How about adding a cooling fan (for the workpiece) to the sanginololu? Do I need additional hardware or can I attach it directly to some free pins?
If you’re just looking to connect an always-on fan, just hook it right up to the 12V input. If, however, you need a (PWM-)controlled fan for printing PLA, you’ll have to add a driver (e.g. based on a BC547) via the expansion port.
And don’t forget a freewheel diode or the transistor will not last long.
@Thomas_Sanladerer AFAIK the ultimaker cooling fan is turned on when the ?first? layer is printed. Thats more like on/off than a PMW-control, right?
@F_Malpartida_fm Does this have to be added in any configurations? Because I have an existing cooling fan for the E3D-hotend, which is connected to the12V input.
If your fan is constantly on you don’t really need the diode but you will have some spikes (back EMF) on the rail as the coils cross over segments and when you switch off the motor. The real problem comes when you PWM the motor as you are constantly energizing and de-energizing its coils.
Some motors come with a diode builtin, but you never know.
DC fans are actually brushless DC motors with their own driver circuit and all, so spikes aren’t as much of a problem as with regular motors. In fact, i’ve been running a 80mm fan with a BC547 for many kilos of filament without any hiccups - granted, not on PWM and with just one on/off per print.
Besides adding a driver, you’ll also have to tell your firmware which pin you connected it to - the Sanguolululu’s section in “pins.h” will likely have a “-1” for the fan pin, change that to reflect the actual pin you used and you should be good to go.
According to some folks on the Eventorbot forums, you can tie the PWM pin of a 4pin pwm fan to an extra pwm pin on the sang board. I tried but my fan was on 100% all the time, despite updating my pins.h accordingly and shutting the fan off through gcode. I didn’t really troubleshoot too much, so it’s probably my mistake as others swear they have gotten it to work. I just go full on with a smaller fan
This is helpful: http://www.thingiverse.com/thing:22202 although I didn’t go through the hassle of the perfboard instead just wiring up an IRF510 MOSFET. Gate to D12, drain to the ‘-’ of the fan, source to GND, and +12v straight to ‘+’ on the fan. I didn’t find the diode to be necessary but a capacitor across the + and - of the fan helps on noise if you are using PWM to change the fan speed.
@Brian_Evans don’t forget the pulldown resistor on the gate!
On my board, there is a dedicated pin and driver to control a fan with its FET an freewheel diode connected to a PWM pin of the AT90USB.
On other boards you will have to configure it in the board config file.
Careful with the cap as the entire circuit’s RLC could resonate.
Thanks for your replies. 
Hm, can the print quality be raised when I add a PCB-controlled instead of an always on fan?
Now I realise that I should have asked this before… Sorry
Depending on the fan, If the fan is always on it can cool TOO much and your prints will suffer there. I like to have the fan on 100% for bridges and otherwise I tend towards just fast enough to spin up, often around 50-60%.
I use always on, but unplug on first layer. You can block the flow with paper to lower the flowrate
For abs I don’t use any layer fan, however for PLA a gentle fan blow of 40-60% does make a difference.
On my ABS prints if I use the layer fan they do tend to warp and when they don’t warp the prints don’t look any better.
Do you folks use layer fan with ABS?
No fan on ABS for me. However, the PLA printers i’ve built were almost impossible to get printing right without a fan.
@Thomas_Sanladerer @F_Malpartida_fm @Eric_Moy @Brian_Evans For my understanding: When you print PLA you control the fan manually?
Not really… you set it up in whatever slicer your using. In Slic3r its under filament settings > cooling. The slicer will adjust the fan speed within the min and max settings that you provide depending on what its printing. Try to print something pointy without a fan - it will be a molten blob. You want your slicer to speed up the fan when the layer time decreases.
@Thomas_Sanladerer OK, but I have another stupid question: I don’t have a heated bed. Could I connect these unused pins to my fan?