text: Basically, you can use the same light switch, without any modification to the Sonoff device.
  Just change the default mode of the device when it is being powered up.
  Go to eWeLink App, Select the device > Setting > Device Power On State = ON.
  To turn the light on if it has been switched off by the remote app, turn the existing Switch OFF and ON again, the Light will be turned back On.
  Sonoff Device will be re-connected to WIFI after few seconds, after it is being switched on.
  If I solder a push button switch to the momentary pause button to trigger the relay output can I have trouble if someone holds more than 7 seconds it tight? as an example reset sonoff?
  201605041837...
  Yes you can reset using this method.
  I have about 7 sonoff switches connected throughout my home with wired push buttons(bell type switches)
  i use alexa for turning on and off light,fan,ac,geyser etc
  In case my network connection fails, i can still use my equipments using the wired buttons.
  this is by far the cheapest smart home soultion if you buy these in bulk.
  I directly soldered two wires to the Sonoff's switch and fed them outside the case where then I've just put a terminal(or whatever is called?) on the ends to connect wires coming from the switch.
  @max doro. Agree, but still required a physical contact to the switch to turn it back on, so why not use the original switch instead of making another switch and open the device up And soldering mods if it is not really needed. Yeah the mod makes sense but is it worth te efforts. I just wanted to share the info so that other members aware of the options, because I was looking for way to turn the light on without my pbone, and without the mod, simple settings achieved that.
  No need to make any modifications. The device can be controlled using existing light switch. See screenshots for settings. 1) go to ewelink app 2) select device from the list 3) click Top right icon to show menu 4) click Settings 5) Power on settings = On How to use existing light switch 1) When light is on - Switch light switch off, 2) when light is off - flip switch to on to switch light on.
  Nice idea. Could you please advise what kind of wall switch I can use?
  I know it must be momentary one, but how to find one.
  Just thinking about this one, but it's not low voltage
  https://www.aliexpress.com/item/EU-Standard-2-Gang-1-Way-Remote-Control-Touch-Switch-Remote-Wall-Light-Switch-With-Cystal/32693735504.html
  Ang Weilie
  Hey Arik, you happen to be from Singapore? According to our population, the chances are quite slim hahahaha... but how thank you for the helpful tip! Just to clarify, this is using the Sonoff Basic? So you install a Sonoff Basic behind every wall switch?
  @max doro. I believe the objective is to be able to switch the light on/off using the manual switch instead of the app (in case u dont have tbe phone with u or your wifi isnt working). The App settings feature allows you to use the existing wall light switch to turn light on/off when you dont have ur phone or wifi. You could try to use the wall switch to turn off light, it would be off right? k, but whe u turn it on again, it would not work? unless u use the app. So by changing the default setting- Power on stage = on. You can control the light by existing wall light switch. Try it and it will makes sense. Cheers.
  In other word. You dont need a secondary switch to turn on the light if it has been turned off by the app. Use the existing light switch on the wall to turn on the light if it has been turn off by the app. E.g.switch position would be On but the light is switched off by the app. So if you have changed the default settings, you could flip the switch to Off (wait 3 secs), flip it back to On Position and the light would be turned on again.