Greetings,

I've been reading Myke's book (programming and customizing PICmicro
microcontrollers).  I found myself fascinated with the Analog clock
project.  It looks like a pretty nifty 1st project.  But, of course,
I couldn't copy it straight from the book!  What would be the fun in
that!

So, with that in mind, I decided that the clock would be even cooler
with LEDs for the seconds.  Of course, I could add 60 more LEDs and 4
more driver chips and a PIC with more outputs (expensive and messy).
How about using nifty bi-color LEDs.  But how to drive them?  Instead
of using 4 x 1-of-16 demuxs like in the book, how about logically
using an 8x8 matrix with 2 x 1-of-8 demuxs?  This would provide room
for 64 LEDs.  Then I started wondering how to drive the bi-color.  I
figured out how to use a pair of tri-state demuxs (one with
transistors) on each end of the matrix, but it seems messy.  Then I
realized that I could use 3 lead LEDs and simply consider it as an
8x16 matrix.  I'll use program code to duty-cycle between the seconds
LED and the minutes LED.  So I'll only need one of the LEDs lit at
any instant.

(By the way, I think that this approach swaps 3 of the big 1-of-16
demuxes for 1 smaller 1-of-8 demux.  Instead of 60 resistors, you
would only need 8 [on the 1-of-8 demux].  You may need to add 8
transistors - see below.)


But I'm a bit stumped on a few things.  The last circuit design I did
was college (and that was some time ago)!  Can anyone help with...

CONSIDER:  Since each demux provides 1 LOW and the rest high, I'll
need to invert the output of one of the chips (the 1-of-8 is
smaller).  If I do this, I'll be able to directly light one of the
LEDs.  However, most of the remaining LEDs will have reverse current
applied (some will be HIGH-HIGH or LOW-LOW).

QUESTION:  Since these are diodes, is it OK to have reverse current
applied to all of them?  (With 120 LEDs [in 60 packages]:  1 will
have forward current, 15 will be high-high, 7 will be low-low, and
the rest [97] will be reverse biased.)

QUESTION:  Would it be better to add 8 transistors on the 1-of-8
allowing the selected output to be high or float?  (This would result
in 1 with forward current, 15 high-high, 7 low-float, and the rest
[97] high-float.)

QUESTION:  If I need transistors, can someone recommend a part#?  I
need PNP because the active output from the demux is low, right?


I also have a few questions about Myke's project info.

QUESTION:  Myke shows R1 and R2 being tied to "PIC".  I assume that
this should be "Vcc"?

QUESTION:  The other end of the battery should be tied to GND, right?

QUESTION:  What is the purpose of the 5 caps C4-C8?

--
-----------
Jim Brunner

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