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: Missing Operators: May 2021 Challenge Question   Next in Blog: A suspicious suitcase and a boat (July 2021 Challenge Question)
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
17 comments
Rate Comments: Nested

Which Light Switch?

Posted May 31, 2021 12:00 AM
Pathfinder Tags: challenge question

Here's a brain teaser for the electricians out there.

A lightbulb is behind a closed closet door - you cannot see it. It is currently off.

There are three identical wall switches outside the closet, labeled A, B and C.

You need to determine which switch operates the lightbulb. You can open the door exactly once.

How do you do so?

Reply

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

Good Answers:

These comments received enough positive votes to make them "good answers".

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
11
Randall
Guru

Join Date: Aug 2005
Location: Hemel Hempstead, UK
Posts: 5826
Good Answers: 322
#1

Re: Which light switch?

05/31/2021 5:08 AM

You open the door and try the three switches one at a time.

But I'm sure that the questioner has missed something from the question.

When the question is corrected, the answer is to switch on A wait 5 minutes; switch A off, switch B on and open the door. If the light is on it's B; if it's off then feel the bulb: if it's hot the correct switch is A; if it's cold the correct switch is C.

I think the question normally involves a flight of stairs.

Please feel free to delete this answer and rephrase the question so others can have a go.

__________________
 If you spend all your time looking for people and things to complain about: trust me, you will find plenty to complain about.
Reply Good Answer (Score 11)
Go to Next Good Answer
Rixter
Guru

Join Date: Apr 2010
Location: About 4000 miles from the center of the earth (+/-100 mi)
Posts: 9913
Good Answers: 1141
#2
In reply to #1

Re: Which light switch?

05/31/2021 10:38 AM

But unfortunately, someone replaced it with an LED bulb...
Reply Score 1 for Good Answer
Go to Next "Almost" Good Answer
SolarEagle
Guru

Join Date: Mar 2007
Location: by the beach in Florida
Posts: 33392
Good Answers: 1817
#3
In reply to #1

Re: Which light switch?

05/31/2021 5:01 PM

The question is , what do the other two switches control...?

This is where ghost stories come from....haha

__________________
 All living things seek to control their own destiny....this is the purpose of life
Reply
Anonymous Poster #1
#12
In reply to #1

Re: Which light switch?

06/16/2021 1:45 PM

This is the correct answer. It's a process of elimination. And on your one chance to open the door, either the bulb is warm, the light is on, or neither, and the switch-flicker can deduce which is which.

I see what you mean about stairs. It's not critical to the equation. This is sometimes introduced to explain why you can only see the bulb once (by door restrictions or remoteness from the bulb and room).
Reply
Nano-Oil.com
Member

Join Date: Aug 2009
Location: Petaluma, CA USA
Posts: 9
#4

Re: Which Light Switch?

05/31/2021 10:53 PM

just measure the current ar the switch
Reply Score 1 for Good Answer
Go to Next "Almost" Good Answer
2
Waylan
Active Contributor

Join Date: May 2020
Posts: 14
Good Answers: 1
#5

Re: Which Light Switch?

05/31/2021 10:56 PM

Presumably the other two light switches operate something outside the closet. Turn them on one at a time. The other two will light up (or turn off) something that you can see, outside the closet. The one that doesn't is the closet light. You don't even need to open the door once.
Reply Good Answer (Score 2)
No more Good Answers. Go to first "Almost" Good Answer
Waylan
Active Contributor

Join Date: May 2020
Posts: 14
Good Answers: 1
#6

Re: Which Light Switch?

05/31/2021 11:12 PM

Alternative answer:

Open the door first.

Don't close it.

Test the switches.

Wait, I just noticed Randall beat me to this answer and his better answer I like better than my previous one. Even an LED will be warm. Randall has my vote.
Reply
Stef
Guru

Join Date: May 2018
Location: Under the spreading Bunya Trees, South Burnett, Queensland, Australia
Posts: 809
Good Answers: 65
#7

Re: Which Light Switch?

06/01/2021 3:00 AM

Well an electrician would take out his trusty multimeter on mA or A scale and connect the probes across each switch. Whichever measures current then that is the correct one.

__________________
 Hare today, goon tomorrow!
Reply
EBrown
Associate

Join Date: Dec 2020
Location: Dayton Ohio
Posts: 36
Good Answers: 2
#8
In reply to #7

Re: Which Light Switch?

