# Understanding the following 8x8 led dot matrix schematic

#### u-will-neva-no

Joined Mar 22, 2011
230
Hello everyone, I am confused by the following schematic because the actual product has 16 pins, both parallel to each other. Im confused how to connect these pins to my 74HC595 (I have two of these). I would like to know how to read the schematic drawing to beable to connect the 8x8 dot matrix to the output pins on my chips.

If anyone has links for me to read then thats would be great also!

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#### praondevou

Joined Jul 9, 2011
2,942
The 74HC595 is not suited to drive directly all LEDs. In worst case it'd have to source or sink the current for 8 LEDs on each output!

Or is the 8-dot matrix a special low current model?

You will possibly need (two) 8-channel (LED)-driver ICs or transistor array ICs.

#### ErnieM

Joined Apr 24, 2011
8,200
That device has 64 LEDs, and each is addressable by driving the correct lines. It is best to light each LED in turn very fast to make it appear that any given number of LEDs are on.

Pins 13, 3, 4,10, 6,11,15,16 need to be pulled low.
Pins 9,14, 8,12, 1, 7, 2, 5 need to be pulled high.

You will also need 8 resistors to limit current, can go either side.

To light a LED you need to power the high and low pin that go to it.
Example: to light the top left LED, you apply + to pin 9, - to pin 13.

To use the 74HC595 for this you would shift out a pattern of 1000000 on the high side, and 01111111 on the low side. HOW you assemble, shift, and change for the next LED depends on what is driving those chips.

#### u-will-neva-no

Joined Mar 22, 2011
230
Well im using two of the 74HC595 ICs. I am getting this project directly from a book called "beginning arduino" but the led layout is different to mine.

Im still confused so im going to attach another picture, from the book, which is explaining what I should do. The diagram of 7-2 is basically the same as my first one however it has different numbers.

Say i was wiring output one from the 74HC595. I would connect a resistor in between but my issue is what pin on the dot matrix goes to. I know you said tie all the low to ground and bottom to high but the drawing in my book is different. I'm really bad at this so sorry if you have explained the answer to me and i'm not following!

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#### ErnieM

Joined Apr 24, 2011
8,200
Since the 74HC595 has a ±35 mA rating per pin it can drive the LEDs directly.

I couldn't tell you which pin is which row or column without knowing A) what LED device this is and B) which way you are looking at it. But the good news is you can in essence "rewire" those connection in the software driving the shift registers.

Here's one way to wire the pins:

Rich (BB code):
LED  U1  U2
13   15  -
3    1   -
4    2   -
10   3   -
6    4   -
11   5   -
15   6   -
16   7   -

9    -   15
14   -   1
8    -   2
12   -   3
1    -   4
7    -   5
2    -   6
5    -   7
You DO have some sort of micro planned here to drive the shift registers, correct?

#### praondevou

Joined Jul 9, 2011
2,942
Since the 74HC595 has a ±35 mA rating per pin it can drive the LEDs directly.
Are you sure? Maximum DC-supply current (VCC and GND pin) is 75mA. If all LEDs turn on, each output needs to drive 8 LEDs...

#### ErnieM

Joined Apr 24, 2011
8,200
Are you sure? Maximum DC-supply current (VCC and GND pin) is 75mA. If all LEDs turn on, each output needs to drive 8 LEDs...
Patient said:
Doctor, it hurts when I do this.
Doctor said:
Then don't do that!
The scheme as I have been describing will light just one LED at a time. A list all LEDs that need be lit for a given pattern needs to be repeatedly run thru.

Each LED will be turned on and off individually. A better set of drivers would be needed to drive a whole row or column at the same time.

And as each LED can be expected to be on a maximum of 1/64th of the time you best use nearly all of that 35 mA too.

#### praondevou

Joined Jul 9, 2011
2,942
The scheme as I have been describing will light just one LED at a time. A list all LEDs that need be lit for a given pattern needs to be repeatedly run thru.
Ok. I must have missed that part. Never mind.

Maybe I should get some coffee to wake up.

#### u-will-neva-no

Joined Mar 22, 2011
230

#### ErnieM

Joined Apr 24, 2011
8,200
Put the part on it's back. It will probably be ordered as an IC would be, so the sides are

Rich (BB code):
16 15 14 13 12 11 10  9

1  2  3  4  5  6  7  8
So which side is up? Look at the schematic again. Pins 1 & 16 conduct one way thru a LED, but pins 8 & 9 have back to back LEDS between them and never conduct.

So git an ohmmeter. (1 & 16 conduct when 16 = + and 1 = -)

You can take a power supply & resistor and probe the backside leads and check LEDs. A little hard to see both sides, but you'll like seeing them light up. I know I would be doing that first thing.

#### u-will-neva-no

Joined Mar 22, 2011
230
Thanks ErnieM!!

#### SgtWookie

Joined Jul 17, 2007
22,221
Actually, you can determine pin 1 visually.

Place the display pins down in front of you, then rotate it so that the part number is facing you.
In the row of pins that are nearest to you, the leftmost pin is #1.
Then going to the right, the pins are numbered 2, 3, 4, ... 8.
Opposite pin 8, in the row furthest from you, on the right end, is pin 9.
Then progressing towards the left, are pins 10, 11, 12, ... to 16, which is the leftmost pin in that far row.

#### ErnieM

Joined Apr 24, 2011
8,200
Whoops, I drew those pins looking down on the device. You'll have it on it's back when you probe it:

Rich (BB code):
 1  2  3  4  5  6  7  8

16 15 14 13 12 11 10  9

#### SgtWookie

Joined Jul 17, 2007
22,221
Whoops, I drew those pins looking down on the device. You'll have it on it's back when you probe it:

Rich (BB code):
 1  2  3  4  5  6  7  8

16 15 14 13 12 11 10  9
Yes, this is the way the pins will be when it is face-down, pins up, and the part number facing away from you.