LED multiplexing

Design Concepts
1

Hi! First post here! Loving the clean interface of this forum. Great people, keep up the good work!

I am thinking of using a 74HC595 as described in the LED rings schematic: 11 LEDs per ring with column drivers, and X number of rings with row current sinks.

  1. I believe each pin is sinking too much current. The Fairchild 74HC595 datasheet says 25mA sink or source under Absolute Maximum, MM74HC595 says ±35mA. Given that every 11th LED is on at once, or only 11 LEDs are on at once, this may not be a problem. But it seems like it is awefully close to the Absolute (read danger) rating.

  2. It would be cool to apply a PWM signal the Output Enable (pin 13) of the 74HC595 to globally adjust brightness of the LEDs. This should work, right?

Thanks again!

CausticPhoton

2

Hi,

welcome to the forum! :slight_smile:

These topics are (partly) discussed in following threads:

  1. <___base_url___>/index.php?/topic/12566-dout-max-current-supply-driving-leds-blm&do=embed

  2. <___base_url___>/index.php?/topic/10168-pwm-leds-for-rotary-encoder-position&do=embed

Addendum to 2) so long all LED rings should be dimmned from the same PWM signal, it will work.

This is a simple programming example (for MIOS32) which demonstrates how to generate the PWM from a single timer IRQ:

http://svnmios.midibox.org/listing.php?repname=svn.mios32&path=%2Ftrunk%2Fapps%2Ftutorials%2F007_mios32_timer%2F

there is another (somewhere…) which demonstrates how to generate independent PWM waveforms by using modulo operations (not recommented for PIC)

Best Regards, Thorsten.