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I have a problem that I believe there is a solution to but I'm lacking in the knowledge on how to do so.

There two switches in a building. Both are single pole single throw switches and cannot be changed or replaced.

I need to make these operate like a standard 2 way switch you'd find at opposite ends of a hallway to control a single light.

There are cables for live, neutral and ground, with two travelling wires between both switches.

I've seen somewhat similar questions answered by suggesting the addition of a flip flop switch or a relay, but I'm not confident on how they work so don't want to risk a short circuit.

Would anyone here be able to advise on this? I would gratefully appreciate any input.

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    $\begingroup$ you're out of luck if you cannot replace the switches ... are you even legally allowed to be working on this? $\endgroup$
    – jsotola
    Commented Dec 12, 2023 at 2:14
  • $\begingroup$ We'd need a diagram showing the wiring in each switch box and the lamp. 1. Presumably the circuit originally used 2-way switches? 2. And the light works if both switches are in the ON position? 3. And one of the travelling wires is unused at present? $\endgroup$
    – Transistor
    Commented Dec 12, 2023 at 8:54
  • $\begingroup$ I can't provide a diagram, but you can safely assume it was a standard 2 way switch original wiring. $\endgroup$
    – denismoc
    Commented Dec 12, 2023 at 9:52
  • $\begingroup$ Regards to the legal question, this is not for commercial or residential. Please think of this as just a theoretical problem. I just want to understand a solution before requesting any professional to get involved. $\endgroup$
    – denismoc
    Commented Dec 12, 2023 at 9:54
  • $\begingroup$ Effectively I'm looking for an XOR logic device if it even exists? $\endgroup$
    – denismoc
    Commented Dec 12, 2023 at 12:11

1 Answer 1

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It's not impossible, but might not be a good idea.

schematic

Figure 1. Original, current and future(?) wiring.

Option (c) converts the lines to DC. SW4 will pass positive half-cycles and SW5, negative half-cycles. RLY1 will energise on positive and RLY2 on negative. The relay contacts perform standard 2-way lighting function.

Your technical problem now becomes finding a device that will switch on with half-wave DC current and not chatter violently. Opto-isolators are another option, would be silent and integrate easily with a "smart relay" to do the switching logic.

Your compliance problem is whether or not this is legal and meets local electrical standards.

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  • $\begingroup$ Actually I found this item yesterday after searching for the device you suggested. Salus RM16A Hard Wired 16amp Relay. Would that work? $\endgroup$
    – denismoc
    Commented Dec 13, 2023 at 9:33
  • $\begingroup$ I have never heard of it. $\endgroup$
    – Transistor
    Commented Dec 13, 2023 at 10:42
  • $\begingroup$ Can't you just wire each switch to an SPDT relay, and then treat the relays as 2-way switches, like (a)? $\endgroup$
    – Simon B
    Commented Dec 13, 2023 at 11:56
  • $\begingroup$ That's what I'm thinking this would achieve. (Salus RM16A Hard Wired 16amp Relay). Looks to be the right rating. $\endgroup$
    – denismoc
    Commented Dec 13, 2023 at 12:10
  • $\begingroup$ @SimonB, only if there are two wires from the second switch to the lamp - and that's unlikely. That's why I asked for a diagram. $\endgroup$
    – Transistor
    Commented Dec 13, 2023 at 15:41

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