Let me step back from this specific project for a minute and explain
what my long-term goal is with PIC microcontrollers.  When I was a kid I
was really impressed by moving message signs, whether they used LEDs,
light bulbs, mechanical displays, etc.  A dream of mine, as a child, was
to figure out how to build one myself.  Specifically, I wanted to design
and build a scrolling message sign using a display made up of LEDs that
displayed characters that were 5 LEDs wide and 7 LEDs high.  I even
mapped out the characters for upper and lower case letters and numbers
on graph paper.

A few months ago I found out about PIC microcontrollers and figured
using them would be an easy way to build one of those LED signs.  I know
there is a whole range of PIC chips to choose from.  As a personal
challenge, I would like to use the cheapest and smallest chip practical.
I am currently doing all my experimentation on a 16F628.  It's cheap,
easy to program, has enough memory for my needs so far, and has a fairly
low pin-count and footprint.

So, a big question I have now is, how many individual LEDs can you
multiplex so that the display is bright enough?  What is the minimum
number of components needed to drive 100's multiplexed LEDs?  I would
like to design an 8x1 character display to start with.  For 5x7
characters, I would need ((5 + 1) * 8 - 1) * 7 = 329 LEDs, or 47 columns
of 7 LEDs each .  Can I use 5 4017 chips chained end to end to count
from 1 to 50 to select the columns and light a whole column at a time or
light each LED in each column in turn?

As you can see, I have a lot of questions and not many answers.  I am
just hoping I don't need 47 8-bit parallel-in / parallel-out shift
registers or something crazy like that.

So, back to the topic of this message thread.  Multiplexing the segments
in 4 7-segment displays helps me solve two problems.  First of all, I
need a 4-digit counter displayed on 7-segment displays.  Secondly, if I
can multiplex 28 LED segments and have a bright-enough display, this is
a good start towards multiplexing 47 columns of LEDs.

I hope this is making more sense.  Now that I have revealed what I am
really trying to accomplish, does anyone have anything to comment on
scrolling message signs?  Does anyone know how the real ones work and
how much circuitry they actually use?

Thanks,
-Pat

> > I've already talked about two issues above: eliminate high power
> > drivers or drive the segments even harder if you use high power
> > drivers. Others include maintaining equal segment brightness,
> > lowering the individual LED duty cycle, and reducing component
> > count.
>
> I see how that can be used to overcome the drive power limitations of
> a PIC I/O pin.  What I don't see is how this really gains us anything.

> To get a certain amount of brightness out of the display it's going to

> take a certain amount of "average" input power to the display
> regardless of whether we turn on one segment at a time, or all the lit

> segments in each display digit (or for that matter all the segments of

> all displays at once) .  Is the idea here to not exceed some total
> output limitation of the PIC? (i.e. even though each PIN is capable of

> 25mA, we can't drive seven pins at 25mA simultaneously obtaining 175mA

> output).  What am I not understanding here?




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