> Jinx,
>
> Don't you find that there is a big difference in brightness between
> columns with different numbers of LEDs on?

No, not at all. You know you get little jobs that you fully expect to
fight and fuss over ? None of that, very pleased with the way it
went well first time. Even when using PWM on the strobe line
to fade in/out there's no discernable difference in brightness
between any LEDs or flickering

> It looks to me like a column with 1 LED would run that one
> at 18+ mA (really bright), but with all 8 on each LED would be
> driven at 1/8 that, or about 2.3mA.  This is why the somewhat
> similar scanned array I built (smaller, it's 12x16) uses no
> current limiting resistors and depends on the low duty cycle to
> determine the current.

As I was unsure of what these little LEDs (HLMP-1385) could
take in the way of pulses, I started conservatively with 270 ohms
as the current limiters (even 330 when I ran out of 270, lower 10
in the picture). I may go down to 100R but for now it's plenty bright
enough. I've repaired commercial units that have suffered very
badly from heat stress and wanted to avoid that. They tend to use
packaged drivers like the ULN200x series, 5mm super/ultrabrights
and 47R to 68R resistors. Problem usually is that the PCB under
the driver IC or resistor can't take the heat and joints fail. However,
as I was aiming for a low-current display, heat stresses shouldn't
have been possible. Also, when replacing blown LEDs in these
commercial units it's sometimes difficult to colour-match. Yellows
in particular seem to come in so many tints, and LEDs are one of
the few components that aren't numbered

> I used a transistor for each row & column drive, bases
> driven by the PIC port.  I forget what the scan rate is, but I seem
> to recall it being around 1kHz.

I've seen a couple of shocking displays around town that have
a pronounced angle to the letters as they scroll. That would
seem to be a consequence of using just one micro for the whole
display at a moderately slow refresh rate. I was quite prepared
to use multiple PICs if just one on its own couldn't cope. No point
in being clever and getting just one to do the whole thing if the
end result is rubbish. Luckily that wasn't the case
>
> Downside is that if the CPU latches up whatever LEDs are on
> are in trouble, but that's why I use the watchdog on a short leash!

In trouble ?

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