On 10/31/2015 2:15 PM, Jon Elson wrote:
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
The 74HC595 8 bit shift register has a storage register also. You can
cascade them then update the displays with a single pulse. I'm using
them with ULN2803 8 bit drivers
to drive the incandescent bulbs on my 360/30 panel.
Bob
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