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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).

Detailed scenario:
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.

Thank you!

enter image description here

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    \$\begingroup\$ I mean, we all could list dozens of ideas that come to mind. Your description is far too unspecific. You should come up with one or two approaches yourself, since we can't know what "simplest" means to you. What kind of experience do you have building communication devices (a notification is still communication), what kind of system integration experience? What are the size, cost, energy … constraints? this goes on. \$\endgroup\$ – Marcus Müller Mar 18 '17 at 16:14
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    \$\begingroup\$ You should study the X-10 protocol. \$\endgroup\$ – Dave Tweed Mar 18 '17 at 17:03
  • \$\begingroup\$ If they are only a few feet apart, why not use IR diodes? I know you said you don't want modulated signals, but IR diodes can be modulated trivially (similar to a remote control) using modules. To the micro-controller, it just looks like serial communication (often referred to as RS-232). \$\endgroup\$ – mkeith Mar 18 '17 at 18:19
  • \$\begingroup\$ @mkeith Thanks but IR won't work in my case, I added a scheme to point out why \$\endgroup\$ – i2r Mar 18 '17 at 18:44
  • \$\begingroup\$ You pretty much have to use some form of RF modulation coupled to the power lines. X10 is a good example, though if you only want to send a very low bitrate you could use something a little simpler. This is not a good beginner project due to the hazards of mains voltage and the subtleties of designing circuits that can be safely connected directly to mains. \$\endgroup\$ – pericynthion Mar 18 '17 at 23:08
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ALTERNATIVE APPROACH

There are numerous RF transceiver modules on the market that make this kind of thing easy enough without the fear of frying yourself or anyone else.

In fact there are complete RF remote control systems you can apply to do some complicated stuff.

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enter image description hereYou could connect an LED to device A and a phototransistor to device B, since they're only a few feet apart. Butt the LED right up against the phototransistor and cover them both with tape or an enclosure to prevent ambient light from getting in. Device A simply turns the LED on to notify device B. This is dead simple, dirt cheap, and allows the two devices to communicate while staying electrically isolated from each other. That way you don't have to worry about the safety concerns of linking the two devices.

You say you're a beginner in electronics, so I advise you to tread carefully if your devices handle significant mains power. There are a lot of non-obvious ways a poorly built device can burn your house down. What is it your devices actually do?

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  • \$\begingroup\$ Thank you for the answer but IR can't do the job, I updated the question to explain why \$\endgroup\$ – i2r Mar 18 '17 at 18:45
  • \$\begingroup\$ Is it impossible to even run skinny wires out of the wall box? Or does it have to be radio communications? I don't see why you couldn't do this: \$\endgroup\$ – Matt24 Mar 18 '17 at 22:57
  • \$\begingroup\$ Photo didn't attach. I added the diagram to my answer \$\endgroup\$ – Matt24 Mar 18 '17 at 23:04
  • \$\begingroup\$ Unfortunately I cannot run any additional wires; that would have been a simple problem to solve, not involving the AC power lines anymore \$\endgroup\$ – i2r Mar 19 '17 at 11:31
  • \$\begingroup\$ Not to mention that combining high-voltage and low-voltage wiring in this manner would violate the electric safety code in pretty much any jurisdiction that I can think of. \$\endgroup\$ – Dave Tweed Mar 20 '17 at 15:37

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