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On Thu, Jun 21, 2007 at 09:58:56AM -0300, Gerhard Fiedler wrote:
> I haven't run the numbers, but my gut says you might be right :)
> 
> Peter, if I understood you correctly, so far you've been talking about a
> central unit with some hundred communication+supply drivers for the nodes
> (one per node) plus the nodes themselves. Are you sure that the cost of the
> extra pair of wire is not offset by the extra PCB plus the extra components
> on /both/ sides (node and central)? You want two-way communication, and
> just stringing all nodes in parallel at the central probably needs RF or
> audio communication. The simpler voltage level thing won't work well --
> imagine the currents you need to draw to make the voltage drop. In order to
> use this, you have to have individual drivers at the central, or at least
> in clusters.

Not quite. I'm assuming that the central node will consist of a nothing
more than some PIC with a uart (to talk to the controlling computer) and
a single linear power supply capable of supplying about 5A.

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.

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

If the master wants to communicate with a slave, the strong pull up is
modulated, and the slaves recieve the data.

Now for the tricky bit... If the slave needs to communicate anything
back, the master turns the strong pull up off, and turns the weak pull
up on. The slave will have no problem pulling 5k to ground.

This works if we assume that every slave uses a diode to get parasitic
power. So when the weak pull up is in use, there is nothing pulling the
lines low.'

Of course, lets hope a slave doesn't get stuck in the low position...
But hopefully the watchdog timer will help prevent that, otherwise,
debugging and wirecutters...

> 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.

On the master the leds can be easilly pwmed using 40pin pics which have
~30 usable IO ports each. (36, but we need at least 2 to for serial
comms for instance) I'll use the 40pin ones, as they come in DIP
versions for easy soldering by other groups who might not want to make
up pcbs.

So each pic can drive 10 RGB LEDs. The cheapest 40pin pic is the 16f914,
at digikey for $2.80 assuming you're buying 25 of them. Cost, $0.28/led.

So $0.28/led worth of master drivers, $1.36 worth of cable = $1.64/node.

(I'm deliberately ignored items common to both solutions in the pricing)


For the "smart node" solution:

$0.68 for the PIC, (12f508) $0.15 for capacitor/resistor/diode, $0.50
for the pcb and $0.68 for the cable = $2.01

Only a $0.37 difference, although the latter is a lot more "interesting"
in terms of getting it to work. 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.

The "smart node" solution also allows you to put sensors on the nodes,
the 12f508 pic listed has pins free, and I just had a meeting with the
artists... Sensors are a must, although they don't really need one per
node, so the "dumb node" is still an option.


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?

- -- 
http://petertodd.org
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