Help! PIC Controlled LED Array Circuit - 100 LED's??

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I am new to electronics, but a programmer by trade. I am trying to
achieve a circuit of up to 100 independently controlled LED's (not
segmented display modules). Ideally the circuit would be controlled by
one or more PIC microcontrollers as I have a Willem II programmer. My
application is artistic in nature and requires the leds to be spread
out over a wide area as opposed to being in a grid formation.

I have heard a lot of conflicting reports about Maxim's MAX721xx chips
for this job. Some say it's the one I want, but others (including an
actual Maxim tech-supporter) say it's more suited to the segmented
displays. I have experimented with this chip with little success.

Has anybody achieved anything like this? I have scoured the Internet
and newsgroups for literally weeks looking for a matching schematic
and source module. I have found a few Basic Stamp modules but I'm not
setup for that at all and would rather stick with native PIC. The only
compiler I have is MPLab's MPASM.

Many thanks to anyone that can help

Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
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This is doubtless the cheapest way to control 52 leds from a single 8 bit
port.
http://www.tinaja.com/glib/muse152.pdf
I'll leave it to you to scale it up.

Peter



Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
On Tue, 9 Nov 2004 08:22:43 -0000, the renowned "Peter"

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Better do some calculations first, that method is cute, but scales
HORRIBLY.

I suggest either the simple-Simon static drive by serial SR suggested
by someone else, or a low multiplex ratio n x m array. No more than
8:1 mux ratio. For example, 8 high side drivers and 16 low side
drivers, with a timer-triggered ISR controlling the drivers would work
nicely. At 20mA each, 100 LEDs will draw 2A maximum. The drivers could
be as simple as 4 SOIC-8 dual MOSFETs, 2 D-MOS power shift registers,
and 16 resistors, for driving up to 128 LEDs at full brightness.


Best regards,
Spehro Pefhany
--
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Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
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I have done it using 74HC595 serial to parallel chips. 8 leds off each
chip, and you just chain them together, the whole thing becomes one big
serial shift register requiring only 3 pins from the micro. They will
drive low current LEDs directly or add a ULN2803 driver chip to drive up
to 500mA per output. Cheap and it works.
--
Tim Mitchell

Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
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Hi Tim,

This sounds like exactly the sort of thing I'm after. I google'd that
chip and it does sound like the chip for the job. A few questions:
What PIC did you use? Do you have a schematic of how it goes together?
Do you have a source module available? What language/compiler was
used? (I did say a few questions :) ) Lastly, something confuses the
heck out of me... I can see that you can daisy chain the 74HC595 chips
together, but how do you tell your PIC which one to address?

To everyone that replied, thank you very much. But I am quite the
novice and hoping that someone can provide me with a PIC-compatible
schematic/source-code combo

Thanks heaps

Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
snipped-for-privacy@gmail.com (Scott Hooper) banged on the keyboard until

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Get yourself some 74154 decoder chips. 4 inputs to 16 outputs; active-low
on the output. A couple of these as a matrix would give you 16^2
capability for only 8 output pins (or even only 5 if you commoned the
inputs and used a line for device select).

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Dorsai - Author of Erotic Fiction
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Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
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That sounds pretty good. Are you able to point me to a schematic &
matching code example of that scenario?

Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
snipped-for-privacy@gmail.com (Scott Hooper) banged on the keyboard until

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Google 74154 decoder, you'll turn up the whole datasheet for it. Can't
help you with the code - I'm an Atmel weenie :-)

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Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
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Didn't use a pic, used an 8051. Code was 8051 assembler but it's really
simple.

You address the chips just by clocking the bits out. Say you're
controlling 64 LEDs, you clock out 64 data bits (set the value to the
data pin, take the clock pin low then high, 64 times). The first data
bit goes to the LED furthest from the pic, then each bit goes to the
next LED, the 64th bit going to the LED nearest the pic in the chain.

On the 595 chip, once you've clocked all the bits into position, you
toggle a latch line to set the bits to the outputs (display them).
--
Tim Mitchell

Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
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I've done 96 LEDs using a PIC 18F242 and 6 Allegro A6276 16 bit shift
register/driver chips, which works really well.  I use software
generated PWM to allow each LED to have an independent 8 bit brightness
level, controlled by an RS485 input.

Martin


Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
Have you looked into using some kinda FGPA?

Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
Check out http://www.allegromicro.com/ for some nice LED driver
chips (up to 32 channels each I believe).  They are good with samples too.

Mike Anton

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Re: Help! PIC Controlled LED Array Circuit - 100 LED's??



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Why not several PICs  1 master and the rest slaves.



Re: Help! PIC Controlled LED Array Circuit - 100 LED's??
Hi Scott,
I may have come a little late on this thread but I did this LED thing
just recently. I used an 8-pin PIC (12C675) driving an array of 64 LEDs
using 74HC164 serial to parallel shift registers. These are more common
and cheaper (real cheap) than other types but do not have any kind of
output enable or buffer register so I simply used a P-channel FET to
drive the anodes of all the  LEDs. This allows me to blank the LEDs when
I am clocking plus I can also execute PWM in s/w to control the brightness.

Sure, you require a resistor per LED but when you do it with SMD then it
really is quite simple. There is no problem with chaining this array out
to any practical length.

Note that there is a limit to the total amount of current that you can
pass through the substrate of any IC. For most 74HC devices this is
around 50ma total which means you should limit LED current to about
6ma/LED. As the LEDs I was using were around 200mcd I didn't have a
problem with that.

I can give you schematics and source code if you like.


--
Peter Jakacki

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