On Sun, 24 Jun 2001, Dan Michaels wrote:

> Dale.B wrote:
> >On Sat, 23 Jun 2001, Dan Michaels wrote:
> >
> >> Dale.B wrote:
> >> .....
> >>  The LEDs are driven via row/column
> >> >transistors with no current limiting resistors (reduced parts count)
> >> ......
> >>
> >> Dale, do you really think this is safe?
> >
> >Yep, sure do.  Otherwise I wouldn't have designed it that way.  There
> >actually is some current limiting provided by the transistors, of course.
> >Since the LEDs are only on for a few milliseconds at most, The average
> >current is still well within a reasonable range.  The LED array scanning
> >is interrupt driven using TMR0, and the watchdog is set to prevent any
> >unfortunate results should the software glitch.  The maximum duty cycle of
> >the LEDs in my case is 1/33 (or roughly 3%).  So, yes, I think it's safe.
> >
>
> Yep, I figured this was the reasoning. Every thing is ok ...... as long
> as .......... the duty cycle stays low/etc. However, if something
> unforseen [misc hangup, murphy's law, mouse house, lost penny, what
> ever] does occur, then ........ PSSSSSSST!
> =========

Yeah, that's why the WDT and all...  sooner or later you have to decide
where to trust the hardware to do what it was designed to do.

> >It was difficult to come up with another way to drive a large-ish array of
> >LEDs with acceptable brightness, and without a lot of power dissipation
> >somewhere...  the variable brightness was just a bonus.
> >
>
> Understood - still gives me the willies, as I like to build in some
> kind of basic "guarantee", at the level below the uC. Some of us
> like to live our lives a little further from the edge ;-).

Sure thing...  if you've got any bright ideas I'm open to suggestions.  15
rows of LEDs, 16 LEDs per row, variable brightness, and they've got to be
fairly bright when cranked up.  Other than dissipating a boatload of power
at the transistors or somewhere else (oh, did I mention limited space and
a harsh environment?) I couldn't come up with an acceptable alternative.

Dale
--
A train stops at a train station.  A bus stops at a bus station.
On my desk I have a workstation...

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