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On Wed, Jun 20, 2007 at 10:12:01PM +0200, wouter van ooijen wrote:
> > 50ft from node to master
> 
> I dunno what you pay for wire, but at 50 feet (== 15 meters?) I would
> consider using 3 wires. Or rather 4 (two UTP pairs). I pay E 0.22 for
> UTP cable (@ 400m), split in two (imagine your students with x-acto
> knifes..) that would cost E 1.65 for 15 meters. 

Definetely too much. Wire costs, buying 1000ft spools from digikey, will
work out to be over half the cost of the project ($2.50/50ft) just for
pairs, let alone duel pairs.

> And for the no-intelligence solution: 4 wires can easily drive 3 leds.
> And the wires are colorfull, adding to the display :)
> 
> But I think the largest cost will be assembling and correcting the
> mistakes made during assembling.

Remember though, student labour is free... and part of this grant is
that we're supposed to be developing designs that other groups can use,
groups that are explicitly supposed to be dirt poor artists.

> But are you sure a star is the best configuration? Wiring such a star
> would be a nightmare, IMHO some sort of chain or matrix would be much
> better. Maybe with moderate intelligence for each N led triplets.

My idea with a star is that each node would essentially end up as a 50ft
coil of wire with a pcb at one end. Then all the installers have to do
is put that pcb somewhere, and uncoil the wire until it reaches the
master.

The master would then have 100-odd pairs from the different nodes.
Just solder them all to the circuit board, or even use telco-style
punchdown something. (gotta do some research there...)


I did a project a few years ago that involved a chain topology. I got a
class of 20-odd students, with no electrical experience at all, to build
a floor that would light up when you stood on it. We had about 160 tiles
at $1.5 or so each assembled from corrugated plastic, copper foil, and a
really simple capacitor discharge circuit with some leds. The main
source of mistakes were the really crummy pcb's we etched ourselves, but
given a good "factory" with a QA team each "shipped" tile worked just
fine. I found that the students made very few mistakes actually, arts
students are rather good at boring repetitive tasks, probably a good
thing considering their usual jobs after graduation. ;)

That said having the tiles arranged in chains meant that frequently
whole sets of tiles would go dark due to wear and tear breaking one
link. If it were only one tile at a time breaking, no-one would have
noticed.

- -- 
http://petertodd.org
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