>I am considering building a small LED matrix sign, probably
>using 12 stackable 8x8 LED matrix blocks (approx 2"x2").  This
>would produce a display surface of 8 rows each 96 wide - enough
>for 16 chars of 5x8 with one space between.  (One use would be
>a Caller ID I could read at night with my glasses off :-)

>How is this generally done?  If I did 8 way multiplexing, with
>20 ma current per LED, this would mean about 2 amps draw.  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.  OR I could split up the common side,
>into, say, 3 groups of 4 matrix blocks, each with it's own driver (to
>keep the current in the shared driver down) - but this would mean 24
>common side drivers (plus 96 on the other side).  Is there a preferred
>architecture?

Zhahai

  I used to make LED displays like the one you describe.  Mine worked
as follows:

I used a row of 74HC164 shift registers, all connected in series so
that whatever you clock in at the first one, would eventually get to the
last one.  Then every shift register was buffered by a ULN2803
driver.   If your display is 96 leds wide, then you will need 12 x
74HC164's and 12 x ULN2803's.   There will also be 96 resistors to
limit the current to the LED's.

The eight rows of  96 LEDS are switched on as a whole row, by means
of one of eight TIP31 transistors.   My duty cycle for each row was
about  2mS per row.


>Is 1/8 duty cycle enough?

Depends on the birghtness that you need from your display.  Use only
good bright LED's, and it should not be a problem.

>Is there a prefered direction for scanning, like top row to
>bottom row (vs side to side within each 8x8 block)?

I found that my direction of scanning caused a slant of the font in a
particular direction.  I rather liked a forward slant in it.

Please feel free to ask any more question.  If you want, I can even
provide you with some of my old display boards to do your prototyping
on, but you will have to do the microprocessor part yourself.

Rgds
Werner



--
Werner Terreblanche   Tel +27 21 7102251   Fax +27 21 721278
wterreb@plessey.co.za (work) OR werner@aztec.co.za  (home)