> what easy trick am I missing here....

I should clarify - unless you have control over each of the
40 LEDs in the matrix individually (ie each LED has its
own personal switch), it is not possible to display all possible
patterns statically. Shift registers are used to significantly
reduce the hardware but the downside is that s/w has to
be more complex (well, perhaps not necessarily) because
the display now has to be strobed

What you need to do is start at the top row and strobe
downwards. But you can have the display anyway around
you want - rows/columns, columns/rows, tomato/tomato -
you can strobe top to bottom, bottom to top, left to right,
right to left. Just pick one

Using my schematic as an example, say you've got a shift
register controlling the Row Select Darlingtons on the left and
a shift register to control the column transistors at the bottom

You load the Row shift register with 0000 0001 to enable
Row1 and Row1 only. Then you load the Column shift register
with the pattern for Row1, say 0100 0011. LEDs 2,7 & 8 will
light. You hold that for as long as you want (depending on
taste/LEDs/electrical stresses etc). Then load the Row SR
with 0000 0010 to enable Row2 and Row2 only. Send the
Row2 pattern data to the Column SR and hold. Note, that
in the instant the selected row pattern is showing, all other
lines are actually blank, although to our eyes, because of
persistence of vision, they may not appear so. It's just like a
TV scan, which is where the timing becomes important.

Depending on the installation site, there may be advantages to
using other than red. Our eyes are more sensitive to green
(which is why night-vision equipment is green). Although things
like fire engines are traditionally red (perhaps to incite a sense
of urgency), yellow is a more noticeable colour against most
normal backgrounds and yellow is favoured for emergency
vehicles (ambulances - white & yellow stripes). However, red
LEDs are generally the cheapest and least power-hungry

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body