/11/05)   Next in Blog: Kitchen Bulb: Newsletter Challenge (10/25/05)
Close
Close
Close
13 comments

Light Switches: Newsletter Challenge (10/18/05)

Posted October 18, 2005 7:00 AM

Chris Leonard writes:The question as it appears in the 10/18 edition of Specs & Techs from GlobalSpec:

"Frank" is a skilled but lazy electrician. He has just wired three on-off switches to three separate lights, all of which are upstairs so he cannot see them. He wants to check which switch operates which light, but wants to do this by operating the switches once then making only one trip upstairs. How does he do this?

Click here to view previous Challenge Questions.

Reply

Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.
Anonymous Poster
#1

Easy

10/18/2005 12:43 PM

Send his apprentice up the stairs.

Reply
Active Contributor
United States - Member - New Member Engineering Fields - Aerospace Engineering - New Member

Join Date: Oct 2005
Posts: 16
Good Answers: 2
#2
In reply to #1

Re:Easy

10/18/2005 1:10 PM

Assuming this is a logic problem and not an electrical one (shudder): Flip switch one ON and wait 5 mins. Flip switch two ON and wait 5 mins. Flip switch three ON and run upstairs. Coolest is switch three, hottest is switch one. Just don't burn your fingers. This is assuming they're filament bulbs; if they're incandescent, I'm to square one.

Reply
Active Contributor

Join Date: Oct 2005
Posts: 11
#3

light switch

10/18/2005 2:17 PM

it would be as simple as one switch on, one off, then place a test lamp of equal voltage in line with the last, this would make it burn at half brilliance. thus making each light different. Evar

Reply
Participant

Join Date: Oct 2005
Posts: 1
#4

Quick and easy light switch test

10/18/2005 2:22 PM

Assuming these are standard light fixtures: Turn on the first switch for 2-3 minutes and then turn off. Turn on the second switch and go upstairs. The light bulb that is hot to the touch is connected to the first switch, the light bulb that is off is connected to the last switch and of course the light that is on is connected to the second switch.

Reply
Active Contributor

Join Date: Oct 2005
Posts: 11
#10
In reply to #4

Re:Quick and easy light switch test

10/21/2005 1:46 PM

you have to remember this is a lazy worker.. would he want to bring a ladder up the stairs to check.... for temp change.... he would go for the least amount of work..I would think.

Reply
Associate

Join Date: Sep 2005
Posts: 40
#5

Light Siwitches

10/18/2005 10:49 PM

Poor Frank. If I am Donna Trump I will say "Frank,. You are Fired!. Well, I really have no idea.

Reply
Commentator

Join Date: May 2005
Posts: 93
#6

Light switches

10/19/2005 4:03 AM

Easy (if you apply some lateral thinking!) Frank switches on one of the lights (after noting the switch position), he then goes for a cuppa. On his return, he switches the light off and switches on another (again noting the switch position). He heads upstairs to find that one bulb is off but still warm, one bulb is illuminated, and one bulb is cold......Voila!

__________________
There's no such thing as bad weather....only the wrong clothes!
Reply
Member

Join Date: Oct 2005
Posts: 8
#7

Three lights, one trip, using vom

10/19/2005 3:55 PM

How about this: He turns two lights on. He goes upstairs and notes which one is off (this determines one switch – light correspondence). He removes one light from its socket, leaving the other in place, noting which light is still in its socket. He goes back downstairs. He checks the voltage at the two switches that were turned on. The one that shows no voltage across the screw terminals when the switch is turned off is the one controlling the socket with the light in place. The third is controlled by the remaining switch. QED

Reply
Anonymous Poster
#8

Light Switch Problem

10/20/2005 9:18 AM

Frank, being Lazy.....uses his skills to hire a contractor to do the work and specifies that the swithces must be labeled!!! Just kidding, turn on two switchs for a couple of minutes, turn one off.....go upstairs, he now has one bulb on, one bulb warm and one off....answered! I didn't think of this, I was too lazy to think and used my skill to find it on the internet, maybe I'm Frank!

Reply
Power-User

Join Date: Jun 2005
Posts: 394
Good Answers: 1
#9

Light Switches

10/20/2005 9:51 AM

All the solutions that work assume that the lazy electrician had installed bulbs in the circuits. Installing light bulbs does not require an electrician's skill and a lazy electrician would probably leave it up to the home owner. So without light bulbs, he could leave one switch off, one switch closed and on the third disconnect the switched wire and short it to neutral. Then taking his trusty multimeter upstairs he would test the first socket to see if it was hot. If not he would then switch the meter to measure resistance and see if the circuit was open or shorted to neutral. He would then go to the second circuit and repeat. After checking the second, he would know which switch was connected to which circuit.

Reply
Member

Join Date: Oct 2005
Posts: 6
#11

Crude but effective

10/22/2005 12:04 PM

Frank is so lazy he dosn't carry tools so he slipped a nickel into one of the light sockets providing a short to the circut breaker. This gave him the third variable to the on off switches.

Reply
Friend of CR4

Join Date: Dec 2004
Posts: 1995
Good Answers: 35
#12

And the Answer is....

11/02/2005 11:38 AM

As written in the 10/25 issue of Specs & Techs from GlobalSpec:

"Frank" should turn Switch 1 on for a minute and then turn it off. He should then turn Switch 2 on, leave it on and go upstairs. He can assign the lights as follows:

Switch 1 is for the light bulb that is off and warm.
Switch 2 is for the bulb that is on.
Switch 3 is for the light bulb that is off and cold.

The insight comes in when one realizes that a light bulb is not merely a Boolean (which only has two possible states, on and off), but a device that has another property — heat!

__________________
Off to take on other challenges. Good luck everybody! See you around the Interwebs.
Reply
Participant

Join Date: Nov 2005
Posts: 1
#13

Light Switches

11/08/2005 7:51 PM

Who said the light switches were down stairs? The newsletter says "all of which are upstairs" which I would think includes both the lights AND the switches.

Reply
Reply to Blog Entry 13 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.
Copy to Clipboard

Users who posted comments:

Anonymous Poster (2); Chris Leonard (1); davidt06 (1); Evar (2); Hairy Potter (1); Howetwo (1); Jim H. (1); phrogengr (1); rvguest (1); Simon (1); Stupott (1)

Previous in Blog: Figure Skating: Newsletter Challenge (10/11/05)   Next in Blog: Kitchen Bulb: Newsletter Challenge (10/25/05)

Advertisement