Can I convert light switch into smart light switch without buying smart light switches?

I have an area with 3 way light switch. Is there any way to convert it into smart light switch without buying smart switches? For example, add Sonoff device or any other options? I would prefer to add it directly to my chandelier and not to the switch because there is not enough room in the switch box.

A sonoff mini might be small enough to fit in your chandelier rose.

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Thank you for suggestion, but sonoff mini is On or Off. I need it to behave as a relay. I need it to switch from one position to another one. I have to use it with 3 way light switch.
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So?
3-way switch to you means you have three settings?
Off, on1, on2?

Generally 3-way switches mean you have two switches to control one light, like at the bottom and top of a staircase.

This is 2 way switching. The common does not count as a switch point. ie: there are 2 ways the current can flow. common to 1, common to 2

When I look at sonoff mini, I do not see it having common to 1 and common to 2. It looks like just on and off. Any yes, I think I understand how 3 way switch work.
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I was referring to the diagram of the switch you put up. The bit that the switch arm pivots on in the diagram is referred to the common on a switch. The sonoff is a single through switch so is only on/off, as you said. Using one in this instance can be done but you will lose functionality.

Putting it at the light fitting:
You can switch the light on/off via HA, only if the physical light switch is on.
If the light switch is off, HA loses control

Putting at the light switch:
You can only use it from one location, so if the light circuit is currently switched from 2 locations (2-way switching) then you are again limited.

Okay, sorry for misunderstanding, and yes, I am looking for something like this. Does Sonoff or any other device has it? I need device that I do not have to connect to 5v and can use the power that is connected to the lights.

Lots of options.

So you are wanting to switch this light from 2 physical locations? (existing switches)

I use z-wave so I would put a z-wave switch in the roof and slightly modify the switch wiring to suit using 2 switches with the one z-wave switch. But that is not really for a non-electrician as you need to know what you are doing.

I understand in the US you use ‘3-way switch’ to describe the fact that you have 2 (or more) switches toggling one light (or group of lights). That is entirely possible with a sonoff mini. You just need to have the wiring correct. I use them (and shelly 1’s) like that.

I am trying to build something like this, but instead of that last light switch I want to use Sonoff or any other device. Some device that does not need any additiona (5v) power.

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That did not make it any more clear to me.
What is those boxes? Do you have them currently? Can you take detailed pictures of them?

I have a room with two switches.

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I want to add another device and have this:

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Obviously I will have to update the middle light switch from 3-way to 4-way switch and add another device.

It will be much easier if you don’t use terms that usually means something else when you try to describe something.

As stated before 3-way and 4-way is not count of connection points in a switch.
It states the number of switches controlling the same light.

Your upper picture will in most countries be called a 2-way switch, but in US 3-way. And by that I mean all of the picture, not 3-way + 3-way.
The second image is what would be called a 3-way or 4-way (US).

But what you need is a relay there with common and NC/NO. I don’t think I have seen any, but they can be easily built with an ESP.

If, at the last switch, you have only 2 wires arriving and 1leaving, you can’t use a sonoff mini or Shelly 1. They both need a live and neutral to power them. And since the switch wires are ‘hot’, you can’t use a sonoff mini as the S1/ S2 (the switch inputs) don’t tolerate power.

If you want to use multiple switches to toggle light through some relay, I suggest to use momentary switches.
Existing wires can be reused but you can use infinity number of switches with simple wiring (unlike toghle switches). I strongly recommend such approach as it’s easier, allows to install relay device anywhere due to parallel wiring. Also you get consistent position of knobs

Anyway you can use sonof or shelly1 or anything similar which can react on input change (for configuration with bi-stable switches) or on ramp-up change for momentary ones

Here is diagram for shelly1 used with momentary switches.

gyR8Icz

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To try to help clear things up (and probably add more to the confusion)
The US (and I think Canada too) counts the number of connections on a switch and calls them ‘ways’ (the reasoning is a bit tenuous)
The rest of the world looks at a switch and counts the electrical circuits you can make with it.
So the above “American 3-way” can switch this way or that way (ie two ways) there is no 3rd way
So we should go for common terms
A single pole, - single throw (SPST) is just an on-off (American 2 way - World 1 way)
A single pole - double throw (SPDT) is a change over (American 3 way - World 2 way)
A double pole - double throw (DPDT*) is an intermediate (American 4 way - World intermediate)
Note: the above DPDT* actually has its output ports cross linked to provide the X configuration you see.
As Dave says it is possible to just wire the existing wiring into even a sonoff mini to provide the desired functionality, though this is not recommended for the non-electrically qualified. The problems usually arise when you don’t have 4 wires between all the switches ‘left’, ‘right’, ‘return’ and (of course) ‘earth’ (regional terminology may vary) and three to the lamp L, N + E
The ‘switch’ should have L, N + E which is easy if you have power to the rose where you have the ‘switch’ OR at the wall switch point, where you have the ‘switch’
Any other combination is a moderate nightmare.

Edit: it took me a while to write that and maxym came in with another solution which could reduce your wiring headaches but ‘may’ change the way you operate those switches and/or need different (momentary) switches.

I must say it does frustrate me that the US name things wrong… but anyway…

Swapping the existing light switch for momentary ones is what my solution uses (didn’t go into the detail previously) however there is another option if you didn’t want to go down that path.

You could use the Sonoff to energise a relay and wire the relay to perform the switching of your last switch in the diagram (US ‘3-way’). The thing is though, the Sonoff (and therefore HA) won’t know that status of the light.

I did not know this. Too bad.