Filament sensor

Updated 3 weeks ago ​by Tomáš Chvalina

The optical filament sensor is among the best features we have added to our i3 MK3. Not only does it detect the presence of the filament, but also its movement. That means that we can trigger filament run-outs.

    If you just want to test this feature, then visit MK3 safety features - how to test them.

    Running out of filament

    With this filament sensor, running out of filament is no longer a problem. Therefore, you can confidently let your spools finish to the very last bit of plastic. How does it work? Once you run out of filament, the printer will automatically pause the printing. Move the X-carriage away from the print and unload the remains of the filament.  

    Printer will guide you how to resume printing safely: 

    1. "Please press the knob to unload the filament" = Simply press the knob to unload the remaining of the filament.
    2. "Repeat unloading filament?  [Yes/No] = If everything unloaded properly. Simply press [Yes]. If not, then you can repeat the process by pressing [No]. In case not all of it went out properly; open the Extruder-idler and clear the filament with pliers.
    3. "Insert filament and press the knob" = Make sure to insert the same type of filament. Cut the filament properly (check out the photo bellow for the reference), insert the filament and press the knob. 

    4. "Changed correctly?" [Yes/Filament not loaded/Color not clear] = If everything went just fine, simply press [Yes]. If you think that the filament was not loaded properly, press [Filament not loaded]. That will push the same amount of filament as before. If only the color was not clear, then press [Color not clear]. This way it will push through only a bit of filament. 

    False sensor readings and debugging 

    There are couple of common reasons why you would encounter false sensor readings. We divide them into two groups. 

    Proper setup & Clean surroundings 

    1. Check if the sensor is positioned correctly and connectors are properly seated.

    2. Make sure that there is no dust on the sensor. If you believe that there might be some, you can clean it using compressed air. There is a dedicated hole in the X-carriage exactly for this purpose. 

    3. Make sure that custom printed parts for the extruder carriage are Black. Since the filament sensor is calibrated to operate in a black environment and will give false readings otherwise.
    4. Extreme light conditions & Exotic filaments
      For both of these, it is usually better to turn the filament sensor off, as it might cause plenty of false alarms which can make printing a very lengthy process. 
    5. If you’re printing in extreme light conditions (direct sunlight or very strong LED lighting), you might also encounter some false alarms. Settings - Fil. sensor [on/off].
    6. Some translucent filaments focus more light on the sensor and this can result in a weird behavior. Certain colors like ivory, matte white or lime green can also cause false alarms. If you’re printing with any of these filaments and you’re getting a lot of false alarms, consider turning the sensor off in Settings - Fil. sensor [on/off]

    We would highly suggest trying out a different position for the printer or a change of the lighting condition. Should you still have any questions, please do not hesitate to contact us at

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