# I have weird light switches: A Puzzle

**URL:** <https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769>\
**Category:** Factual Questions\
**Created:** [May 27, 2009, 2:51am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769 "2009-05-27T02:51:01Z")\
**Posts on this page:** 20\
**Page:** 1

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**Author:** ![Klif](https://avatars.discourse-cdn.com/v4/letter/k/f08c70/32.png) [@Klif](https://boards.straightdope.com/u/Klif)\
**Post date:** [May 27, 2009, 2:51am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/1 "2009-05-27T02:51:01Z")

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For the record, the following is a real-world situation.

Wifey and I bought a house recently, and this lighting issue has puzzled me since day one. There are three light switches: one at the bottom of the stairs, one at the top of the stairs at the beginning of the hall, and one at the end of the hall. Let’s call the switches A B and C. I went through and flipped through every combination of up and down for each switch, and noted the light’s reactions. Unless I did something wrong, I got a total of 8 scenarios, as below:

Capital letters are switched UP  
lower case letters are switched DOWN  
Lights On is represented by an \*  
Lights Off is represented by a #

abc = #  
Abc = \*  
aBc = \*  
ABc = #  
abC = #  
AbC = \*  
aBC = #  
ABC = \*

The puzzle is, how the heck are these switches wired? How can I logically think it through so that it makes sense?

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<div class="post-metadata">

**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\
**Post date:** [May 27, 2009, 3:04am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/2 "2009-05-27T03:04:45Z")

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Have a look at [this and see if it clears anything up for you.](http://www.electrical-online.com/4-wayswitch.htm)

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [May 27, 2009, 3:05am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/3 "2009-05-27T03:05:08Z")

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You have a combination of three- and four-way switches, as diagrammed [here](http://www.electrical-online.com/4-wayswitch.htm).

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**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\
**Post date:** [May 27, 2009, 3:07am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/4 "2009-05-27T03:07:56Z")

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If switch C is up, the light is controlled by switch A. Otherwise, switches A and B have to be in opposite positions for the light to be on. I have no idea what that corresponds to in terms of the actual wiring.

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**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\
**Post date:** [May 27, 2009, 3:49am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/5 "2009-05-27T03:49:13Z")

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Here’s one circuit that could produce these results:

```auto

                                      1
                      +---------------o C
                      | <--*--+
              1 | 0 |
           A o-------+---o B |
HOT -----*--> <--*--------------+---- LIGHT
              o-----------o
              0 1

```

Here A and B are 3-way switches, connected in the usual way so that either controls the light. C could just be a normal on-off switch or a 3-way with nothing interesting connected to its 0 terminal.

In this diagram, there are two wires running from A to B and two running from B to C, which is just what you’d expect for an attempted 4-way wiring. It’s possible that B is a miswired 4-way switch (wires running from both ends of one switched connection rather than from one side of each switch); C may also be miswired in this case. I haven’t seen enough actual household 4-way switches to know how easy it would be to do something like this.

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**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\
**Post date:** [May 27, 2009, 4:06am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/6 "2009-05-27T04:06:52Z")

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> [@Klif](#):
>
> For the record, the following is a real-world situation.
> 
> Wifey and I bought a house recently, and this lighting issue has puzzled me since day one. There are three light switches: one at the bottom of the stairs, one at the top of the stairs at the beginning of the hall, and one at the end of the hall. Let’s call the switches A B and C. I went through and flipped through every combination of up and down for each switch, and noted the light’s reactions. Unless I did something wrong, I got a total of 8 scenarios, as below:
> 
> Capital letters are switched UP  
> lower case letters are switched DOWN  
> Lights On is represented by an \*  
> Lights Off is represented by a #
> 
> abc = #  
> Abc = \*  
> aBc = \*  
> ABc = #  
> abC = #  
> AbC = \*  
> aBC = #  
> ABC = \*
> 
> The puzzle is, how the heck are these switches wired? How can I logically think it through so that it makes sense?

The answers so far refer to how a standard three switch circuit is wired but not to how THIS circuit is wired. I am going to say there is a mistake and the output of positions 4 and 5 are inverted.

```auto

Pos A B C ON 
0 abc = # 1 0 0 0 1 
1 Abc = * 0 1 0 0 0 
2 aBc = * 0 0 1 0 0 
3 ABc = # 1 1 1 0 1 
4 abC = # 1 0 0 1 0 
5 AbC = * 0 1 0 1 1 
6 aBC = # 1 0 1 1 1 
7 ABC = * 0 1 1 1 0 

```

Pos 4 should yield OFF and pos 5 ON with the standard wiring. If the table given is indeed correct then I cannot think how the wiring goes. Can you confirm the table is all correct?

