[sdiy] 7-segment LED displays with 4511 driver chip - max current?

rsdio at audiobanshee.com rsdio at audiobanshee.com
Fri Aug 7 08:30:02 CEST 2015

On Aug 6, 2015, at 2:35 AM, Gordonjcp <gordonjcp at gjcp.net> wrote:
> On Wed, Aug 05, 2015 at 10:29:52PM +0200, Rick Jansen wrote:
>> On 05/08/2015 19:34, Brian Willoughby wrote:
>>> In the old days, a TTL chip like the SN74143 would drive up to 22 mA on each of its 7-segment driver outputs, and had what seems to be plenty of headroom since the chip could dissipate 1.4 W total. But that's an obsolete chip that only operated on a 5 V supply.
>>> I agree with Richie: handle the BCD conversion in your code, and wire the individual LED segments so that you can create any pattern you want. Even old product designs from the pre-surface-mount decade of the eighties had direct drivers for the LED segments to save money on parts like special BCD decoder chips. Have fun.
>> The CD4511 is 30 euro*cents* at Conrad!
>> The Arduino is not fast, and it *is* doing timing and interrupt
>> stuff, so if I can offload the display of the numbers, which are
>> just a nice extra, and not crucial, I don't mind doing that with
>> three 30 cent chips.
> Use shift registers.  These will latch the LED pins, allow you to display arbitrary symbols and only take up a couple of GPIOs.

One disadvantage of shift registers is that you cannot run them from a very efficient interrupt. If the shift register is on a shared serial bus, then you have to worry about interrupts occurring in the middle of a message to another device on the same bus. If you're bit-banging those serial shift register pins, then your interrupt will take vastly more cycles. Of course, some chips might actually allow DMA to a serial bus, but it's unlikely that serial bus could be shared with other devices.

The advantage of direct I/O for an LED matrix is that you could handle everything in an interrupt that takes ten or twenty cycles (on PIC or AVR) and be done with it, leaving the bulk of your CPU for other, high-level tasks.

… just trying to encourage some circuits that differ from the typical open hardware being designed these days.


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