Filament sensor

Updated 10 days ago ​by Tomáš Chvalina

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

  1. Running out of filament
  2. Nozzle jams 

Running out of filament

With this filament sensor, running out of filament will no longer be a problem. Therefor you can confidently let your spools finish to the very last bit of plastic. How does it work? Once you ran out of filament. Printer will automatically pause the printer. Move the X-carriage away from the print and unload the remaining 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 believe 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 thru only a bit of filament. 


Nozzle jams 

The sensor can also detect filament getting stuck. Unfortunately, in most cases, it’s not possible to recover the current print. As it is hard to detect the exact moment of the jam happening. However, in a future version of the firmware, we plan to offer a few more options to possibly save the print. 

When the jam is detected, the printer will try to unload the filament. However, this may not always be possible if too much filament was already ground off. Either way, the printer will stop, display a warning about the jam happening, and cool down to prevent any further damage. 

False sensor readings and debugging 

There are couple of common reasons why you would encounter false sensor readings. We can 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 with the use of compressed air. There is a dedicated hole in the X-carriage exactly for this purpose. 



Extreme light conditions & Exotic filaments
For both of these. It is usually better to turn the filament sensor of. As it might cause plenty of false alarms which can make printing a very lengthy process. 

  1. 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].
  2. Some translucent filaments focus more light on the sensor and this can result in a weird behaviour. 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. Would you still have any questions. Please do not hesitate to contact us at info@prusa3d.com.


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