I didn’t think that Sonoffs had a mains voltage switch input (gpio).
These work pretty well (re-flashed them using tasmota wifi hack) and fit them inside the connection box in the ceiling.
I use my ‘old’ switches as i found new (pulse-)switches too expensive (3 times as expensive as the dimmer itself
).I solved it simply by using the old switches with a piece of foam rubber stuck under it on one side
I’m yet to see a diagram showing how this is achieved using multiple switches (ie: converting a 2 or 3 way switched light circuit…[that’s using normal international terminology, not the US]) without changing the original wiring or doing something dodgy
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I’ve never used Shelly’s but looking at the diagram above it seems they have an LV switch input (rather than ELV like many smart devices) so that is fine. This is not possible with something like an Aeotec Z-wave switch as they have ELV switch inputs. I thought Sonoffs had also ELV GPIO inputs.
They don’t and they don’t need to like I said.
One way, two way, three way, there’s no difference in how you wire it up if it’s normal UK style wiring. The Sonoff is connected to the wires that used to go to the first switch, the switch wires are then connected to the gpio and become low voltage. Simplicity itself if you can fit the Sonoff behind the light fitting.
If I’m not mistaken I did in fact say previously that it can’t be done without changing the wiring slightly, which your method (and in fact what I have at home) is what I was referring to. But this shouldn’t really be done by someone who isn’t an electrician. My reasoning is this:
At least in Australia, each light switch circuit is not necessarily wired completely independent of others in the same area. For example, if there are two switches on one switch plate for say a kitchen light and a dining room light then those will likely share an incoming unswitched active wire. Therefore you can’t change one to ELV to signal a smart device without changing the wiring of the other switch as well. Secondly, you now potentially have ELV and LV in the same wall box without segregation (if you did have enough wires to change only one to ELV and leave the other switch normal).
Also, if the OP is in the US… then be careful since from what I’ve seen the wiring is rather sub standard in a lot of places (eg: wire nuts… they aren’t even legal for use in Australian wiring standard)
Never come across that, sounds like Australian wiring standards are somewhere in-between what we’re used to and the horrors the US allows, plenty of banter on here in the past along the lines of that’s why they only trust them with half the voltage etc.
Wasn’t argueing about whether you change wiring or not. Technically anything that doesn’t plug into an outlet is changing it, even if it’s just replacing a.switch.
You definitely do need to be sure of what you’re doing though which is where what can seem Draconian standards come in handy 
The trick with the dimmer is, you don’t need the SPDT/DPDT switch, you can wire them as SPST and put them all parallel 
Which is the same as posted by maxym.
I only did not replace the switches, I just ‘made’ them a pulse switch by placing some foam-rubber in the switch (which is the ‘cheap’ version 
)
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I only use dumb light switches but smart lights (Phillips Hue and innr bulbs on Hue Gateway, but which system is used doesn’t matter in that case).
That way I can control all lights smart but when the smart system goes down for some reason I can just switch the lights on / off manually on the dumb switches.
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I have just ordered on of these Zigbee 2 way controllers to try out.
@HorizonKane
Not sure about Hue/innr lamps, but Ikea Tradfri turns on when flipping the dump switch momentarily.
But…when switched off with the dump switch, there is no way to turn it on by remote (as there is no power) Guess in that case you still need a pulse switch to do the trick (normally on, pulsed off)
Of course my dumb switches are always on as I usually use HA or HUE app to control them.
When they are switched off, you have to switch them back on to be able to control through the smart system of course.
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My ‘cheapskate’ DIY solution for SPST switch
stuck inside my SPDT switches
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Cool solution!
My problem with this is that I want to be able to turn on lights from remote (i.e. by scene) and if the physical switch is turned off that’s not possible… and it’s a bit unreasonable (I think) to ask people to not use the switches, I mean it’s pretty much human nature to use a light switch at this point in society 
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Thanks for your replies they are very detailed and help a lot.
I should have added that I am indeed in the U.S. and every light switch I’ve ever opened has used wire nuts inside sigh
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Very late to the party here but noticed you’ve used the stitchy as well. Had a sparky mate install some of the switchy dimmers for me but can’t for the life of me get them to dim via HA except for jumping through 600 hoops. Did you use the dimmers or just normal ones? They dim perfectly through Google assistant though
I have had no problems with them working with HA via the Phillips hue integration. I have not tried using with a zigbee usb stick. I have both dim and switch units my only problem is the current model dimmer doesn’t dim via the switch but a new model with built in push button is coming soon
Those looks great, definitely on the wish list. Thanks for your response too, appreciate the insight.