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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

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    \$\begingroup\$ Draw a circuit would you please \$\endgroup\$
    – Andy aka
    Apr 13 '13 at 19:52
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    \$\begingroup\$ Are you providing power to the Arduino itself? \$\endgroup\$
    – DimKo
    Apr 13 '13 at 19:54
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    \$\begingroup\$ Please draw a proper circuit. Edit your question and hit Ctrl+M, the current image is unreadable. Check your breadboard, most rows along the side are have one or two interruptions half way (Arduino's GND connection). Possibly add a picture of your setup aswell. \$\endgroup\$
    – jippie
    Apr 13 '13 at 20:16
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    \$\begingroup\$ You drew an PNP transistor and gave a datasheet for a NPN transisitor. Which one is it? \$\endgroup\$
    – jippie
    Apr 13 '13 at 20:21
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    \$\begingroup\$ Are you sure about the diode polarity? \$\endgroup\$
    – jippie
    Apr 13 '13 at 20:21
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The diode in your drawing is backwards.

As such, all the current is flowing through the diode, and you're not getting enough voltage to even turn the fan on.

enter image description here

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