06/01/2021 4:41 AM

Not necessarily. You would need to know that the other switches aren't drawing current first. I would say "turn them all off, but some may be three way switches. I think I also like Randal's answer the best. Electricians are smart enough to do it the simple way. Open the door and see which switch turn it on.

__________________
 Don't follow in the footsteps of someone who you wish to emulate. Instead, seek what they sought.
Reply
HiTekRedNek
Guru

Join Date: Oct 2006
Location: 44.56024"N 15.307971E
Posts: 8277
Good Answers: 270
#14
In reply to #7

Re: Which Light Switch?

07/27/2021 7:41 PM

That will only measure current if the switch is off..you will not measure any current if the switch is closed,and you will not know if the other switches have a load associated with them,so you could not tell using this method.If you knew the exact wattage of the bulb in question,you could look at the current and calculate the wattage,and if there are no other identical loads on the other switches,it would work.

The simplest method is to open the door and leave it open while you test the switches.

__________________
 "A man never stands so tall as when he stoops to help a child." "Never argue with a stupid person.They will drag you down to their level and beat you with experience" "To create an apple pie from scratch, first you must create a universe"
Reply
horace40
Guru

Join Date: Jan 2006
Location: Tamworth, UK.
Posts: 1782
Good Answers: 45
#9

Re: Which Light Switch?

06/01/2021 8:49 AM

...open the door and try each switch...(as others have said), but along these lines of thought... a fridge light comes on when the door is opened...but does it go out when the door is shut....

__________________
 When arguing, remember mud-slinging = lost ground.
Reply
BroAdam
Active Contributor

Join Date: Mar 2020
Posts: 12
Good Answers: 1
#10

Re: Which Light Switch?

06/01/2021 2:33 PM

If this has been done by most good electricians and DIYers and handymen, the switch closest to the door, or at the top is the one for the closet. Just the way most of us have been taught.
Reply
someone
Participant

Join Date: May 2020
Posts: 4
#11

Re: Which Light Switch?

06/01/2021 5:25 PM

Once the question is fixed, open the door, remove the light bulb, screw in a 100 dB horn, either close the door or leave it open, try each switch and listen. Or just look at the gap under the door to tell if the light comes on. Or if there isn't a gap, bust a hole in the wall to see. Then you don't need to open the door even once. This is really easy.
Reply
kkn
Active Contributor

Join Date: Oct 2020
Posts: 24
Good Answers: 2
#13

Re: Which Light Switch?

06/22/2021 8:06 AM

I open the door, let my brother go into the room, and I close the door.

I operate the switches, and my brother screams when the ligth goes on.
Reply Score 1 for Good Answer
No more "Almost" Good Answers.
PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#15

Re: Which Light Switch?

08/27/2021 5:33 AM

Who actually needs a <...lightbulb...behind a closed closet door...> (rhetorical question - NNTR)?

__________________
 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Reply
Randall
Guru

Join Date: Aug 2005
Location: Hemel Hempstead, UK
Posts: 5826
Good Answers: 322
#16
In reply to #15

Re: Which Light Switch?

08/28/2021 4:57 AM

People who make up logic puzzles?

__________________
 If you spend all your time looking for people and things to complain about: trust me, you will find plenty to complain about.
Reply
hitekrednek2
Power-User

Join Date: Mar 2019
Location: 3rd rock from the sun,a small dust mote,in a small insignificant spiral galaxy referred to locally as the Milky Way
Posts: 167
Good Answers: 3
#17
In reply to #16

Re: Which Light Switch?

03/01/2022 5:02 AM

Research statistics has shown that most jokes and puzzles come from prisoners since they have nothing else to do,except keep the rope on their bar of soap.

Remember, 27/32(84.375%) of statistics are made up on the spot.

__________________
 There is no darkness except for ignorance..Shakespeare
Reply
Reply to Blog Entry
Have a Different Question?
17 comments

Good Answers:

These comments received enough positive votes to make them "good answers".

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (1); BroAdam (1); EBrown (1); HiTekRedNek (1); hitekrednek2 (1); horace40 (1); kkn (1); Nano-Oil.com (1); PWSlack (1); Randall (2); Rixter (1); SolarEagle (1); someone (1); Stef (1); Waylan (2)

Previous in Blog: Missing Operators: May 2021 Challenge Question   Next in Blog: A suspicious suitcase and a boat (July 2021 Challenge Question)

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Section
General Discussion:
Go

Search this Blog
Challenge Questions:
Go

Recent Blog Entries

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (61 right now)
Show Members
Advertisement