My question relates to cost and practicality - I'll be needing to convert 120vDC down to 12v or 5v or 3v quite often; can this be done (for low current) *very* cheaply - like - with nothing more than a stack of 1-cent diodes or something?
Well, engineering on superlatives is not entirely as easy as engineering on cold hard numbers, so what does "low current" and "*very* cheaply" translate to in numbers?
Personally, I'd add a separate 12V bus, as this takes quite a bit off the total cost, as you'd then only need to regulate down to 5V or 3V from that the 12V bus can be made by switching down from 120V only once in a central place (i.e. less loss and less outlay).I hope that you're aware that 120VDC is a tad more dangerous than AC, since if you touch it in a way that you get a muscle contracted, the DC won't let it loose again.
The stack of diodes that you'd need to go from 120V to 12V is around (120-12)/0.65 = 166 diodes and that will enable a current that the diodes can safely handle and it won't be very stable - changing voltage with a changing load.
Does anyone know if there's a small range of *cheap* components out there that could be assembled to impliment the "control unit" for the lights -
Well, it's way cheaper for AC. For DC the leanest method (minimum loss) is switch mode converters and that is probably the most expensive way in components.
I'm guessing that 120v AC halogen globes need nothing more than a suitable power transister to turn them on/off or "dim" them,
Are they true 120V lamps or do they have a circuit inside, with the lamp itself being run on a lower voltage?
and LEDs should in theiry be as easy as the transistor to control -
LED's are current operated devices, so they need a bit more.
but I'll need some kind of digital-to-analog converter to drive these (at least). I've seen a few cheapies around - like audio ones maybe?
Err, why would you use D/A-C to run a LED?
I'll also need some kind of "counter" or other chips to hook up to some components to "listen" for the signals on the BUS that indicate the need to turn on or off - and whatever other things need to hook onto the DC to convert the signals for those chips. And I'm going to need to buld several hundred of these things.
Either use addressable devices (I assume that's your point with the counters) or use small microcontrollers programmed to respond in a similar way, with an identity code and a command code.
A PIC10F2xx microcontroller is tiny, lean, cheap (from around $0.40) and lends itself nicely towards these kind of apps.
Could I manage this for $10 or less do you think?
You have to specify which part of it you expect to make for $10?
An addressable unit made with the above mentioned 10F2xx should come in below $10, if you need at least 100 (based on your own design and PCB layout).
Will everything get "fried" in the first electrical storm that hits me?
Yes... If you don't build it right
As for switches... I'm guessing that a modification to the light reciever idea, which I can "poll" might work; address the switch, then listen for it to send back it's "current state" answer.
Yes. Each switch will need an equal addressable unit as well, but with a clever design, the PCB and most of the circuit would be identical in both switches and lamps.
In a really clever design, every bit will be identical, so that you just need one type to connect up with either a switch or a lamp.
The reason that this may be good (even if it means a power transistor or similar not being used in half the units) is, that you may get PCB and assembly done in Bulgaria at a very low price and get the assembled units ready to install shipped to your door.
If you haven't made any PCB's or assembled any electronics before, take my word that producing 150..200 ready boards will take you a fair bit of time and some may be faulty.
Mechanical breakers should be included, perhaps one in each room, or at least one that can take out all lamp groups, one for all outlets and one for kitchen/bath hardware, so it's still possible to cut a group if some of the electronics go haywire and keep the rest working.
I don't mind spending more on the complicated bits, like the motors and more sophisticated sensors -
Lucky 'cause you're going to.
possibly wiring those via ethernet, so only their DC comes from the "bus" - but any/all advice about using 120v DC would be greatly appreciated!
The control signals could be modulated onto the DC, but may break into sensitive equipment not meant to run this way and will be hard to modulate onto a low impedance bus.
I wouldn't use Ethernet for that. If you by this just mean to use eg. Cat5 wire, then fine.
2 sets of twisted pair wire running through each room, carrying the control signals, will make life so much simpler for you.
[...] in which case I'm hoping that I can simply "rectify" my 240volts with some really *fat* diodes and capacitors, to get my 120v DC that way).
You can't - if you add diodes to the 240V, you will get a pulsating "DC" that looks exactly like when you draw the sinewave of the 240V on translucent paper and fold it along the length in the middle of the curve Something like "UUUUUU" (flip vertically for the most common way to view it).
Instead of connecting to the grid, you might wanna consider a generator, if it's mostly for a back up supply.
To sum up, my recommendations are:
Convert to 12V at a centrally located place (to keep lead lengths as short as possible), use a switch mode converter for this and feed it to each room.
+5V and +3V supplies could be made the same way if you need them in great numbers, but if you need only 1..3 each room, local voltage regulators may be just as good a choice.
As a side note, I would run the system in duplicate, with half the load on each part, to avoid a total blackout in case anything gives (and sooner or later, something will).
Make a small addressable unit that can be put in any switch or lamp to take care of signaling back and forth and the control of the lamps.
Sit down with pen and paper and make a realistic assessment of what's needed in each room in terms of 3V, 5V, 12V and 120V and the total currents of each voltage group.
Consider what you may have that requires an close to 100% stable supply (perhhaps just some emergency lights, perhaps a fridge/freezer or whatever.
This will be quite an outlay, but don't let that convince you to use the cheapest solutions, as they may be expensive in the long run. And do make sure you use proper solutions, to avoid electrical fires, downtime and general annoyance.
A UK magazine, either Practical Electronics or Electronics Today International (don't remember which from the top of my head), now merged (with yet another mag) ran a series of articles on a single wire home control system sometimes in the eighties IIRC - perhaps your local library have or could get them.
I think that series would give you a good foundation for what you have set out to do and perhaps you could use some of the ideas for peripheral add ons.
Which country do you reside in?
The above is just my take on it and some people may give different advice - lot's of ways to shave that goat.