I have a light that is switched at one end of a room but want to put a switch at the other end without running cable. What components would I need to do that?
If you have two sonoff devices, you will figure out by yourself :) You connect your light to the Sonoff basic or Sonoff mini. Behind your switch, you install Sonoff Mini (another one) or one of other Sonoff wall switches. Then the most important part, on your eWelink app (that controls sonoff devices), you creaste Scene with a setup someting like "if WallMini is on, turn other Mini /Basic on". Search for ewelink scene on youtube.
So what do I use for the second light switch that has no power supply?
In case you cannot provide power to the switch, then you need RF devcies. You connect your light to the SONOFF RFR3 relay (see below)
and then to switch the relay on and off you use RF Remote (see below)
To control RF devices remotelly /via the Internet, you need one RF bridge see
You find more informations from youtube J