@enhydra yes, that’s totally possible. If you change the bed pin to -1 and your fan pin to the original bed pin, you can even have your slicer control it as needed.
@Thomas_Sanladerer Cool, thanks for your reply. 
In this case, do I need also a driver? (e.g. BC547)
No driver needed. The Sagululu already has a powerful MOSFET transistor for the bed output, which doesn’t care much about whether it’s switching a 140W bed or a 2W fan.
@Thomas_Sanladerer You just made my day!
@enhydra , I control my fan manually because I have a sang board and use the fet to control a heated bed. I had tried to hook up a pwm fan to one of the pwm pins but the fan was always on full blast. I think it may have been a faulty fan, as I even tried shorting the pwm Corel pin to ground and the speed never changed. I just got impatient and power it manually. PLA prints improved ten fold
@Eric_Moy Are you talking about the yellow wire on 3-wire fans? If so that is a signal wire in the fan that sends a pulse from the fan (usually to a computer) to know how fast the fan is spinning. Its not used to control the speed of the fan at all. The yellow wire can be completely removed from the fan for these purposes. Instead, you hook up the black ground wire to the source pin on an N-channel MOSFET with its sink pin to ground and gate pin connected to one of the PWM pins on the sanguinololu - D12 IIRC. The red wire on the fan is then connected to either +5v or +12v depending on the fan.
@Brian_Evans higher-end fans usually come with a fourth wire, through which they accept a low-power PWM signal.
@Eric_Moy it’s been a while since I’ve dug into that topic, but have you looked at PC’s PWM signal as a reference? Two things that might have been off are a) the voltage level the PWM signal needs to be at (12V vs. the Arduino’s 5V), the accepted voltage might differ from one fan to the next; and b) whether or not fans require an inverted PWM signal or not, since they need to work even when the PWM input is not connected.
@Thomas_Sanladerer I’ve just changed the settings from “pins.h” and tried to upload the firmware. No luck! I’ve got instead a bunch of errors:
missing/deleted image from Google+
Did I miss something? Do I have to alter other settings?
@Thomas_Sanladerer , I had shorted the 12vdc line to the pwm control line, no change, shorted to ground, no change, seemed to be full blast all the time. The fan has a minimum speed that is above zero according to the data sheet, but I gotta believe it was full blast as it was like a little hurricane.
@enhydra you might also need to remove the thermistor definition (and possibly other parts as well) in the main configuration.
@Thomas_Sanladerer That was easy! I just had to set the “TEMP_BED_PIN” to -1. Now it works. Thanks