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[on:]

Hi everybody !
I would like to know if one can use analogs demultiplexers on PWM outputs, to use the PWM on many LEDs independents. My objective is to use the RGB Matrix of Sparkfun with Arduino Board. I use the PWM and the demultiplexers to put the color of LEDs of the row (3 PWM, one for the red, one for the green and one for the blue).
Thank you  

[Reply #1 on:]

i guess you could use a analog demultiplexer (4051) to light up rbg colour leds, but  with a De/Multiplexer you can just light up one led(one line) at the same time, so you have to update your matrix quit fast and you will lose a lot of the brightness of the leds, further more you can not use long delays anymore.

there are better ways to ligth up a LED matrix, the best would be a LED-driver (max7219) or i don't know how does your matrix look like but you could also youse the pwm as the the source and your other digitalpins as the ground and build your own little matrix. If you write a digitalPin to LOW they will be direklty conectet  to ground.  you also have to update this matrix quit fast but not as fast as the demultiplexer matrix, so you will not lose so much brightness.
In the past had taking place a lot of discussion about LED matrixs in this forum so check out the older post and discussions.

ha det bra
tomek

[Reply #2 on:]

Thank you  
In fact, the column is connected to the ground with digitalPin, and I put the color in the first line, then the second ... I don't understand how to use a max7219, I have only three PWM, so, I can put the color of one led only at the same time.  

http://vincentlenoir.d.free.fr/schema.png

[Reply #3 on:]

In fact, the problem is to add PWM outputs on the Arduino board  :-/

[Reply #4 on:]

hi

Without looking at whether the multiplexer circuits are correct, I can see the big probelm is current drive... the problem you will find with that schematic is that essentially you're using the Atmega8 to provide current drive for the segments. this means a maximum (before thie chip melts down!) of 40ma from the PWM pin; divided by 8 that makes 5ma per individual led colour, 15ma total per led. In sum,  it's not goign to be very bright, especially when yo're multiplexing it as well! You should try using some driver transistors with appropriate resistors for each led column... have look at this diagram and you'll see what I mean... instead of using the PWM pin as the current source, you need to use the pwm pin to drive trnasistors that switch on much greater currents.

D

[Reply #5 on:]

Ok, but on a PWM, only 1 led is put on in the same time, so the current is not divided by 8 (moreover when I test the current on a PWM, I have 85mA, it's sufficient)

Thank      Â