Front panels

Bob Rosenbloom bobalan at
Sat Oct 31 17:35:16 CDT 2015

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.


Vintage computers and electronics

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