I am trying to build a communications system for a car, for turning gadgets on or off. One idea was to parasite on power lines, but what kind of components and signaling could be used for this? I am thinking pretty low communication speed is acceptable, such as below a megabit, but more than 10 kilobits per second.
Is there some suitable standard for a hostile environment such as a car?
There are two commercial solutions that I'm aware of; the first is PLC4trucks (SAE J2497) which is (or at least was) based on generally difficult to obtain Intellon ICs, the second is through Yamar Electronics who offer a range of devices that would seem to meet your requirements.
I would not try to add signals on car power since it can be very noisy.
Your problem is exactly what CAN was invented for. You run power and CAN to everything, then the rest is done by sending messages.
Silicon chips are cheap and light but copper wire is expensive and heavy. The car industry decided long ago it made more sense to run a single power wire, for example, to a tailgate assembly and send messages to tell the various lights to turn themselves on and off than to run separate switched power lines to each fillament.
CAN is differential and therefore has good immunity to external noise. For short distances like a car, it can do 1 Mbit/s. CAN has been so common for so long that there are plenty of microcontrollers with CAN capability built in. You will find lots of info out there with the search string "CAN bus".
I realise this is a relatively old question but in case your are still on the problem, take a closer look at the work of Miloslav Trnka.
In his publication "Development system for Communications over DC power lines" he treats exactly the problem you describe. I cannot quite remember the data rate he achieved but given that he used a FSK modulation with frequencies in the MHz range, tens of kilobits seem possible.
Miroslav dealt with the noisy environment cited by Olin Lanthrop in his answer by sending the same information in parallel on a variety of carrier frequencies. This way even if part of the frequency spectrum becomes too noisy for communication, other parts are likely to remain available.
Note that this was research at laboratory stage, so it is no standard. - But nevertheless a nice idea.
