Hi all,
I'd like to use the 'Super-simple' led driver as shown at instructables http://www.instructables.com/id/Super-simple-high-power-LED-driver/
But
I'd like to power it with an external power-supply (e.g. 9v battery) and use my arduino as the 'switch'.
What are the extra steps I need to know?
Plan to use a 3w led.
Thanks 
Probably that 9V batteries are expensive and don't last very long when powering a 3W LED. Especially if most of the energy is turned into heat.
On ebay you'll find very affordable high efficiency LED drivers for very little money.
What are the extra steps I need to know?
Connect the arduino output to the base of a transistor going through a resistor. Connect the + of the circuit to the +9V and the - of the circuit to the LED. Then connect the other end of the LED to the collector of the transistor and finally the emitter of he transistor to ground.
By the way it is a crap circuit.
Thank you for your responses.
Call it intuition, but I'm getting the feeling that perhaps the approach I was taking could perhaps be done in a better way.
Okay, so maybe I should change my question to this:
I would like to drive 1 (maybe up to 6) 3 watt leds using a battery (something like a 9v) for power and an arduino as a switch. The whole thing will be like an intermittent strobe in that it flashes 4 times, pauses and then flashes again on and on.
What would be the most straightforward approach for me to take?
Go to Ebay and put:-
led driver 3W
into it and you will see modules that will do this for you that can be connected directly to the arduino.