more rookie questions about shift registers and LEDs (more ways to skin a cat &
Fred Hawkins
Posts: 997
I am learning to use 74HC595's and RGB LED's.
(favorite explanations:
http://www.parallax.com/Portals/0/Downloads/docs/prod/stamps/BS2Appnotesv1_9.pdf··· superior! explains 595's from the logic diagrams and up
http://www.parallax.com/dl/docs/books/sw/exp/sw23b.pdf·········· good for cascading)
So my questions are:
1) Can I power 595's with the appropriate voltage to skip using LED resistors?
I am thinking a simple pair of resistors could adjust the voltage to suit. Mostly the Red LED which needs to be fed about 2 volts. Green and Blue work fine on 3.3v.
2) Or am I missing the point about the resistors? To wit,·LEDs need them to limit current, not voltage.
3) Or·can·I·just use PWM to manage the current?
4) Where is DeSilva when you need him? (I miss his grumpy help)
(favorite explanations:
http://www.parallax.com/Portals/0/Downloads/docs/prod/stamps/BS2Appnotesv1_9.pdf··· superior! explains 595's from the logic diagrams and up
http://www.parallax.com/dl/docs/books/sw/exp/sw23b.pdf·········· good for cascading)
So my questions are:
1) Can I power 595's with the appropriate voltage to skip using LED resistors?
I am thinking a simple pair of resistors could adjust the voltage to suit. Mostly the Red LED which needs to be fed about 2 volts. Green and Blue work fine on 3.3v.
2) Or am I missing the point about the resistors? To wit,·LEDs need them to limit current, not voltage.
3) Or·can·I·just use PWM to manage the current?
4) Where is DeSilva when you need him? (I miss his grumpy help)
Comments
2) Yes, LEDs need them to limit current
3) No
4) ???
Once the LED forward voltage threshold is exceeded, it will conduct as much current as is available
until the heat produced destroys something. Even with PWM, you need some kind of current limiting
because there are peak current and thermal effects that can damage or destroy the device.
Post Edited (Mike Green) : 6/16/2008 3:46:23 PM GMT
1) Alas.
2) Eventually I will learn this stuff.
3) More alas, maybe. Why complicate one's life?
4) Haven't heard from him since a month after his leaving post. (deSilva, say hi now and then)
Post Edited (Fred Hawkins) : 6/16/2008 8:39:33 PM GMT
There are shift registers designed specifically for LED control, do you know about them? The one I like to use is the TLC5940 by Texas Instruments. The chip has 16 outputs and each output is 12-bit of control of the intensity of the LED. I used this chip to design the LED hardware for a couple LED mood lighting applications. This chip may be a little too fancy depending on your application. There are chips that at easier to interface too.
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Timothy D. Swieter
www.brilldea.com·- uOLED-IOC, RGB LEDs, TLC5940 driver PCB
www.tdswieter.com
One little spark of imagination is all it takes for an idea to explode