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LJ12A3-4-Z/BX Inductive Proximity Sensor Switch

K-TECHNICS
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KSh600.00

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The LJ12A3-4-Z/BX is a cylindrical, M12-sized inductive proximity sensor primarily used to detect metallic objects without physical contact. It functions as a sinking NPN, Normally Open (NO) electrical switch, making it highly popular for 3D printer auto-bed leveling, CNC limit switches, and automation robotics. 
Technical Specifications Overview
  • Sensor Series: LJ12 (Cylindrical M12 barrel with a 12mm thread diameter)
  • Detection Distance: 4mm (Optimal for thin gaps or sensing iron, tin, and aluminum plates)
  • Supply Voltage: DC 6V to 36V (Standard versions require a minimum of 6V to power the internal circuitry reliably)
  • Output Type: NPN NO (Normally Open)
  • Output Current: Up to 300mA maximum load capability
  • Target Materials: Ferrous metals like iron/steel give maximum range; non-ferrous metals like aluminum or brass can be detected at slightly reduced distances.
Understanding the NPN Normally Open (NO) Operation 
Because this sensor is an NPN Normally Open configuration: 
  • Idle State (No metal present): The signal output is left floating (high resistance).
  • Triggered State (Metal within 4mm): The sensor activates an internal transistor that pulls the signal output line LOW, effectively connecting it to Ground (0V).
  • Visual indicator: A built-in red LED on the back of the cylinder turns on when metal is detected, allowing for easy calibration. 
3-Wire Wiring Configuration 
The sensor features a standard industrial color-coded three-wire layout: 
Wire Color Function Connection Target
Brown Positive Supply  Connected to DC +6V to +36V
Blue Negative Supply (GND) Connected to Power Supply Common / Ground
Black Signal Output Connected to microcontroller input pin or relay
Important Consideration for Arduino / Raspberry Pi (5V / 3.3V Logic)
If you plan to connect this sensor directly to a 5V microcontroller (like an Arduino) or a 3.3V system (like a Raspberry Pi), do not connect the black wire directly to your input pin if you are powering the sensor with 12V or 24V. Because the sensor operates on a wide DC voltage range, it can send high voltage down the signal line when switching or via leakage, which will damage your board.
Safe Interfacing Solutions:
  1. Voltage Divider Resistors: Drop the signal voltage on the black wire down to a safe logic limit.
  2. Optocoupler Isolation: Use an optocoupler board to completely isolate the high voltage loop of the sensor from the logic loop of the microcontroller.
  3. Internal Pull-up Technique: Since it is an NPN sinking switch, you can power the sensor at its required voltage, use a blocking diode on the black wire to prevent back-voltage, and enable the internal microcontroller pull-up resistor to handle the logic change.

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