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Prusa MK3S IR Filament Sensor

prusa mk3s ir filament sensor

Your Prusa 3D MK3S IR Filament Sensor may need a bit of tweaking to work properly, as I describe in this article.

My new Prusa 3D printer has a really neat filament sensor. The sensor provides two benefits. First, the printer knows when it runs out of filament. When this happens, the printer pauses until you replenish the filament. Second, you can use this sensor to automatically unload and auto-load new filament.

The heart of the Prusa MK3S IR Filament Sensor is an infra-red sensor module. Here’s how it works, as you can see above.

Loading filament into its channel pushes a small steel ball to the side. You can see the steel ball then pushes a small plastic lever to the left. When this happens, the plastic lever breaks into the infra-red sensor, triggering it.

Two opposing magnets are used to move the lever to the right when there is no filament in the channel.

All in all, as you can see above, this appears to be a relatively simple and reliable design, as the IR sensor is triggered by a lever, rather than by the filament itself. This means you can use transparent filament with the system.

Prusa MK3S IR Filament Sensor Challenges

Unfortunately, a lot of Prusa kit users have had trouble getting their sensors to work reliably. Normally, the sensor reads “1” with filament in place, and “0” without. However, many report the sensor getting stuck on “1”. This happened to me, as well.

The normal fix is to place a very small spacer under the IR sensor board, to raise it up by about half a millimeter. You will find this helps the lever not to trigger the sensor when filament is out.

However, most of us find it difficult to print such as small space, which is thin and only 4 by 8 mm. You can resolve this challenge by making the spacer just a small part of a larger print, and then cutting it out with a knife or scissors.

I took this approach and it worked just fine. By the way, the animated GIF file shown above is courtesy of the Prusa Knowledge Base.

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