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At my workplace there's a room with two entrances, each featuring a push button light switch, A and B. There are two ceiling mounted lights, 1 and 2.

Switch A switches both lights on/off. Funny enough, if both lights are on, switch B switches off a single light (2). Now in this state, switch A "flips" the state of the two lights (2 goes on, 1 goes off).

SWA==on  AND SWB==on  ==> Light1==on  AND Light2==on
SWA==off AND SWB==on  ==> Light1==off AND Light2==off
SWA==on  AND SWB==off ==> Light1==on  AND Light2==off
SWA==off AND SWB==off ==> Light1==off AND Light2==on

I don't know if these are 3-way or 4-way switches.

Just out of academic curiosity: how exactly is this - obviously flawed - circuit possibly implemented?

(Disclaimer: I'm a physician, not any kind of engineer)

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Is a the question is this possible with switches? The answer is Yes. Is the question can I fix it or how do I get a circuit to have this operation? It sounds like you have a shoddy electrician. Somebody probably installed the switch wrong. – laptop2d Feb 4 at 18:24
Well yes, evidently it works, it's just I can't wrap my head around how the circuit would look like, so the question is really how you do it, not whether it can be done. – Tim Gumma Feb 4 at 18:27
You start by making a (truth) table. A column for each of the two switches and a column for each of the two lamps, then you fill all possibilities. – jippie Feb 4 at 18:38
Jippie, to be more precise, I am actually rather looking for a circuit drawing, an actual implementation. I know the circuit for a two-switch-one-light-scenario, even as a physician, but I can't wrap my head around what the circuit must look like for my problem. – Tim Gumma Feb 4 at 18:43
What I really don't understand is how actuating switch B can functionally transform switch A from a two-way-switch (on/off of both lights) into a three-way-switch (either one or the other light is on). – Tim Gumma Feb 4 at 18:47
up vote 8 down vote accepted

Rewriting your truth table a little more succinctly:

| SW-A | SW-B || LAMP1 | LAMP2 |
| on   | on   || on    | on    |
| off  | on   || off   | off   |
| on   | off  || on    | off   |
| off  | off  || off   | on    |


simulate this circuit – Schematic created using CircuitLab

Figure 1. Mis-wired two-way switching circuit.

Does this fit?

In Europe this is referred to as a two-way switching circuit. In the US the switches are referred to as 3-way as they have three wires.

It looks like LAMP1 is wired to the wrong terminal on the switch.

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Possible connection scheme (you have given partial information so this may not be correct)- SWA and SWB are '3-way' (by North American definition) switches


simulate this circuit – Schematic created using CircuitLab

With switches in the shown configuration, SWB turns LAMP2 on and off, LAMP1 is on. SWA will turn both lamps on or both lamps off.

However with SWB in the opposite position, LAMP2 is off and LAMPa is on. Flipping SWA will reverse that (LAMP2 on and LAMP1 off).

To fix this, change the wiring to SWB so the lamps are in parallel, though I'd be tempted to leave it just to confuse people.

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Beat me to it by a minute or two but I think you have the lamps swapped. :^) – transistor Feb 4 at 18:59
Yes, you're right, I didn't pay much attention to the lamp numbers. Swapped around. – Spehro Pefhany Feb 4 at 19:00
I think this circuit does not quite fit the requirements that I now clarified in the original question in monospaced font, if I am thinking it through correctly: SWA should switch both lights on and off simultaneously if SWB is one state. SWA should flip the two lights oppositely if SWB is the other state. – Tim Gumma Feb 4 at 20:31
Just swap the names of lamp 1 and 2 (the way I had it originally!) and consider the switches to both be shown in the 'on' positions, and I think it works exactly. – Spehro Pefhany Feb 4 at 20:42
Okay had to think it through one more time and you're right, I apologise! Thank you! – Tim Gumma Feb 4 at 20:47

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