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On Wed, Jun 20, 2007 at 07:36:11PM +0200, wouter van ooijen wrote:
> You will always need a form of regulation at the slavess, unless you can
> ensure that the voltage loss in the wires and the voltage drop due to
> depletion of local charge storage (the local elco's) does no harm. So I
> would go for a voltage that permits local regulators. If the '0' voltage
> is still enough for the local regulators you won't have to worry about
> big storage capacitors in each slave. But this all depends on current
> consumption, number of lasves, length and type of wire, etc.

I'm planning on operating the line at 5V, using PICs that are okay down
to 2V. I've also found that most LEDs don't have very much brightness
change at the "top end" of the voltage you give them.

> > So what would the node need to detect a HF signal? High-pass 
> > filter w/ a capacitor?
> 
> PLL? Or an IR diode + TSOP receiever?

Ahh, sounds complex for what I'm thinking.

> > Note that for installation reasons, the nodes really should 
> > be in a star configuration, by strings do you mean strings of nodes?
> 
> Ooh, you should have said so! And what are the other parameters? (wire
> length, wire resistance, current consumption, slave oparting voltage,
> cost/reliability balance, etc)
 
50ft from node to master

CAT3 works out to 1.5ohms for 50ft, 1.2V voltage drop at 20ma

20ma, maybe 60ma if we use three color leds...

Slaves should be able to run fine from about 3V to 5V, and not reset
until 2V

Cost is way more important than reliability. I'm expecting nodes to get
broken right out of the box, and in that case, all that happens is you
get one less point of light. Installation lifetime will be something
like 1-3 months, then it all gets thrown out. 

As an example, say if we do go with three color leds, each color needs a
different forward voltage to operate properly. I'm planning to deal with
that by simply limiting the PWM duty cycle in software such that average
current won't get too high after making some measurements of voltage
drop. Anything to save a resistor. Heck, I'll likely do the same thing
with a monochrome setup if needed.

Heck, for my "conformal coat", I may very well end up using something
like wax simply to avoid hassle from the artists about "toxic chemicals"
It'll last long enough.

- -- 
http://petertodd.org
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