I'm trying to power a 12v fan using a darlington so I can control the speed from an Arduino. When I wire up as below nothing happens, even though I'm sending a PWM signal:
Circuit diagram http://img809.imageshack.us/img809/5261/wireissue.png
Have I wired up the transistor incorrectly?
Transistor: http://www.bitsbox.co.uk/data/transistor/BD681.pdf