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I want to add a switch to an existing lighting circuit to toggle the function of the circuit.

Currently, I have a 'standard' 3-way lighting circuit with two switches at opposite ends of a space, controlling a pair of lights in the space -- one light is near each switch. I seek to be able to switch between this mode of operation (i.e. where both lights are controlled, together, by either switch), and the mode of operation where each light is independently controlled by only the switch nearest it.

To be clear, I do not want to change this permanently to two separate simple light-with-single-pole-switch circuits. Rather, I want to add a switch to the circuit to make that mode be one of two selectable modes of operation (with the other mode being just as it is now.

Extra credit if I can make it such that, when operating in the mode wherein the lights are decoupled, the switches for each are always in the proper orientation (i.e. toggle up is on, down is off).

The extra switch can be 2-way, 3-way, or even 4-way, and can be placed anywhere. How can I wire this? (Technically, I'll use a 'Shelly' internet-controlled switch to toggle between modes of operation -- they come in SPST, SPDT, DPST, and DPDT configurations, as far as I know, but I don't think that is germane to the question -- I think it can be done with an ordinary switch just-the-same.)

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    Unless you use smart switches and momentary physical switches, the toggle position will always be confusing. Commented Feb 26, 2024 at 11:11
  • You'd have to isolate the three way function of the circuit and make it two single toggle ones. You'd need a few miles of wire and many DPDT break before make relays. Just add two lights and two single switches on a seprate circuit. Commented Feb 26, 2024 at 13:19

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Vague, but specific question, so I'll give a vague, but specific spit-ball answer:

IF the power comes into one of the 3-way switches (and isn't supplied at one of the lights)...

  1. Install a new DPDT switch on the power supply side before it reaches this part of the circuit.
    • flipping this switch completely removes power from the 3-way circuit & lights and will, instead, supply power to a new set of wiring installed as follows:
  2. Install new wiring (made hot by flipping the switch in the step above).
    1. One set of "always" hot goes to a second switch at the hot supply side. ("Always" hot when the DPDT switch is in the appropriate position, that is.)
    2. One set of "always" hot goes to a new switch by the other 3-way switch. ("always" as above.)
    3. Each of these two new switches then gets additional wiring to the appropriate light.
    4. At the light, each set of wires feeding the light would need to flow through a circuit capable of handling 120v @ 15 (or 20) amps. This circuit would prevent power from back-flowing into the other wiring.
      • without this circuit, turning on one light would flow power through the wires connecting the two lights and they'd both come on.
      • In a DC circuit, this would be implemented with a diode capable of handling the appropriate voltage/amperage. I don't know how this would be implemented in AC.

All in all, this is going to be quite difficult and will require opening up your walls to run a bunch of new wiring.

Since you're looking at using smart devices (I'm a fan of the Shelly relays, as well, make sure you're getting the UL listed versions), it may be easier to replace the 3-way switches with simple 2-way switches, each in combination with a Shelly relay, then use automation and a digital-only toggle to determine whether flipping a switch controls one bulb or both.

To do this will require running always hot power from the current hot supply to both switches. Depending on the current topography of the wiring, this might be easyish or difficult. If you choose to go this route, post a whole new question asking how to separate the switches and lights to get always hot power to both. I believe you might be able to do it without pulling additional cable, but you'd need to supply pics of the wiring at each switch and each light.

Or, for an even easier solution, simply replace the existing bulbs with smart bulbs that can be controlled by your home automation software, leaving all the existing wiring as-is. Maybe replace the dumb 3-way switches with smart 3-way switches so that a physical switch press still works and you don't have to worry about someone turning off power to the smart bulbs by hitting the switch.

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  • So you'd be adding three switches? Commented Feb 26, 2024 at 14:15
  • Yup, @JACK, I would. In a general, vague way, that's my spitball answer. Or, more likely, I'd rejigger (that's a technical term!) the existing setup to make it all software controlled instead of hardware controlled, especially since the OP has already mentioned using smart switches. Commented Feb 26, 2024 at 14:18
  • Added a better, even easier option at the end. Commented Feb 26, 2024 at 14:20
  • Thanks for the response. Adding a collection of switches and wiring, leaving some switches dead at times is a non-starter. There has got to be a way to do some switching between the two 3-way switches to make this work. I can't believe I haven't found some post including the solution already. I think you're on the right track with using 2-way switches and letting Shelly smart relay(s) handle all the logic. I just haven't figured out if there is a simple mode for that built-in. Handily, I think one of the two 'carrier' wires in the 3-way circuit can simply be repurposed as Shelly power. Commented Feb 27, 2024 at 10:55
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    I'm about 99.999% sure that this isn't as easy as you want it to be @Jimbo1987 with just plain dumb switches, or people would do it. With smart switches I'm sure it's an obtainable goal, but I doubt that there's a simple, built-in function for that, either. I use HomeAssistant and I know I could make it happen with automations and a helper sensor or two, but I'm pretty sure there's nothing "canned" to make that happen. It's just not a common scenario. If you use HA, check out their Discord server - loads of help for sorting out the automation. Commented Feb 27, 2024 at 12:51
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It seems it isn't possible with a dumb switch of any kind.

However, I found that the "Plus" and "Pro" series of the Shelly switches can use mJS scripting to create any mode of operation your heart desires. I'll get a Shelly Plus 2PM and write an mJS script to handle the logic. I'll repurpose one of the carriers (the two wires that alternately carry the current between the 3-way switches) to be a full-time power supply to the Shelly device, installed in the ceiling box, and the two 3-way switches become plain-old toggle switches that I'll program to act as "edge" switches when in the "3-way mode" and act as regular (persistent state) toggle switches when in "independent mode". This covers all bases, including putting the orientation of the switches right whenever it's in "independent mode". Whoot!

References:

Lastly, I found a repository online for example scripts . This application might be too complex for their liking, but I'll submit it there when I'm finished with it. Maybe they'll add it to their repo.

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    Basically, this is what I recommended. Commented Feb 29, 2024 at 13:54
  • @FreeMan, Ahhh, indeed. It is more-or-less your middle solution. Not understanding how the smart switches worked prevented me from 'grokking' what you wrote. Many Thanks for putting me on the right track! Commented Mar 1, 2024 at 12:48

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