Front panels

[Mike Stein]

As Jon says, not really that complicated. Google
DIY LED signs for lots of ideas; e.g. one of mine
uses 75492 MOS LED drivers and 40158 shift
registers plus a few current limiting resistors 
and
driver transistors.

Don't forget to share the finished project!

m

----- Original Message ----- 
From: "Jon Elson" <elson at pico-systems.com>
To: <General at classiccmp.org>;
"Discussion at classiccmp.org:On-Topic and Off-Topic
Posts" <cctalk at classiccmp.org>
Sent: Saturday, October 31, 2015 5:15 PM
Subject: Re: Front panels


> On 10/31/2015 03:11 PM, Charles Anthony wrote:
>> What I don't know how to do is drive 500 or so
>> LEDs.
>>
>> I am guessing a bunch of shift registers, but
>> I've pretty much reached my
>> design limits. I need some guidance on locating
>> and understanding the
>> technology to run that many LEDs.
>>
> it really isn't that complicated.  The simplest
> might be a byte shift register, ie. a bunch of
> octal D-FFs like
> the 74HC374.  Given a byte-wide group of GPIOs
> on the Beagle Bone, you could send out 63 8-bit
> words
> with one additional GPIO to act as a clock for
> the FFs.  The LEDs could be driven directly from
> the FF outputs with a resistor.  With the
> current generation of high-efficiency LEDs 10 mA
> would be plenty of current, and so the FF
> outputs would still be close enough the specs to
> drive the next stage.  One downside of this
> scheme is if the serial transmission was slow,
> you'd see a blink each time the Bone sent a new
> light pattern.
>
> If you want to get more complicated, you could
> have one HC374 for the shift register and one
> HC374 as the latch.
> You'd shift all 63 bytes through the byte-shift
> register, pulsing the byte clock 63 times, and
> then pulse the latch clock once to latch all the
> 5xx bits of light info into the latch register,
> which would allow the LEDs to be updated without
> any flash as the shift reg is being shifted.
>
> Now, another way to do this is with
> multiplexing.  You could maybe have 8 64-bit
> words that loaded to a small RAM, and the RAM is
> scanned to load data to banks of 64 LEDs.  This
> reduces the number of drivers to, say, 64
> cathode drivers and 8 high-current anode
> drivers, but complicates the rest of the thing a
> fair bit.  It will also cause the whole panel of
> LEDs to flicker at the multiplexing rate, which
> could be annoying when you flick your eyes
> across the panel.
>
> Jon