Peter Todd wrote:

> You're right about the voltage drop, except... Who says the big beefy 5a
> power supply is always connected? 
> 
> There will two things driving the data/vcc line. A high-side fet,
> connecting directly to the 5A supply, the strong pull up, and an IO pin
> from the master PIC, connected through a 5k resistor, the weak pull up.

Yes, this works if you can shut down supply when you need to communicate
back. OTOH, it has some potential of disaster: if a slave, for what reason
ever, tries to communicate while the supply is on, at least that slave's tx
driver will pop.

> Normally the weak pull up is disabled and the strong pull up is enabled.

FWIW, the weak pull-up can be always enabled. Simplifies the switch-over.


>> Depending on your requirements, there may also be the possibility to use
>> some 5 or 10 sub-centrals, controlling 10 or 20 LEDs each. These can then
>> be connected to the central with more wires (separate supply and comms). 
> 
> Well assuming that I simply wire each RGB LED up directly to the master
> I'll need 4 pairs. At rock bottom prices that's going to cost me about
> $1.36/50ft.

Did you mean 4 pairs or 4 wires?


> Only a $0.37 difference, 

Well, if cost minimization is your goal... You didn't count the PCBs,
though. At this end of the price scale, the number of PCBs does matter
(unless the plan is to etch them at home). 

> although the latter is a lot more "interesting" in terms of getting it to
> work. 

Definitely :)

> The former though would be more labour in making it, I bet plugging parts
> into a pcb is easier than shrink wrapping four leads per led.

I don't think so. Populating 100 PCBs with small micros is probably more
work than populating a bigger one with a few big micros or a few bigger
ones with one micro each, even considering the mounting of the LEDs. Don't
forget the chance for mounting errors. Using the latter approach (a
two-level star) would get the wire cost still down (even though it seems
that your node cost is determined mostly be the LED).


> On another note though... Cheapest RGB leds at digikey are $3.50 each
> (qty 100) and they are going to be non-stocking. Eek! Someone said
> they'd gotten really cheap RGB leds, like $0.50 each or soemthing,
> where?

Can you use individual LEDs?

Gerhard

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