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Topic: Shift Registry Question (Read 6279 times)previous topic - next topic

CrossRoads

#15
Oct 16, 2012, 07:07 am
I think you need more than a diode - you need to have the WS2801 drive a transistor to sink current thru the LEDs.
See the figure 8 discussion on pages 9 & 10 of the datasheet.
WS2801 is still powered from 5V, the LED string with current limit resistor is powered from 12V.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Computer991

#16
Oct 16, 2012, 07:11 amLast Edit: Oct 16, 2012, 07:18 am by Computer991 Reason: 1
@Osgeld:Okay thank you very much!

I think you need more than a diode - you need to have the WS2801 drive a transistor to sink current thru the LEDs.
See the figure 8 discussion on pages 9 & 10 of the datasheet.
WS2801 is still powered from 5V, the LED string with current limit resistor is powered from 12V.

Ah okay does it need an independent transistor for each color?(RBG)
I've read through those several times I'm a bit too newbie to completely understand that.

Also is my diagram wired up correctly?

CrossRoads

#17
Oct 16, 2012, 07:44 am
Yes, transistor for each color.
Use a resistor from arduino pin to base of transistor as well - driving the base is like driving an LED to Gnd with similar voltage drop, ~0.7V.
So (5V - 0.7)/20mA = ~220 ohm resistor

What diagram?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Computer991

#18
Oct 16, 2012, 08:09 am
Oh alright thank you for your help I think ill just stick with the other solution since this would require 3 transistors.

The diagram to the other solution I was thinking of was this one https://dl.dropbox.com/u/31949989/Idea1.PNG

frollard

#19
Oct 16, 2012, 11:49 am
There are different versions of the ws2801 chip; some for 5v and some for 12v.

Daniel Garcia made a good library to use SPI EXTREMELY efficiently to drive most led strands (in the megahertz)...at the cost of some chip functionality because of how low level the code is.
http://code.google.com/p/fastSPI

Also
LadyAda made a great library for SPI/and bitbang control of ws2801 - it's not nearly as efficient but it easily outputs hundreds of frames per second on a 64 rgb strand.
https://github.com/adafruit/Adafruit-WS2801-Library

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