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<div class="post-metadata">

**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\
**Post date:** [May 27, 2009, 4:15am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/7 "2009-05-27T04:15:26Z")

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> [@sailor](#):
>
> The answers so far refer to how a standard three switch circuit is wired but not to how THIS circuit is wired. I am going to say there is a mistake and the output of positions 4 and 5 are inverted.

The first two answers do seem to assume that, but **ultrafilter** and I refer to the truth table as posted in the OP. I’m guessing the OP’s table is correct, because if the switches acted like a normal XOR (like a correctly-wired 4-way switch setup) he probably wouldn’t consider it unusual.

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<div class="post-metadata">

**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\
**Post date:** [May 27, 2009, 4:33am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/8 "2009-05-27T04:33:01Z")

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> [@Omphaloskeptic](#):
>
> Here’s one circuit that could produce these results.

Maybe I am analyzing it wrongly but I do not get the same results with that circuit.

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<div class="post-metadata">

**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\
**Post date:** [May 27, 2009, 4:41am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/9 "2009-05-27T04:41:41Z")

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> [@sailor](#):
>
> Maybe I am analyzing it wrongly but I do not get the same results with that circuit.

Arrgh, you’re right. Well, I was _trying_ to refer to that table, anyway. How about this one:

```auto

                                      1
                      +---------------o C
                      | <--*--+
              1 | 0 +---o |
           A o-------+---o B | 0 |
HOT -----*--> <--*---+ +---- LIGHT
              o-----------o
              0 1

```

This one looks even more like a miswired 4-way at B.

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<div class="post-metadata">

**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\
**Post date:** [May 27, 2009, 4:53am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/10 "2009-05-27T04:53:39Z")

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> [@Omphaloskeptic](#):
>
> Arrgh, you’re right. Well, I was _trying_ to refer to that table, anyway. How about this one:
> 
> ```auto
> 
> 1
> +---------------o C
> | <--*--+
> 1 | 0 +---o |
> A o-------+---o B | 0 |
> HOT -----*--> <--*---+ +---- LIGHT
> o-----------o
> 0 1
> 
> ```
> 
> This one looks even more like a miswired 4-way at B.

I need to get some sleep and do not have time to analyse it but if it does work that way then rather than “miswired” I would call it “ingenious” because it obviates the need for a 4-way switch.

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<div class="post-metadata">

**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\
**Post date:** [May 27, 2009, 4:58am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/11 "2009-05-27T04:58:49Z")

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> [@sailor](#):
>
> I need to get some sleep and do not have time to analyse it but if it does work that way then rather than “miswired” I would call it “ingenious” because it obviates the need for a 4-way switch.

Well, except that the light can’t always be switched on/off from all positions: e.g., if A and C are on, B doesn’t do anything (the light stays on no matter what).

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**Author:** ![j\_sum1](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@j\_sum1](https://boards.straightdope.com/u/j_sum1)\
**Post date:** [May 27, 2009, 5:09am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/12 "2009-05-27T05:09:28Z")

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> [@Omphaloskeptic](#):
>
> Well, except that the light can’t always be switched on/off from all positions: e.g., if A and C are on, B doesn’t do anything (the light stays on no matter what).

Agreed.  
The situation as described matches the circuits linked to by **HongKongFooey** and **Q.E.D.**. But IMO is less than satisfactory for the reaqson stated above. I have often wondered how to set up a simple circuit with more than two switches so that any switch can reverse the state of the light no matter what the configuration of the other switches is. I have never succeeded.

---

<div class="post-metadata">

**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\
**Post date:** [May 27, 2009, 5:18am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/13 "2009-05-27T05:18:31Z")

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> [@j\_sum1](#):
>
> Agreed.  
> The situation as described matches the circuits linked to by **HongKongFooey** and **Q.E.D.**. But IMO is less than satisfactory for the reaqson stated above. I have often wondered how to set up a simple circuit with more than two switches so that any switch can reverse the state of the light no matter what the configuration of the other switches is. I have never succeeded.

The circuits linked to above are the right way to do what you want to do (and presumably what the electrician wiring **Klif** ’s house wanted). It requires special switches, though (the 3- and 4-way switches described there).

