Fixing a dodgy two way lighting circuit

[tidal star]

I recently got a nice new fuse board fitted in my house (UK 1970s property) with individual RCD breakers on each circuit. Unfortunately, this unearthed that my two way switched upstairs landing light was wired up badly. The original wiring of used a live from the downstairs circuit, and a neutral from the upstairs circuit. This is exactly the setup described here as the alternative "not-recommended" way of wiring up a two way switch. This was always dodgy, its just that the new fuse board picked up on it - tripping out whenever the light was used. For now the circuits have been properly separated, and the upstairs and light and switch have been re-configured into a typical single switch setup.

The frustrating thing is the cable going between the two switches is only twin and earth, to do a two way switch properly a multi-core cable is needed. I can't easily run a new cable between the switches without causing a lot of damage. However, I can easily get twin and earth from both switches up into the loft, so I think I should be able to implement some kind of relay solution to get two way switching back.

Has anyone here done something similar before? I ideally would like something that would work as a two way switch without any dependency on a network or hub, with integration into HA as an optional extra. I have used some Shelly devices before, but only in basic configurations. I suspect a Shelly device with a dual relay could do this? i.e. Shelly 2PM or Shelly Dimmer, but it doesn't seem to be their primary use case, so I am not that sure.

I am aiming for something good enough to last until I have an opportunity to re-pull cable i.e. the floorboards are up and I am redecorating. Any thoughts or suggestions would be great.

[timid fog]

There's no way to do it with smart tech without any network or hub... not sure how that could possibly work, you'd need some way to communicate

The next question would be to ask what wiring you have at each of the two switch locations

The second location has a single twin and earth to the first location? What does the first location have? Specifically, does it have live and neutral, or does the neutral stop at the ceiling rose?

If you have live and neutral at switch 1 then there's a super easy setup available to you... just put a smart relay at location 1 which physically switches the light

Then use the T&E to send live and neutral to location 2, where you'd put a second relay or smart switch which you use to remotely trigger the relay at location 1. It wouldn't physically switch anything

[tidal star]

I have live, neutral and earth at the rose of the light and have easy access to it from the loft so could install a junction box above it if needed. I don't have a neutral at either switch. I have twin and earth going from the rose to the upper switch, and another twin and earth going from the upper switch to the lower switch. I have been able to drop an additional twin and earth from the loft into the upper switch to effectively extend the twin and earth connection from the lower switch all the way into the loft. So I now have: live and neutral at the rose, and a twin and earth down to both switches. I am pretty sure I could do this with two smart relays, one connected to the light and one of the switches, and the other just communicating to the other. What I am hoping is I can avoid the dependency on a network through the use of a device that can work with two switches (a dual relay) such as the Shelly 2PM or Shelly Dimmer. Does that make more sense?

[timid fog]

Does the twin and earth to each switch go directly from the rose? If so, you can just set it back up as a classic two way switch, surely?

If it goes via one switch then to the other, I don’t think you’ll be able to do this without at least one switch relying on network connectivity of some description between them

[tidal star]

It wasn't originally, but I can essentially get get twin and earth from each switch to the rose. In my understanding this isn't what you want for a classic two way switch, for a classic two way switch you want three cores going between the switches. Thanks for all your input.

[timid fog]

You can do it with two cores between the two switches... but I wasn't accounting for the need to take the L/switchedL to them

Although thinking about it, this is all overcomplicated - as long as you have two cores from the rose to each switch, I believe you can do it with a single Shelly 1L, if you set it to use them as toggle inputs not on/off switches

Or possibly this, I'm not quite sure what the difference is between the two setups (one uses Sx, the other uses L directly)

[tidal star]

The diagrams are really helpful thanks. I think we are on the same page now, and confirms what I was hoping. I think this is possible with the Shelly 1L (and maybe the the Shelly 2PM) as it has two switch connections on it. The first diagram is for when you have a neutral at the Shelly, and the other is for when you don't and use a Shelly bypass. As I would be doing this in a junction box above the rose I can have access to a neutral.

What made me unsure is the Shelly 1L's primary use case is around dimming, and the 2PM seems to be about managing blinds. So they appear to have two switches for specific up/down purposes, not to do two way switching on a single bulb. So I was unclear if the config options would easily support this use case - will probably just have to take a punt. Have you got any opinion on if the 1L or 2PM would be a better bet? Thanks for all your help.

[timid fog]

The 1L definitely has an ability to set the switches to toggle, I’ve never tried it as a two way switch

I feel like it’s worth a shot though