I would like to make a small board with a simple MCU that just repeatedly sends a message via a modem on one end of a power line wire (14awg or so). On the other end will be a similar device for reading that message. The only requirement is not interfering with/cutting the wire or isolation in any way. There will not be access to a plug or any way to directly tap into the line like PLC home networks do. The device will be powered from an external source (3.3 or 5V).
I've done some reading on this topic and the ST7540 seems to be a good option, in my case I would not need to filter out the 50/60Hz because there would be no electricity on the line. I am mainly uncertain about the induction, possibly using some kind of coil around the power line. What I basically want is to use the power line wire as an antenna between two modules (approx 3m or 10ft apart). I can't use full wireless (315MHz for example) for various reasons.
What is the best way to inductively transmit data (only one direction is needed, tx to rx) over a power wire that has no voltage/current and what kind of parts would be required? Are there modems specifically made for something like this? I remember reading about something similar being used in under-water applications in the past.
Hypothetical example: There's a 2-lead mains wire permanently plugged into an outlet in my room. The wire leads to the next room and it powers a lamp with a switch (relay). I can't go into the next room. I want to induce a modulated signal into the mains line by means of a transceiver/MCU while the lamp is off (no voltage/current in the mains wire), the signal goes through the mains wire, which acts as an antenna. There's a receiver/MCU on the mains line in the other room, which reads the signal inductively and turns on the relay, lighting the lamp. I do not want to physically manipulate the mains wires in any way, but also can't use a regular RF link (I know this could easily be done wirelessly).
Best Answer
Consider PPM....pulse position modulation.
Couple pairs of spikes into the hot-wire. For ZERO, place the spikes 5 uS apart. For ONE, place the spikes 10uS apart.
Send the bits every 100uS.
Or, to have a very narrow-band datalink, so you can narrow-band filter to reject more noise, for ZERO, send +spike,+spike, For ONE, send +spike,-spike.
Examine some Fourier signals and pick your spacing and modulation. Here are useful waveforms and spectra from Signal Wave Explorer. I used the JBStage (canned example) with trapezoidal input (50%, 50%, no plateaus)
Notice how narrow-band (centered at 200KHz) the output spectrum is.
By the way, the powerline can easily kill you. Are you thinking about transformers wrapped about the HOT wire?