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IR Filament sensor detection (MK3S)

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Last updated 3 years ago
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With filament sensor detection, the printer will be able to recognize if the connection with the sensor was interrupted (e.g. broken wire or disconnected cable) and take appropriate action based on your settings.

This feature was activated with firmware version 3.9.0 and requires revision 0.4 of the IR filament sensor. The compatible version of the sensor has a red PCB and was introduced in 2019 (picture below).

For those using a printer with a modified filament sensor (i.e. with a LED), we are unable to test every possible modification and their feedback, and can therefore not guarantee correct behavior.

 Revision 0.4 of the sensor chip is the only one that is red.

Possible states

In the Support -> Fil. sensor menu, information about the revision of the sensor will be displayed. Three possible states are defined:

  1. 0.3 or older (Not supported)
  2. 0.4 or newer (Detected and supported)
  3. unknown status (Sensor detection not completed)

It measures the voltage from the sensor which corresponds to a certain state:

  • No filament: 4.2 - 5 V
  • Inserted filament: 0.1 - 0.5 V

In the Support > Voltages menu, a new value will now be displayed regardless of the version of the sensor: “IR: x.xxV”. This value represents the actual voltage measured on the pin of the filament sensor.


In the Menu > Settings > HW setup > FS action you can set the printer's response to an unexpected change in the filament sensor state:

  • Pause - Will pause the print and show an onscreen message, awaiting user input.
  • Continue - Will ignore the issue and continue printing.


The filament sensor detection procedure is an automatic part of the Self test or Calibration Wizard. However, if you skip the wizard there is a new submenu found in LCD menu ->Settings -> HW settings -> Fsensor Detection. Here the sensor revision recognition can be triggered manually for the MK3S printer. The MK3S with the MMU2S unit must run the Selftest.

The process is fully automatic and the printer enables the new feature for the revision 0.4 only when all of the following conditions are met:

  • The printer is in an idle state (e.g. not printing, not heating up, …)
  • Only if the InfoScreen (home screen) is displayed
  • The sensor state is either “Unknown” or “Older version”
  • In the settings menu, the Filament sensor is set to “ON”
  • The correct voltages were measured (No filament: 4.2-4.6 V - Inserted filament: 0.3-0.5 V). If your voltage differs and you have revision 0.4 you should double-check the connection to the EINSY board.
In the rare scenario in which you decide to downgrade the 0.4 sensor to an older revision, it is necessary to run the detection procedure, to inform the printer about the change.
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