LED matrix 8x96 + 74HC595 + ULN2803

[rascupanamuha]

I am trying to build a 8x96 LED matrix, but i dont know how. I have been searching, and found that a lot people dont even use transistor..

That would be connected to arduino, but i dont know how to connect that all properly?
I have common anode 8x8 matrices that i will conect together (12 of them)

I saw this: https://www.electro-tech-online.com/attachments/macmux™-8x128-matrix-png.34625/
but it doesnt use transistors and, also i dont understand it...

So is there a easy schematics to do that what i want?

 

[dougy83]

In that schematic you linked there is a FET on each row. Each column doesn't require any additional buffering as there's only ever [at most] a single LED in each column being driven at a time.

You will need a transistor/buffer if you're driving up to 96 LEDs at a time. You can get away with not using one if the current requirements are reduced (e.g. reducing the drive current to almost nothing, or reducing the number of LEDs)

 

[Dr_Doggy]

port b is power for each row, so each (ANODE?) goes to the port b pin for that row,

the ground pins go by column to each register, so each Q pin of the register go to the cathode of each diode(LED) for that column,

it should work, after you connect your 8 data pins and 2 clk pins, then code arduino.

one thing i did notice also is that the shift register there are 4 data pins aswell to arduino from SER, be careful cos on the other 4 shift registers are wired a little different

 

[rascupanamuha]

I dont think if i understand you dougy83. Are you talking about refreshing rate 1/8, so first draw all LED i need in the first row, than the second, third and so on? So on each row there will be maybe 10 or 20 LEDs active, and only one column will be active at the time? I am doing this for as LED message display.

And, what are SER pins of 74hc595's connected to? To rows or what? What do you think about this schematics, and would it be easy or hard to program to show what i want on it with arduino?

EDIT: I think i just found one exactly i wanted: **broken link removed**
Isnt this better than the first one i posted? And i wont have 1 x those 2 chips on the right, but 12 of them. What do you think about it?

 

[dougy83]

I dont think if i understand you dougy83. Are you talking about refreshing rate 1/8, so first draw all LED i need in the first row, than the second, third and so on? So on each row there will be maybe 10 or 20 LEDs active, and only one column will be active at the time?

Each row is active for 1/8 of the time, and only a single row is active at a time.

There will likely be many columns active at the same time, but only a single LED in each column will be active at a time. This makes it possible for the 74HC595 to have a chance of driving the LEDs without a buffer. According to the datasheet, each output shouldn't draw more than 70/8 = 8.75mA, which will give an average LED current of ~1mA (which may not be very bright).

The SER pins are connected to the rows, and are multiplexed with the row drive function - the displays are disabled while updating the column contents.

The instructables diagram looks good too and has the advantage of higher LED current capability and fewer microcontroller pins being used.

 

[rascupanamuha]

Yep, thank you, i would focus than on that second option because i want bright leds. So in my case, i just need to add some more chips and conect them together (as 2 of 74hc595 in schematic are)? And with the same code as here https://www.instructables.com/id/LED-Dot-Matrix-Display-1/ 4 page i thing, i would get the result i wanted (just need to put somewhere that i dont have 8 row but 96 of them?

 

[dougy83]

Yes, the SEROUT (or H8+) or whatever it's called goes to the SERIN of the next chip. The clocks for all devices are connected together i.e. all RCLK use a single wire and all SCK use another wire.

You might want to rewrite the code to suit.

 

[rascupanamuha]

Thank you very much !
I will come back here if i have some questions, and i will post the final result

one more question. I thought it would be easier not to have anything on row driver. I didnt have anything on my 8x12 matrix. So i was writing latters col by col and then change the position. You helped me with that
And now i have to write first (down) row of all latters, then the second, etc, and when the cycle is over, change the position. That is right? And in code in that link i think he is changing as i was in 8x12, col by col, not row by row?

 

[dougy83]

Great! All the best

With this circuit you will have to write all column values for the current row, then the row select -- these are shifted in/out using the SERIN and SCK lines, then latch the values from the shift registers to the output registers -- this last part is done using the RCLK line.

 

[rascupanamuha]

So i write column values for bottom of latters, then the same to next row... And refresh rate will be 8ms, and brightnes will be 1/8, right? I think i would then put variable resistor to make it 1/1 bright or just to change from 1/8 to 1. Would that be ok?