It is already known for a long time that the electric grid can be used
for the transmission of signals. The signal frequency must be higher than the mains frequency (50Hz),
so that the mains have sufficient impedance for it and lower injection current is needed.
For the transmission over short distances (within a house or apartment) the suitable frequency of about 1 to 25kHz.
In our case, the transmitter has frequency about 5kHz, which is 100x
more than the power line and can therefore be easily separated by highpass filter. The oscillator consists of an integrated
circuit 555 and power amplifier with two transistors, which are protected by diodes
against the voltage surges. The output is connected to the phase voltage via the capacitor with the capacity
about 220n to 470n, which must be rated for sufficient voltage, ie 250V AC, Class X2. The transmitter can be
powered from battery or the AC/DC adapter, or through simple supply using capacitor reactance to reduce the voltage.
The transmitter is controlled either by switching its power supply or by logical
1 to the 4. pin of 555.
The remote control over power line receiver has highpass filter at the input. This separates the signal with a small amplitude from the much larger mains voltage. The transistor is permanently open via the base resistance 220k. This transistor is shorting the current of the triac gate and triac is thus closed. When receiving a signal the transistor is closing for some part of the signal period and thus via two resistors 820R the capacitor 470u is charged and triac opens. The triac operates in II. a III. quadrants. Directly to the triac can be connected lamp or another load with 4A maximal current. The receiver is powered directly from the mains via 330n capacitor. It has also a 330k discharging resistor and 470R resistor, which limits the current peak at power on.
The signal of the remote control over power line transmitter must not reach out of an apartment or house, but it is usually not necessary to use any filters, as the coil of electricity meter attenuates the higher frequencies significantly. The problem can mean the classic fluorescent lamps (not talking about compact saving ones) as they have power factor capacitor (if they have one :-)). This capacitor could absorb signal. The simplest solution is to disconnect it (it only reduces current flowing, but not the power, for which you pay). Another solution is to connect a serial RF chokes.