There are not powerline-carrier devices, but hardwired (temporarily). I have my holiday lights flashing at about 1Hz. X10 devices are too slow for this. I have the relays linked using 6-wire telephone-type cable. The relays themselves are installed in plastic electrical boxes along with the controlled receptacles. They are located inside and plugged into a GFCI, with cords going out windows. The control signal comes from an old computer (Pentium 166) through a simple RS232-level buffer I built with a MAX233 IC.
BTW, The wires are assigned like this:
1 (white) exclusion line. This is on when the lights are and can be connected to NC relays to disable things that shouldn't be used at the same time as the holiday lights.
2 (black) lights on (+12V from wall-wart will be present from sunset to 10PM).
3 (red) flash 1. Output from computer serial port to flash lights. The computer switches the DTR line (connected to this) to say "Happy Holidays" in Morse code.
4 (green) flash 2. Logical NOT of above. However, both these lines can be made active to turn all lights on to take still pictures.
5 (yellow) ground. As you might have guessed, I originally planned this for 4 wires.
6 (blue) [reserved for future use]
These wires carry no voltages outside the range of -10V to +12V. The normal current is no higher than the few mA needed to operate the SSRs.
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