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Sensors are how the printer knows what's happening in the physical world. When sensors fail or drift, the printer makes bad decisions: missed homing, thermal runaway, layer shifts. Understanding what each sensor does and how to diagnose it lets you fix problems instead of guessing.
Every sensor on a hobby printer.
Use these three in order. Each builds on the one before.
In one paragraph, explain why thermistors are the most failure-prone sensor.
Walk me through how a BLTouch probe builds a bed mesh and how the firmware uses it during printing.
Design a sensor system to detect a print failure (spaghetti, layer shift) automatically — what would you add?
THERMISTORS (you have at least 2):
- Hotend thermistor: reads temp inside the heater block
- Bed thermistor: reads bed temp from underneath
Type: usually NTC 100k 'EPCOS-style' or PT1000 for high-temp hotends.
Failure mode: open circuit → controller reads 0°C → emergency stop.
Short → controller reads max → emergency stop.
Drift → temps look stable but are wrong → bad prints.
Diagnostic: probe with a multimeter. At room temp NTC 100k = ~100kΩ.
ENDSTOPS:
Mechanical switches (microswitches) or optical sensors.
Report 'home' position on X, Y, Z.
Failure mode: stuck closed → printer thinks it's always at home → crash.
Stuck open → never finds home → infinite slide.
Diagnostic: M119 in Marlin/Klipper terminal shows endstop states live.
BED LEVEL PROBE (newer machines):
BLTouch / CRTouch (pin deploys, makes physical contact)
Inductive sensor (detects ferromagnetic bed; needs metal bed)
Strain gauge (load cell — Prusa MK4, Bambu)
Used to build a 'mesh' of the bed for Z compensation.
Failure mode: drift → first layer is over- or under-extruded.
FILAMENT RUNOUT SENSOR:
Microswitch or optical; on the path between spool and extruder.
Triggers a pause when filament runs out.
Failure mode: false triggers from filament wobble; doesn't detect breaks
inside the bowden tube.
ACCELEROMETER (newer printers with input shaping):
ADXL345 or similar; mounts on print head.
Used once during setup to measure resonance freq of X/Y motion.
Then removed (in most setups; some keep permanent).
CURRENT MONITORING (advanced):
Some boards monitor driver current to detect stalls (Trinamic StallGuard).
Used as virtual endstops on some printers.