LJ12A3-4-Z/BX NPN Inductive Proximity Approach Sensor 6-36V DC 1m Lead with LED for Metal Detection

📌 Code Examples & Usage – LJ12A3-4-Z/BX Inductive Proximity Sensor (NPN)
📋 Overview

The LJ12A3-4-Z/BX is an inductive proximity sensor that detects metal objects only (steel, iron, aluminium, etc.) without physical contact.

Key features:

Detection distance: ~4 mm

Output type: NPN (normally open)

Supply voltage: 6–36 V DC

Output: Open-collector (switch to ground)

Rugged, industrial-grade sensor

Common uses:

Object detection

Limit switches

CNC / 3D printer homing

Automation & robotics

Counting metal objects

⚠ IMPORTANT – Voltage & Interface Warning

🚨 This sensor runs on 6–36 V. You must NOT connect it directly to a microcontroller input.

The output is NPN open-collector, meaning it pulls to GND when active.

To safely use it with Arduino / ESP32 / Raspberry Pi, you must use one of these:

✔ Pull-up resistor to 5 V or 3.3 V
✔ Optocoupler module (PC817)
✔ Voltage divider + protection (less ideal)

🔌 Wiring – Sensor Colours
Wire Colour Function
Brown +V (6–36 V DC)
Blue GND
Black Signal output (NPN)
🔌 Recommended Wiring – Arduino (Safe Method)
Option 1: Using a Pull-Up Resistor (Most Common)

Brown → +12 V (or other 6–36 V supply)

Blue → GND (common with Arduino GND)

Black → Arduino digital input (e.g. D2)

10 kΩ resistor from Arduino input to 5 V

This ensures the Arduino only ever sees 0 V or 5 V.

🧪 Arduino Example – Detect Metal Object
const int sensorPin = 2;

void setup() {
pinMode(sensorPin, INPUT);
Serial.begin(9600);
}

void loop() {
int state = digitalRead(sensorPin);

if (state == LOW) {
Serial.println(“Metal detected”);
} else {
Serial.println(“No object”);
}

delay(100);
}

Logic Explanation

LOW = object detected (sensor pulls line to ground)

HIGH = no object (pulled up by resistor)

This inverted logic is normal for NPN sensors.

🧪 Arduino Example – Using Internal Pull-Up (Lower Voltage Only)

⚠ Use this only if the sensor output is isolated and never exposed to the supply voltage.

const int sensorPin = 2;

void setup() {
pinMode(sensorPin, INPUT_PULLUP);
Serial.begin(9600);
}

void loop() {
if (digitalRead(sensorPin) == LOW) {
Serial.println(“Metal detected”);
}
}

👉 External pull-up is still strongly recommended for industrial environments.

⚡ ESP32 Example

ESP32 inputs are 3.3 V only, so this method is even more important.

Pull-up resistor goes to 3.3 V

Sensor output never exceeds 3.3 V

const int sensorPin = 15;

void setup() {
pinMode(sensorPin, INPUT);
Serial.begin(115200);
}

void loop() {
if (digitalRead(sensorPin) == LOW) {
Serial.println(“Metal detected”);
}
delay(100);
}

🧪 Example – Count Objects (Simple Counter)
const int sensorPin = 2;
int count = 0;
bool lastState = HIGH;

void setup() {
pinMode(sensorPin, INPUT);
Serial.begin(9600);
}

void loop() {
bool state = digitalRead(sensorPin);

if (lastState == HIGH && state == LOW) {
count++;
Serial.print(“Count: “);
Serial.println(count);
}

lastState = state;
delay(20); // debounce
}

Perfect for conveyors or rotating metal parts.

🧠 Notes & Best Practices

✔ Detects metal only (not plastic, wood, water)
✔ Detection range depends on metal type and size
✔ Steel gives best range
✔ Output LED on sensor lights when object detected
✔ Always share GND between sensor supply and MCU
✔ For noisy environments, use:

Optocoupler isolation

Shielded cable

Pull-up resistors ≤10 kΩ

Additional information

Weight	60 g
Dimensions	260 × 160 × 20 mm