I am thinking about art installation where I would use 3 or more of these: http://led24.de/flexible-rgb-led-leiste-500cm/ Can I PWM these with arduino and ULN2803 or must I use something else in-between? Thanks!
Those strips draw 3 Amps at 12 Volts (36 W / 12 V = 3 A). The ULN2803 can handle 50 V but only 1/2 A. That might be a problem.
You could switch them with a logic-level N-Channel MOSFET like http://www.sparkfun.com/products/10213. You might have to provide a heatsink for the MOSFET.
Can they handle 8x1A or total load of 1A?
On the page for MOSFET they are mentioning 100Hz... it does not seem much for PWM, one could see it flickering.
ULN2803 can handle 500 mA per channel, 8x0.5 = 4 A maximum per chip. Even it possible to connect channels in parallel to get more current, I don't think it's a good idea. Better way to use power transistor, MOSFET's are good up to megahertz, some of them up to 100 MHz, so 100 Hz is completely not true, or you can get BJT, TIP120 - // - TIP 147
jakovn: On the page for MOSFET they are mentioning 100Hz... it does not seem much for PWM, one could see it flickering.
The first response to that comment says: "100 Hertz is trivial for this part, it has a turn on time of 140 nanoseconds and turn off time of 100 nS."
That's a total cycle time of 240 nanoseconds, roughly a quarter of a microsecond. That's four cycles per microsecond, 4000 per millisecond, and 4,000,000 per second (4 MHz).