Sensor detects presence or absence of the filament. Openscad 3D Printable models for filament sensors with cotactless Hall sensor or with mechanical microswitch.
Applies very small force to the filament, minimizing friction.
prusa has this logic in its original IR filament sensor
sensor OUT:
0 (LOW) LOADED
1 (HIGH) EMPTY
IR sensor pinout
+ +5V
O OUT
- GND
MK2S rambo13a 10-pin connector looking at PCB pins
---------
x x x x x
x s x - +
---------
-=GND, +=+5V, s=signal
MK3 einsy10a 10-pin connector looking at PCB pins
--- ---
x x x x x
s x x - +
---------
-=GND, +=+5V, s=signal
HALL sensor Honeywell SS411P or SS495A
on package is written 411P or 95A
pinout looking at pins, plastic case at the back
_______
/ + - o \
---------
connect
- -
+ +
s o
Prusa MK2.5S firmware can be loaded to MK2S
Support -> Sensor info
Fil. sensor: 1-LOADED, 0-EMPTY
Magnet (5x5 mm Neodymium cylinder) is the only moving part. Solid state Hall sensor (Honeywell SS495A or similar for 5V) in 3-pin transistor plastic case directly connects to printer motherboard electronics.
When sensor is empty, magnet is placed at the position in channel close to the empty filament guide. Inserted filament displaces the magnet.
Best location for the Hall sensor is where magnetic flux reverses sign when filament is inserted, so the sensor reliably reads logical 1 or 0. To invert logic, flip both magnets or flip the sensor.
Flipping or using different sensor may need adjustment, experiment with position along the channel, in case of false runout, reduce the X parameter in steps of 0.1:
hall_pos = [X,Y,Z];
Hall sensor used is SS411P digital output with bipolar triggering in range of +50/-50 Gauss or SS495A analog rail-to-rail output with high sensitivity 3 mV/Gauss = 30 V/Tesla. AH3377 with unipolar triggering in range of +115/+90 Gauss can be used too but I haven't tried it.
Cut small piece of universal PCB, solder 3-pin connector on top side, sensor on bottom side. Insulate sensor middle terminal (-) as it crosses over (o) terminal. Default sensor orientation is flat side towards filament.
When filament moves thru the guide, it can either slide over the magnet or rotate the magnet in the channel. In either case, friction is very small.
When filament is removed, magnet returns to its default position by its own weight or by attractive force from the second magnet.
Second magnet should be used if filament sensor is not always vertical. It provides a more reliable return but it increases force, increasing the friction too. Friction can be tuned by varying distance of second magnet from the filament detection magnet.
Attractive force holds magnets together so they won't leave sensor easily. To remove inserted DxH=5x5 mm magnets, insert filament, try to shake it out, touch it with some steel or approach external magnet DxH=10x5 mm from the side and drag it.
Applies more friction to the filament than the Hall sensor.
Mechanical switch with roller applies its designed force to the filament. Roller should be precisely positioned relative to the filament to reliably trigger so there is not much space to tune the force.