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Just a basic query;

On my understanding of LED Matrix's, The following would light up 4 LEDs.

But what if I only wanted to light up just 3 of those LEDs? The circuit still needs the same inputs, since power will continue across all columns/rows

Thanks

Tom

enter image description here

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    Multiplexing means you are powering only one of them and you have to set corresponding columns (in this case more like reset). If you're doing it for all rows fast enough, your eye won't see flickering (only if you move display) – KIIV Sep 20 '17 at 9:10
  • BTW, you took this picture out of some place where multiplexing is explained. I deduce this by the leftover of the image title "What is multiplexing": did you actually read that article? – Roberto Lo Giacco Sep 20 '17 at 10:12
  • I was trying to find out on the internet and it looks like I was close to the answer. Maybe I didn’t think multiplexing would work over large LED arrays. Or let’s assume LED screens with 255 levels of brightness for each colour. The number of cycles or computing needs to be quick to support the display. – tommylux Sep 20 '17 at 10:42
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Multiplexing is the answer.

enter image description here

If you want to, as an example, light up only LEDS 1, 3 and 4, you will:

  • activate line A
  • activate line X
  • de-activate line X
  • de-activate lines A and B (yes, B was not active)
  • activate line A and B
  • activate line Y
  • de-activate line Y
  • de-activate lines A and B
  • start again from top

Do this so that the loop takes less than 10 milliseconds and you are actually refreshing the entire matrix at 100Hz, fast enough to trick the eyes of any human being as if the LEDs 1,3 and 4 are lighted up at the same time: persistence of vision is your friend.

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This is very simple. You can turn on each LED one at a time and repeat this over and over again. The easiest way of achieving this is to put this code in the loop section.

Persistence of vision is defined as "the retention of a visual image for a short period of time after the removal of the stimulus that produced it" - dictionary.com

Even though the LED is on for only 1/3 of the time, it is fast enough that the human eye will not be able to tell that each LED turns off. Hence the viewer sees 3 LEDs on at the same time.

This allows any combination of LED patterns in your LED matrix.

7-segment displays also use this technique. You probably have some in a microwave or DVD player that you never realized that they were cycling through each digit, using persistence of vision.

Resources:
http://www.circuitstoday.com/interfacing-8x8-led-matrix-with-arduino (recommended) http://tronixstuff.com/2013/10/11/tutorial-arduino-max7219-led-display-driver-ic/
https://playground.arduino.cc/Main/LEDMatrix
http://forum.hobbycomponents.com/viewtopic.php?f=43&t=1489

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