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Topic: LED cube shift register power (Read 1 time) previous topic - next topic

ematson5897

I built an LED cube (8x8x8) with some cheap LEDs. I am going to control it with 8 74HC595 on the columns. Can I skip resistors and power the shift registers with the LEDs voltage spec (2v)? I pulled some LM317 off of an LED strips so I can do that. Because it would make thinks alot easier.

Thanks for any help,
ematson5897

Osgeld

sure, it will eventually kill your led's and your shift registers, but you can

I havent given any thought to it, but it might be better to set the 317 to act as a constant current device
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL


Osgeld

http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

ematson5897

Yea. For cheap leds, they have survived a lot though. I have had them on a 1/8 duty cycle on 5v through a shift register.

Grumpy_Mike

This is why you always need something to limit the current with an LED.
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

ematson5897

Yes I have seen your website. For some reason I was under the impression that if an LED is being fed its fwd voltage, it will only take the current it needs

Grumpy_Mike

Yes that is a common misconception that is why I made that page.  :)

ematson5897

Ok we'll thanks for the help. So I guess I'll just go buy 64 resistors

Grumpy_Mike

No for an 8 by 8 matrix you only need 8 resistors.


Grumpy_Mike

It depends then on how many LEDs are going to be on at any one time and how you wire them.

Docedison

An Led requires a CONSTANT CURRENT for proper operation, regardless of Duty Cycle the current must be limited. Above the turn-on point (voltage ~2V for red and ~3V for green/blue) the led is a short circuit and current MUST be limited. Short Duty cycles apparently work... They DON"T, instead you wind up spiking the 5V source every time you enable a non current limited LED. If you limit the current you don't NEED to shorten the Duty Cycle. Don't forget to BY-PASS, BY-PASS, BY-PASS. Cheap insurance and often times the answer to what looks and acts like a bad sketch. Nice thing about a Bread board is that you can just PLUG them in... IMO

Doc
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