Simple educational puzzle

[PBC_1966]

Here's a simple little educational puzzle. It doesn't involve anything at all complex, but for those just starting out is still a good little test of logic.

Here's the scenario: You have four unidentified conductors running between two distant locations (say four black singles pulled through a conduit, or something similar). You need to identify the four conductors at each end, and for the sake of this exercise we'll say they need to be labeled A, B, C, and D.

The only test equipment you have available is a basic ohmmeter or other continuity indicator, and starting at one end, you must have all four conductors correctly identified at both ends by making no more than one trip to the far end and back again. You don't have any resistances, diodes, or other components to help you, and you certainly don't have a modern multi-core cable ID tester! You have no way to make any connections to earth either.

How do you go about it?

 

[Apache]

can my continuity tester have a buzzer that I can hear from the remote location?

 

[PBC_1966]

can my continuity tester have a buzzer that I can hear from the remote location?

Ooh, didn't think to cover that one!

No, you can only see or hear the meter/tester when you're at the same end of the cable run to which it's connected. And you don't have any test leads long enough to reach from end to end either, before you ask.

 

[Apache]

OK start at first end and temporarily label cables a,b,c,d. Link a+b, go to other end.

Test cables for continuity and we can then say we have continuity between and the wires are a or b and the other two are c or d. Label the cables and the far end A B C D. Connect B to C at the far end. Go to start remove initial link but remember where it was!

We know when we have continuity we have found the B C linked wires. Wire D is the only wire without continuity that wasn't connected initially. The wire that was connected at the start, but now isn't and doesn't have any continuity is A. The wire that was part of the a, b pair that has continuity is B and the remaining wire is C



Now how do I get to the other end to remove the link?

 

[spinlondon]

Connect 2 wires together at one end.

Go to other end find the 2 wires by checking which 2 wires have continuity.

Lable these 2 wires A & B, lable the other 2 wires C & D.

Connect B to D, go to the other end of the cable and disconnect the 2 wires. Check fo continuity between the wires to find B & D, lable the disconnected wire A, and the last wire C.

 

[SPECIAL LOCATION]

OK start at first end and temporarily label cables a,b,c,d. Link a+b, go to other end.Test cables for continuity and we can then say we have continuity between and the wires are a or b and the other two are c or d. Label the cables and the far end A B C D. Connect B to C at the far end. Go to start remove initial link but remember where it was!

We know when we have continuity we have found the B C linked wires. Wire D is the only wire without continuity that wasn't connected initially. The wire that was connected at the start, but now isn't and doesn't have any continuity is A. The wire that was part of the a, b pair that has continuity is B and the remaining wire is C



Now how do I get to the other end to remove the link?

You could try..

phoning a taxi?

or ring Deke... he'd pop over and do it for you....

probably charge

 

[PBC_1966]

You got it! You don't need to worry about removing the far-end link, as the only condition for the one trip was to identify all four conductors at both ends.

When returning to your starting end, you can also test between the linked wires and the others and then remove the link to continue testing if you wish.

Now........

How do you do it with five conductors? (Let's refer to them as A, B, C, D and E.) Other rules all the same.

 

[Apache]

First end create two pairs and leave 1 end disconnected.

Go to to other end - label first pair A and B, C has no continuity and label second pair D and E. Link A and E and B and C.

Go to other end. C is the wire that wasn't as a pair previously. D is now open circuit, the wire that was paired with D is E. The wire with continuity with E is A. B can be worked out by elimination, but has continuity with C. If there was no second link between B and C you would have 3 'open' wires and be unable to decide between B & D.

 

[Apache]

Can I just thank PBC 1966 - I do love these things. Even though I do know it wasn't meant for Vets!

I have sent a scoob.

 

[PBC_1966]

O.K., got another one!

I think these simple little puzzles are good exercises to keep the mind thinking logically. The trap that a lot of people seem to fall into with this puzzle is in thinking that with "all those wires" to identify, they need to positively label at least some at the first end before going to the other end.

Now, can you do the same thing with six conductors?

 

[PBC_1966]

No takers for the six-conductor problem then?

Hint: Start by connecting three wires together.

 

[codamol]

I shall try my best but if it goes bad day explode thenI'm runnin for the hills.

Connect 3 wires, 2wires and leave one spare at start end. At second end id the 3connected as A,B and C, the two wires as D and E and the spare as F.

Connect F to E and C, D to B and leave A spare.

Back to origin, id each cable from testing for short circuit, as you can see if it was originally in the 3 or 2 connections then you can id each cable.

e.g. B would be in the 3 connection and now would test s/c with and id'ing D from the two connection, and A is the only one without s/c reading.

 

[Apache]

I shall try my best but if it goes bad day explode thenI'm runnin for the hills.Connect 3 wires, 2wires and leave one spare at start end. At second end id the 3connected as A,B and C, the two wires as D and E and the spare as F.

Connect F to E and C, D to B and leave A spare.

Back to origin, id each cable from testing for short circuit, as you can see if it was originally in the 3 or 2 connections then you can id each cable.

e.g. B would be in the 3 connection and now would test s/c with and id'ing D from the two connection, and A is the only one without s/c reading.

That works!

 

[kme]

Ok.. you`re too good at those.

You`re standing in a room with a 3 gang light switch. you know each switch controls a light in one of three rooms beyond the room you`re in; but the self-closing doors are too far away for you to reach them from the switch. You can move to each of the three rooms in turn; but once you walk away from the switch, you cannot go back. Can you explain how to identify which switch controls which room?

 

[PBC_1966]

Connect 3 wires, 2wires and leave one spare at start end. At second end id the 3connected as A,B and C, the two wires as D and E and the spare as F.Connect F to E and C, D to B and leave A spare.

Back to origin, id each cable from testing for short circuit, as you can see if it was originally in the 3 or 2 connections then you can id each cable.

e.g. B would be in the 3 connection and now would test s/c with and id'ing D from the two connection, and A is the only one without s/c reading.

That's the way. For more detail on the last part:

After returning to the first end you know that the single wire must be F. Disconnect the three wires (which you know are A, B & C in some combination), then measure between each of those wires and F in turn. The one without continuity is A.

Remove the link between the other two wires (D & E), then test again between the unknown B/C wires and F. The one which still has continuity is C, and therefore the other one must be B. Now all that remains to measure between the D/E wires and F. The one with continuity is E, and therefore the other one must be D.

 

[PBC_1966]

Can you explain how to identify which switch controls which room?

Leave one switch off, turn two on, and wait a few minutes. Then turn one of the switches off, and go to each room in turn.

One lamp will be on, one will be off and cold, and one will be off and warm.

 

[Badger]

Leave one switch off, turn two on, and wait a few minutes. Then turn one of the switches off, and go to each room in turn. One lamp will be on, one will be off and cold, and one will be off and warm.

:Applaud :Applaud

 

[kme]

Applause indeed.

 

[a1]

Gents,

Can you start a new thread for each one that you have please?

Many many many many many Thanks.

 

[a1]

multi-core cable ID tester

Can I have one please?