I am intending to use the Sonoff Mini R4 connecting a 3-way connector whose central pin I will connect to the S2 input. Input S1 will be free, as shown in the diagram. I have no condition to pass other cables.The load is 127V.
However, the line (let’s call it L1) that will go into connector S2 is different from the line that will go into Lin (let’s call it L2). Both lines are 127V, but they have different phases. Am I clear?
Any problems with this connection? I mean, does it matter if Lin and S2 use different phases? Will it work? Or it will explode? 
Thanks.
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There is no problem if they are from different lines. It would be a problem if they were different phases! Or more precisely, the actual L1 of your installation, it should be L1! If you have three phase voltage and meet the two phases it will explode!
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No problem if you use an intermediary switch in the middle (4-way) and:
1- This circuit with a 220V lamp. L1 and L2 only, without Neutral. Bluewire is L1.
2- 127V lamp. L1 connected to switch and to Lin of Sonoff Mini R4.
In two cases, the L1 from swith must be connected to S2 of Mini R4 and the S1 must to be open !!!
I have the option 2 at my home with Sonoff Mini R4 and works very wheel.
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Thanks, for the help.
This is the case, L1 and L2 are different phases. In my Electrical Box, I have 2 different phases and 1 neutral.
One phase is connected to some part of the house and the other phase connects to other parts.
I am sure they are different phases because my shower is 220V and it is connected to both phases.
So, I can’t connect the way I was thinking…
But, if I must leave S2 open, how can I turn on/of the Mini R4? I know that S1 is short-circuited to Lin. Isn’t it?
Sorry I corrected the text. The S1 must be open and the S2 connected to the phase returning from switch.
This is shown in manual. Take a look at the fourth figure, to do what you want just add the intermediary switch (4-way) between the others:
The wiring of switchs has to be like this:
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Whatever you do, you are in danger of encountering both phases! In my opinion, come up with another option.
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Thanks for the attention.
I am trying to set like the last diagram you sent. A 3-way connecting to S2. My doubt, like I said, is if the phase connecting to S2 can be different from the phase connected to Lin.
My electric box has 2 phases, but the one that comes from the 3-way is different from the phase that is next to the last 3-way connector.
Thanks, again.
I agree.
The safest option is only one phase, as the forth figure shown in manual.
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In my understanding, you can´t use two phase and one neutral.
Most smart switches on the market may have isolated regulators (ex. for 3V3), but do not have isolated inputs (S1, S2) and that is the reason you should not do this.
Thats my oppition, because I had not seen the schematic of Mini R4. Someone who know about Sonoff devices should confirm it.
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I don’t believe there is because this is not a standard connection and it is dangerous and the device will get bigger and the idea is to make it smaller.
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The option I can think of is to replace the electric switches with pulse ones. And not to use one phase.
Here I made a diagram, it may help you if the cables in your installation are correct.
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The problem in the fourth diagram is that in the existing electrical wiring the phase is on one side and the lamp cable is on the other side. In principle, there is no zero, but suppose that somehow a neutral wire is pulled to the console where the lamp is.
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Simply use your multi-switch circuit to connect S1 and S2 as in the top right illustration in Julhio’s response (you can have as many two- / three-way switches in series as you like – two are illustrated). The only power applied to your Mini R4 is at the Mini R4’s “L In” terminal, and that is used to power the load (and check whether or not there is a closed path between S1 and S2).
If your switches are connected to something else, as well, that is not illustrated in your initial diagram. To simply use three switches to control one light does not require that they have independent power, only that they have a way of completing the circuit between S1 and S2.
I apologise if I misunderstand your configuration.
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on one side of the electric switch there is phase L1 and on the other side there is a neutral wire and a different phase L2
I see! There is no possibility of pulling a third wire so that the control has to be done with only two wires. It may be better to use a “remote” switch like the R5 (no wiring needed) or the S-MATE2 (power from the “local” wiring).
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It can also be done with R5 1. But if he wants to use the traditional electric switches, the alternative is the scheme I made, the electric switches must be replaced with pulse ones. Maybe you didn’t pay attention to the scheme I proposed.
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Oh, I did! I was just offering options. 
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paulojam I don’t understand why you changed your question? That way it comes across as you know what’s going to happen, but you’re not sure, but you’re asking anyway! More precisely, you make us look like fools, to go crazy and at the end of what you found out, add it to the question! I thought it was made abundantly clear that you shouldn’t do that. But if you don’t believe, just act!
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If L1 goes to Switch 1 and there is two intermediate wires linking switch 1, switch 2 and switch 3, I think the best solution is buy a long wire and pass L1 insite conduit of Switch1 throught the other side of Switch 2, and connect to Mini R4:
It will take more work but it’s guaranteed.
I have two connections with 3 switchs and Mini R4 exactly like that at my home and work very well.