Been thinking about distributed garden water management systems. Plenty of ways to get the sensors and actuators, but in any complex property, the parts are spread over a large area. Very typically, they use low voltage AC wiring (24V). If we were to Arduino control, we would have several systems spread out, and they need to communicate. Powering the Arduino from the AC would be easy. But how should we modulate the AC for data? Has to be cheap, and don't need much data rate.
You are probably going to use some sort of zero crossing detection (The point where the AC sine wave crosses 0V) And send a signal at the cross. Ive never seen it done with low voltage, but i don't think that would be a problem.
You might want to start here Arduino Playground - ReceiveX10 It is a link to the X10 plalyground post. X10 sends signals through your houses mains. It should give you some ideas.
Can't really help you further though. Good luck.
That was helpful, although it doesn't talk about how big the 120Khz modulation tone is (voltage), how much power it takes to drive or how it's coupled (probably capacitive). Probably enough to experiment with tho, unless anyone else knows what X-10 does. The detector sound pretty easy: zero detector and 120KHz bandpass filter. Probably the encoding is easy in software. I wonder how much of the bandpass can be done in software?. Hmmm, zero detector. Capacitive couple, zeners to limit voltage, DC bias with resistive divider to get it all positive, ADC. Hmmm.
Think of old school modems, say 300 baud that have two tones. You can AC couple these onto your AC power lines with capacitors. Start off with 0.1uF.
You can probably pick up some cheap modems as no one wants them today.