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [May 27, 2009, 5:51am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/14 "2009-05-27T05:51:55Z")

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Am I the only one who thinks that maybe **Klif** should hire an electrician to look over the whole house? Whoever wired those switches obviously didn’t really know what he was doing, and who knows what else in the house he didn’t know how to do and wired some crazy way. This particular wiring is harmless (if baffling and perhaps somewhat inconvenient), but maybe the next thing he miswired isn’t.

---

<div class="post-metadata">

**Author:** ![Klif](https://avatars.discourse-cdn.com/v4/letter/k/f08c70/32.png) [@Klif](https://boards.straightdope.com/u/Klif)\
**Post date:** [May 27, 2009, 9:39am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/15 "2009-05-27T09:39:29Z")

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> [@Chronos](#):
>
> Am I the only one who thinks that maybe **Klif** should hire an electrician to look over the whole house? Whoever wired those switches obviously didn’t really know what he was doing, and who knows what else in the house he didn’t know how to do and wired some crazy way. This particular wiring is harmless (if baffling and perhaps somewhat inconvenient), but maybe the next thing he miswired isn’t.

Ha! That’s what -I- was thinking!

I can confirm, by the way, that the table in the OP is correct.

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<div class="post-metadata">

**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\
**Post date:** [May 27, 2009, 10:52am UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/16 "2009-05-27T10:52:19Z")

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> [@j\_sum1](#):
>
> I have often wondered how to set up a simple circuit with more than two switches so that any switch can reverse the state of the light no matter what the configuration of the other switches is. I have never succeeded.

As noted, you just need 3-way switches at each end of the chain, with a succession of 4-ways between them. Then, whatever the state of the switches, flipping any one switch will change the state of the light.

The intervening 4-way switches effectively change the parity of the whole arrangement, so that instead of the 3-ways at the end needing to have the same orientation, they must have the opposite orientation. Flipping a second 4-way switch, or any even number of 4-ways, returns the parity to its original state.

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<div class="post-metadata">

**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\
**Post date:** [May 27, 2009, 1:46pm UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/17 "2009-05-27T13:46:44Z")

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> [@Omphaloskeptic](#):
>
> Well, except that the light can’t always be switched on/off from all positions: e.g., if A and C are on, B doesn’t do anything (the light stays on no matter what).

Right. Looking at the circuit though it seems like all the wires are in place and the only thing needed is to replace switch B with a 4-way switch. Other than that the circuit is correct so it does seem like someone was missing a 4-way switch and just put a 3-way switch instead.

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<div class="post-metadata">

**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\
**Post date:** [May 27, 2009, 3:27pm UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/18 "2009-05-27T15:27:41Z")

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I should post a similar thread about my parents’ basement. There are at least a dozen switches controlling three or four banks of lights, and it’s seemingly completely random. I’ve never seen anything like it. I swear I’ve seen one switch control two different sets of lights.

To be fair, it seems that the lady who built the house was a class A bitch, and the builders hated her with a passion. This isn’t the only example we’ve found.

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<div class="post-metadata">

**Author:** ![jkirkman](https://avatars.discourse-cdn.com/v4/letter/j/ac8455/32.png) [@jkirkman](https://boards.straightdope.com/u/jkirkman)\
**Post date:** [May 27, 2009, 5:47pm UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/19 "2009-05-27T17:47:27Z")

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> [@Smeghead](#):
>
> To be fair, it seems that the lady who built the house was a class A bitch, and the builders hated her with a passion. This isn’t the only example we’ve found.

Her name wasn’t Laura Croft, by any chance?

Because, you know, she has to solve these kind of puzzles so many times during her work day that maybe she put this in her house just to warm up while her morning coffee is brewing…\*\*

I worked my way through the entire TR series, but never could solve the puzzle of some of the banks of switches in my parents’ last house.

\*\*For the non-gamers out there, this is a reference to a character in a series of video games

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<div class="post-metadata">

**Author:** ![shiftless](https://avatars.discourse-cdn.com/v4/letter/s/e9c0ed/32.png) [@shiftless](https://boards.straightdope.com/u/shiftless)\
**Post date:** [May 27, 2009, 6:57pm UTC](https://boards.straightdope.com/t/i-have-weird-light-switches-a-puzzle/497769/20 "2009-05-27T18:57:25Z")

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Your house was actually the first electric computer. Sure, back in those days, it took a device the size of a house just to calculate a single binary, and it took a small team of programmers to work the switches, but it was a start.