> > I am considering building a small LED matrix sign, probably
> > using 12 stackable 8x8 LED matrix blocks (approx 2"x2").

> >  This
> > could be done with 96 high side drivers sourcing 20 ma each,
> > 96 current limiting resisters, and 8 low side drivers sinking
> > about 2 amps each (fairly hefty).  Or reverse this, and have 96
> > low side drivers (at this current, could be PICs directly) and 8 x
> > 2 amp high side drivers.
>
> Wait I'm missing something. Aren't these displays internally
> multiplexed?

No, I believe that the 64 LED's in each of the twelve blocks are
arranged in an 8x8 matrix.  16 pins.  8 are cathodes for columns,
8 are anodes for rows (also available vice versa).  To me
the obvious way to scan them is to power one anode (of 8) at a
time, while driving the appropriate subset of cathodes (depending
one which LEDs in this row should be lit currently).

For 96 wide by 8 high (27" x 2", twelve x8x blocks), this means 96
individually controlled column drivers, and 8 massive row drivers.

There have been some great leads regarding low side drivers (for the
96 sinks).  What about the 8 high side (in this regard) high current
drivers, many amps each?  I now read that the peak current for some
of these LED blocks (per LED, 1/10 or less duty cycle) can be 80-100
ma!  (by 8, would be 640-800 ma per block, via the row anode, times
twelve is up to 10 amps!).  I don't have to have absolute maximum
brightness, so I could compromise this to much less current.  But it
seems that I still need 8 high current transistors for the
rows/anodes.

I have read that logic gate power MOSFETs when used high side
(between power and the LED anodes) take several volts (eg: 5)
above the power supply to switch!  That makes it hard to simply
interface them (as high side drivers) to PIC outputs.

How would I use a PIC output to switch approx 5 volts at 2-10 A
(most to least compromise) into each of the 8 anode/rows?  Only
one (or none) at a time is needed.  At LED multiplexing speeds;
no relays please :-)

I guess this also brings up the question of appropriate multiplexing
speeds.  Would a 16 msec cycle, with 2 msec per row, be a good
frequency?  Or does it need to be faster to avoid visible flicker?

Thanks for the help!
    Zhahai


@ Zhahai Stewart       zhahai@hisys.com
@ A Meme Gardener      http://rainbow.rmii.com/~hisys/zhahai.html
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