7 X 21 led matrix

Hi all,

I know this question might have been asked a million times before, but I can’t find a sufficient answer (probably very bad searching from my part), but I’m gonna ask it.

I’m going to build a 7x21 led matrix (normal red leds).
The columns (21) will be the + side of the leds and will be driven by 3 daisy chained 74HC595 and the rows (7) will be connected to a ULN2803.

Will this setup work?
Or will it cause problems :slight_smile:

(see also attachment)

Thx for the answer

7x21 led matrix.JPG

Yes, it should work. It won’t be very bright. OK for indoors I guess. The reason is that 74hc595 have an overall maximum current of 70mA, so you must use current limiting resistors chosen to limit the current to 10mA per led column. With multiplexing this would mean just over 1mA on average for each led. With modern high brightness leds, this it enough except in bright surroundings.

There are better ways to drive the matrix.

c8343c0e2772b231870577553f12957c3bd5ca4f.jpg

PaulRB:
Yes, it should work. It won’t be very bright. OK for indoors I guess. The reason is that 74hc595 have an overall maximum current of 70mA, so you must use current limiting resistors chosen to limit the current to 10mA per led column. With multiplexing this would mean just over 1mA on average for each led. With modern high brightness leds, this it enough except in bright surroundings.

There are better ways to drive the matrix.

c8343c0e2772b231870577553f12957c3bd5ca4f.jpg

Thanks for the answer.

What would be a better and appropriate way to drive such a matrix?

Use three MAX7219. Use 21 of 24 available columns with 7 LEDs in each column.
Can even work on your code with 3 8x8 matrices while you build up the 7x21.
Or just don't use the 8th LED. Or do, you have to send 8 bits for each column anyway.
Then you can use the parola.h library as well.

The classic solution would be to use 3 x max7219 chips, each one driving a 7x7 part of the matrix. This has many advantages. Increased brightness, lower component count, simplified sketch, fewer Arduino pins...

EDIT Bob you beat me to it again!

Thanks a lot!

The Max7219 was indeed the solution.
I had some led matrix laying around and I took the max7219 out of it. Very easy to use and program.