Filament sensor with optical encoder.
Required parts:

• optocoupler KY-010;
• BMG roller with needle bearings;
• bearing 623;
• screw and nut M3;
• microswitch (suitable from computer mouse);
• spring (from the fountain pen);
• JST 2.54 4-pin connectors;
• screws.
  Before printing an encoder ring from opaque filament (I used white), it is desirable to cover a segment of the filament (~ 10 cm) with a black marker.