I was looking for a cheap solution that will allow me to use my existing wall switches to control lights in my flat.
I decided to use cheap Xiaomi Smart Wireless Switch (~7 USD) and Sonoff Wireless Smart Switch (~7 USD). I modified a bit Xiaomi button by adding two external wires and removing unnecessary plastic “button”. Size of the modified switch is very small so it allowed me to put it into my wall together with my previous wall switch.
I got confused with one thing, why do you need (where did you use) the sonoff?
From what i see, you only applied the xiaomi switch to the previous wall switch, right?
So you only used the xiaomi switch because it’s smaller and fits into your wall? Because otherweise you could have simply installed the sonoff into your wall and connect your switch to the GPIOs of the sonoff.