xmasdreams:
Thanks everyone for your replies. Unfortunately, it still doesn't answer my question !
I understand the limited current capacity on the outputs of the shift register, which is why I asked whether a transistor, or Darlington pair, could be used to pump up the current.
It seems to be perfectly capable of passing on PWM to RGB LEDs, and I have seen projects driving high powered LEDs through 595s.
I guess maybe therein lies the answer.
I'm reading your posts, are you are asking about using PWM through a '595 or similar? My first inclination is that the amount of shifting required to have a reasonable base frequency wouldn't leave much else for the Arduino to do. Then I found this thread suggesting using a TLC5940 instead of software, [this thread](http://this thread) discussing driving PWM through a normal s2p (serial to parallel) shift register with software, and finally [this library](http://this library) for software PWM using interrupts through a normal s2p shift register for up to a 75Hz PWM on each shift register output.
LEDs are just used as examples. Lets say you have a large, complex pluming project for a fountain display. In your fountain display you have 30 valves that you need to control and want to use an Arduino UNO to run your show program. Daisy-chain four s2p shift registers and only use 3 pins on the Arduino to control up to 32 divices (valves, lights, curtains, audience zappers, etc.). You will need to have driver circuitry for each of the devices to provide the voltages and currents required to actuate them, but the actual control signal comes from the s2p shift registers. Just the first example that came to mind.
Drat... Now I have a desktop fountain display VU meter concept trying to take root in my mind. Control the height of each fountain jet based on amplitude of a frequency range. Water is (and mechanical valves are) fairly slow to respond so PWM may not be needed, just fast switching. Each fountain jet is normally illuminated with a green LED except at peak when it gets illuminated with red... Anyone else is welcome to take this idea and make an instructable out of it... 