How do I design the circuit for a led screen made up of leds?

Thread Starter

Ernesto Monge

Joined May 17, 2017
14
Im trying to make a controlable 310x488mm led screen made up of many 8x8 led matrices. The controller for this project willbe a rasberry pi. I need to be able to put simple geometric shapes on the screen since it will be used to teach kids with down syndrome. It will also have a touch panel on top of screen for interactive learning but for now i need to know what is the best way to go about the circuit design. I was considering using a whole lot of demultiplexers ,arranging them on x and y to select the desired led that will turn on. The rasberry pi doesnt have enough pins to control all the multiplexers .So will i have to use another set demuxes to control the demuxes controlling the leds? Or is there a simpler way?
 

WBahn

Joined Mar 31, 2012
26,304
They physical dimensions are very important at this point. What are the dimensions in terms of pixels?

What kind of refresh rate do you need?

Is this a school project?
 

BobTPH

Joined Jun 5, 2013
2,598
I would use LED strips with the adressable WS2812 chips. You can then control all of them woth one pin.

How many pixels and how quickly must they update?

Bob
 

philba

Joined Aug 17, 2017
960
You sure about those dimensions? 310 isn't divisible by 8.

ws2812 - that would be about 2 FPS if a string that long could be supported. I shudder to think how much current that would draw.

HC595 could do a lot better but would still need the screen to be subdivided into say 10 groups of 31 lines of 488. Each would be driven by a micro controller. A master controller would hand each group controller the line data. For a monochrome screen, you would only need 18,910 HC595s and 2363.75 8x8 LED matrices. Triple that if you want RGB. The good news is you can get those parts for really low prices at that volume. The bad news, that's about $17,000 for mono.
 
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WBahn

Joined Mar 31, 2012
26,304
You sure about those dimensions? 310 isn't divisible by 8.
He didn't say that it was 310 LEDs in that dimension, he said that it was 310 mm x 488 mm.

Without knowing the pixel pitch in each dimension, we can't tell what the display format is. That's why Bob and I both asked the TS to provide more information.
 

philba

Joined Aug 17, 2017
960
He didn't say that it was 310 LEDs in that dimension, he said that it was 310 mm x 488 mm.

Without knowing the pixel pitch in each dimension, we can't tell what the display format is. That's why Bob and I both asked the TS to provide more information.
hehe, you're right, I just saw 310x488.
 

WBahn

Joined Mar 31, 2012
26,304
Yes if it is completed i will recieve credit for it later on.
Thank you. Since it is a project for credit, the expectation is that you will do the bulk of the work since the primary objective is for you to maximum what you learn from it. Our role is to look over your work and help you get past specific hurdles by giving hints and suggestions or pointing out things you might be overlooking.

Now, a couple of people have asked how large your array is in terms of how many LEDs by how many LEDs. The physical dimensions of the display really don't tell us anything.

We've also asked how frequently you need to be able to update the display.

These are not the only two factors that drive major decisions regarding your project, but they are two of the biggest.
 

WBahn

Joined Mar 31, 2012
26,304
112x72 is what i will use in leds ,and 50 hz refresh rate
Since you are using 8x8 modules, you are looking at 14 x 9 modules.

Do you have enough data lines to provide 14 bits of data plus a few control signals?

What is the range of values for each LED? Just on/off? Just one color?
 

Thread Starter

Ernesto Monge

Joined May 17, 2017
14
Since you are using 8x8 modules, you are looking at 14 x 9 modules.

Do you have enough data lines to provide 14 bits of data plus a few control signals?

What is the range of values for each LED? Just on/off? Just one color?
I have 17 input/output pins
 

WBahn

Joined Mar 31, 2012
26,304
0-3.3v ,one color, just on and off ,i have 17 data lines, I answered your questions up there
Ah, my apologies. The link took me to your reply to my post and I didn't realize there was one further up with the information.

How many of the 17 data lines can you devote to driving the display output? All 17, or do you need to do other things, as well?

You know how many LEDs you have in the display. You know how many bits of information are needed per LED. You know how frequently you need to update the display. Given all of that, what does your data throughput need to be (bits/second) in order to keep up with the display goals?
 

Thread Starter

Ernesto Monge

Joined May 17, 2017
14
I can devote 12 of those lines ,I dont know how to answer your last question,I need to keep a geometric shape on the display until the child traces over it ,not much activity is going to be going on the display ,it will also have a simple game selection menu.
 

WBahn

Joined Mar 31, 2012
26,304
I can devote 12 of those lines ,I dont know how to answer your last question,I need to keep a geometric shape on the display until the child traces over it ,not much activity is going to be going on the display ,it will also have a simple game selection menu.
How many LEDs are in your display? ___________

How many bits of data need to be sent to each LED per frame? __________

From these two numbers, how many bits of data need to be sent to the display per frame? __________

How many frames are there per second? ______________

From these last two numbers, how many bits do you need to send each second? ___________

Answer it as best you can.
 
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