Hi.

I have done quite a bit of research into this a few years ago. I developed a complete system for hearing impaired people that flashed lights whenever the phone rang, baby cried (microphone), the doorbell was pressed etc. etc.

The whole system had addressable modules that had a house code so that if the house next door had one they would not interfere, and it all communicated through the power wiring in the house. It worked very well, too I might say. I used a power line modem chip from National Semiconductor (I can't remember the number right now) and I think the chip is no longer in production, but I know there are other companies making them now. It used Carrier Current and interfaced the mains power cable with a current transformer. It doesn't matter what the power line is doing, whether it's on or off, as long as there is a connection. All the warnings should be heeded though, it can be dangerous developing such a system, because you have to have high voltages exposed throughout the testing process. I can photocopy some of my work on this project and send it to you, and I have some schematics and code (schematics were done in OrCad for DOS).

But.....I have to say I used a PIC chip, not an 8051. Still, you could use any micro just about.

The other thing is that with power line communications you can't have a high bit rate. My system was 750 Bits/Second. If that's not fast enough for you, you should probably look at radio.