Dimming many LEDs (that have external Powersource)

Hi

i'm building a security system for my studio. with some RF-Remotes from ebay, and lots of applescript (to detect motion on a webcam, call my phone via SIP, etc).

now i want to have something like this, just for the lighting:

when the alarm is on, the LEDs should shine like 30%, enough so the webcam detects motion, but the LEDs wont take too much power.

when motion is detected, the LEDs should go to 100%, so the webcam has enough light to expose the thiefs on the images.

when the alarm is off, the LEDs should be off too :)

now, to do this with a small array of LEDs is quite easy, using the PWM Pins.

But I was thinking of wiring about 10 piece 3W LEDs around the room. Problem is, arduino doesn't have enough power to power all the LEDs.

Is there a way to have an external powersource for the LEDs but still control the output via PWM on arduino?

you can use a transistor to drive the LED's, or maybe one transistor pr. LED.

You just need to make sure you chose a tansistor that can handle the current needed for the 3W LEd's.

High power LEDs need extra care, see this discussion:- http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234273497