Thanks to both of you for all of your input!
Shift registers can be used in many ways, but is it useful ?
Pushing 24 bits into the shift registers is very fast. I think there is no need to change it.
I'll leave it as is then. I think that I have a board with 4 shift registers that I can play with until I actually get this one made.
Led strip resistors:
The led strip is for 12V. Often 3 leds in series are used, with a resistor, to make it work for 12V. With a led strip, you don't need extra resistors.
This is also good news.
To get all the RGB colors, a PWM signal is used for every 'R', 'G' and 'B'.
Since you use the shift registers, using PWM is a lot harder and not so smooth anymore. Did you know that ?
I had seen it eluded to but not specificly talked about. I think for my application I might not need the fade part, but I'll do some testing and find out.
I see you have the 12V at the led connector, and the ULN as driver.
The ULN chips have a maximum current per output and a total maximum current.
Can you check that ? The current depends on the length of the led strip.
The datasheet for the texas instruments chip located here: http://www.ti.com/lit/ds/symlink/uln2803a.pdf
mentions 500mA peak collector current. Does that mean for the entire chip or per IO?
Maybe late to the game, but I would just use qty 3 TPIC6B595, in place of the HC595 and ULN2803. 150mA current sink per output, rated to 50V. 3 LEDs/strip are generally 20mA, so 21 LED strips would be supported.
There are other versions of TPIC6x595 that can sink more.
I have boards on hand already that have arduino functionality and 3 TPIC6B595s already made if you want to buy a bare board, or an assembled one. Will post a pic when I get home. All thru hole, easy to assemble.
Nope not too late. I'll do some looking and try a layout to see what I can come up with.