What is the simplest way in which a device A can notify another device B, both hooked up to AC power lines in a home, a few feet apart but separated by a wall, in a circuit having other low-power consumers that must not be disturbed (granted though that no consumer stands between A and B).
Before devices [A] and [B] came into the equation, the manual switch [SW] used to directly control (On/Off) device [D] (max 200W). [B] is actually a WiFi based relay (a SSR commanded by an ESP8266 module, to be precise) that I introduced for certain needs and I can also extend (both code and additional components if required).
What I need is to further use [SW] to toggle [D]'s on/off state. How this will exactly happen in [B]'s logic is not part of the question, I am rather looking for a way to get [B] notified whenever [SW] is used. I assumed a new device [A] might be necessary (but I might be wrong as well) in order to send some sort of sign over the mains.
Please consider that I don't need to send meaningful data, just a notification sign. If possible, avoid frequency modulation or other heavy processing and don't require [A] to use a microcontroller. Size should be enough to fit in a wall box, design cost should also be kept at a minimum.