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Topic: Understanding the components of a LED matrix (Read 1 time) previous topic - next topic

Grumpy_Mike

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That's 8x20mA sourced on a single pin (the walking bit), and 20mA sinking on each column.

No it is the walking bit that needs the help in sourcing or sinking the current. So 20mA sinking is fine but 8X20mA sourcing needs extra help.
Normally on an arduino you can choose if you want to supplement  the sourcing or sinking capacity. Then the way it is scanned is chosen so that the low capacity current pin only carry the current for one LED.

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If those were connected straight to the arduino,

Yes so don't.

Hippynerd

Technically, you can make a single color 8x8 matrix without any ics. You use 16 arduino pins, 8 resistors, and 64 LEDs. I've made a 64 LED cube (4x4x4) with 64 LEDs, 4 resistors (16 resistors works better), wire, solder, and an arduino.

You can also do this with charliplexing, You could even do this with RGB LEDs if you use charlieplexing. Check out asher glicks charliecube.
http://aglick.com/charliecube.html
Im working on doing some documentation for this, he shows you what you need to accomplish, but not so much about how to get there, and its a bit tricky.
Technically, this design should be doing damage to something Im unclear on weather its the LEDs or microcontriller at risk, but I've been running them over a month, without failure, and no appreciable differences in brightness or color, or speed.

If you really want to use shift registers, then you should check out ShiftPWM:
http://www.elcojacobs.com/using-shiftpwm-to-control-rgb-leds-with-arduino/

I made a few examples that ran straight off the arduinos 5v pin, plugged into a 5v USB phone charger (about 500mA), I only ran them for a couple days, and technically its probably too much current for the arduino, so it *may* be doing some kind of damage.

Have you considered constant current drivers? They make chips just for controlling many LEDs, it makes things a lot easier, less stuff to deal with, less stuff to screw up, and tidier too.

If you want more examples of people using multiplexing with transistors, also check out cubes, and LED strips.
Here Ladyada is using Mosfets and Darlingtons to power strips.
http://learn.adafruit.com/rgb-led-strips/usage

Im still trying to figure out sinking and sourcing to my 2 other cube setups (one with shift registers, one with constant current driver), but I've been busy with other things. The charliecube was a lot of work soldering, but I had it working within a week, my other cubes are still not finished, and I started them over a month ago.

M4573R

#7
Dec 24, 2012, 11:05 pm Last Edit: Dec 24, 2012, 11:19 pm by M4573R Reason: 1

No it is the walking bit that needs the help in sourcing or sinking the current. So 20mA sinking is fine but 8X20mA sourcing needs extra help.


What I'm trying to get at is that I can't source OR sink with a 74HC595 since 8x20mA on one pin is too much, and 20mA on 8 pins is also too much. I'm completely new to this, so if I am not understanding some fundamental rules of current flow, I apologize.


Technically, this design should be doing damage to something Im unclear on weather its the LEDs or microcontriller at risk, but I've been running them over a month, without failure, and no appreciable differences in brightness or color, or speed.


That is a bit settling, but it's what I'm trying to avoid, hehe.

dhenry

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20mA on 8 pins is also too much.


It isn't too much unless you are building for NASA.

In that case,  you can add a swich (npn/pnp, depending on your design) on the 595's output.

M4573R


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20mA on 8 pins is also too much.


It isn't too much unless you are building for NASA.

In that case,  you can add a swich (npn/pnp, depending on your design) on the 595's output.



The 595 is listed as 70mA max current across the chip, how long should I expect it to last?

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