> I'm working on the basis that I can implement that each node 
> will have something like the following part list: (digikey prices)
> 
> $1 - 10f200 SOT23-6 package (maybe, depends on solderability by
> 		students)
> $0.25 - led of some sort
> $0.10 - diode
> $0.10 - capacitor
> $1.75 - 1"x1" pcb, qty 100
> $?.?? - conformal coat of some sort
> $0.00 - student labour to assemble it all...
> 
> $3.20 a node.

Your prices seems too high. For 100+ I would calculate:

$0.50 - 10f200 SOT23-6 (maybe DIP is better..)
$0.10 - led of some sort (a low-cost LED is more like 0.05)
$0.01 - diode
$0.10 - capacitor
$1.40 - 1"x1" pcb, qty 100 (Olimex) (seems a bit large for just these
components, and it is the largest cost factor)
$?.?? - conformal coat of some sort
$0.00 - student labour to assemble it all...

There is no transistor to short the power, so I assume this is one-way
communication. If the voltage drop over the wire is no problem (let's
say 5.2V at the source, no less than 3.0V at the destination) the above
list should be enough. 

For this kind of application you should not worry about reliability of
the message itself - reliability is by repetition. Who cares if one of
the LEDs is on or off a little bit longer.

Do not feed the LED from the capacitor, feed it directly from the power,
so the capacitor can be smaller.

For the brave: leave out the diode, feed the PIC via one of its IO pins,
put the capacitor at the power pins.

But back to your purpose: you want to control a bunch of LEDs, connected
in a star configuration, right? So you have 2 wires to each LED/slave?
So you not connect them all to a big matrix and do without the local
intelligence?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: www.voti.nl/hvu
 
 

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