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I am completely new to Arduino tinkering and am trying to make sense on how to wire a 8x8 LED matrix using the following diagram:

enter image description here

This chart is supposed to correspond to this 8x8 LED, 12x12pin LED matrix:

enter image description here

enter image description here

Now, the question:

  • The physical led matrix has 12 pins on top, 12 pins on the bottom.
  • The diagram shows 16 pins on top, 8 pins on the left.

How am I supposed to make sense of this? I highly appreciate an actual answer as opposed to someone pointing to arbitrary articles because reading articles on this is all I've done the past two evenings.

Side-notes:

  • Yes, this is homework, but we have no book for the course and we haven't learned to read diagrams like this yet.
  • The components used are: Arduino Uno + MAX7219 + 1588ABEG 8x8 LED Matrix
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  • \$\begingroup\$ There is a small 1 at the bottom right of the first image. All you have to do is count. \$\endgroup\$
    – someonr
    Sep 25, 2014 at 15:15
  • \$\begingroup\$ Yeah, I figured out where pin 1 was located, but I don't understand which out of all the 24 pins I need and should use for a one-color display. \$\endgroup\$
    – krystah
    Sep 25, 2014 at 15:18
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    \$\begingroup\$ Take a look at the shematic. The black and the white diodes correspond to one color. If you want to make a single LED light up, all you need to do is to supply a current the at the two corresponding pins. (example: bottom right LED = pin 1 + pin 13) \$\endgroup\$
    – someonr
    Sep 25, 2014 at 15:21
  • \$\begingroup\$ Our professor explained that last week, however the led driver chip (MAX7219) only has 8 DIG and 8 SEG pins, which is obviously not enough to cover all 24 pins, so I have to choose, but which? \$\endgroup\$
    – krystah
    Sep 25, 2014 at 15:22
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    \$\begingroup\$ @krystah did you read my comment? Connect only the black (green) or the white (red). That way you will only use half of the LEDs. \$\endgroup\$
    – someonr
    Sep 25, 2014 at 15:24

2 Answers 2

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The first picture you posted is called a schematic diagram and shows how the component is wired logically. It doesn't necessarily show how the pins are physically arranged. It does show where the LEDs are, though.

To make out how the pins are arranged, you need to look at the pin numbers, locate pin 1 and count from there. Be careful when counting from pin 12 to 13 - you must follow a clockwise direction, that is, pin 13 is the one one the upper-left side of your 2nd picture. Pin 24 is the one on the upper-right corner.

So, if you want to use just one color, choose red or green symbol from the legend (green has the LED symbol filled) and pick the pins coming in and out of those LEDs only.

In either case (picking red or green), you'll always have to wire pins 1, 2, 3, 4, 21, 22, 23 and 24 to the positive terminal of your power supply. These are the LED common cathodes. They are called common because they are shared among each pair of red and green LEDs. If you choose to use the red LEDs, you'll also have to wire pins 13 through 20 to your negative supply. On the other hand, if you pick green, you'll wire pins 5 through 12. These are the LED anodes.

In other words, to light up the red LED on the lower-left corner, you must apply voltage (with limited current), to pins 1 (+) and 20 (-). To light up the green LED at the same position, you must apply voltage to pins 1 (+) and 5 (-).

Don't forget to wire a current limiting resistor in series with each LED.

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    \$\begingroup\$ Although both answers were immensely helpful, the part about cathodes/anodes cleared up so much for me. It's up an running fine now, you guys saved my weekend! \$\endgroup\$
    – krystah
    Sep 25, 2014 at 16:38
  • \$\begingroup\$ Glad we could help! If you have more questions, feel free to post them here on the EE.SE site (as separate, specific questions - as per site rules). \$\endgroup\$
    – Ricardo
    Sep 25, 2014 at 16:40
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The numbers in the circles on the top diagram (schematic) are pin numbers. Pins are numbered starting at one, and go counter-clockwise when looking from the LED side.

See How do I identify Pin 1 on a chip with no corner mark if you feel you need more help

If you connect any row pin (left hand side pin number) to a positive voltage, say 5V from your Arduino, and then any column (top pin numbers) via a 1k resistor to ground (GND), a LED will come on.

Use that to figure out what is connected to where. It is pretty quick if you gently ease the LED into a breadboard. Mark on the edge of the breadboard (with a Sharpie, or tape) pin 1, 12, 13 and 24, to make it easier to track what is happening.

You will be able to see which are Red or Green.

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