LM393 Light Sensor with Arduino: Complete Wiring & Code Guide

How the LM393 Light Sensor Works

The module has two main parts:

1. Photoresistor (LDR): A light-dependent resistor. Its resistance decreases as light intensity increases (typically 1kΩ in bright light, 1MΩ in darkness).
2. LM393 Comparator IC: Compares the LDR voltage against a threshold set by the on-board potentiometer, then outputs a clean digital HIGH/LOW signal.

The module provides two outputs:

• Analog (AO): Direct voltage from the voltage divider, proportional to light intensity. Connect to Arduino's ADC pin (e.g., A0).
• Digital (DO): Goes HIGH when light is above the threshold (or below, depending on jumper). Connect to any digital pin (e.g., D2).

Wiring Diagram

Connect the module to your Arduino using 3 jumper wires:

LM393 Module          Arduino Uno
─────────────          ──────────
VCC  ────────────────  5V
GND  ────────────────  GND
AO   ────────────────  A0
DO   ────────────────  D2

Optional: LED on D13 for visual feedback

Arduino Code

Copy and paste this code into the Arduino IDE. It reads both the analog and digital outputs and prints them to the Serial Monitor.

// LM393 Light Sensor with Arduino
// Reads both analog (AO) and digital (DO) outputs
// More tutorials: https://comparator.online

const int analogPin = A0;  // AO connected to A0
const int digitalPin = 2;  // DO connected to D2
const int ledPin = 13;     // Built-in LED

void setup() {
  pinMode(digitalPin, INPUT);
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("LM393 Light Sensor Ready");
}

void loop() {
  // Read analog value (0-1023)
  int analogValue = analogRead(analogPin);

  // Read digital value (HIGH or LOW)
  int digitalValue = digitalRead(digitalPin);

  // Convert to voltage (0-5V)
  float voltage = analogValue * (5.0 / 1023.0);

  // Convert to light percentage (inverted: lower value = brighter)
  int lightPercent = map(analogValue, 0, 1023, 100, 0);

  // Print to Serial Monitor
  Serial.print("Light: ");
  Serial.print(lightPercent);
  Serial.print("% | Voltage: ");
  Serial.print(voltage, 2);
  Serial.print("V | Digital: ");
  Serial.println(digitalValue == HIGH ? "BRIGHT" : "DARK");

  // Visual feedback via built-in LED
  digitalWrite(ledPin, digitalValue);

  delay(200);
}

Calibrating the Threshold

The on-board potentiometer sets the digital output threshold. To calibrate:

1. Power up the Arduino with the code running.
2. Open the Serial Monitor to see live values.
3. Place the sensor in the lighting condition you want to detect (e.g., shade for "dark" detection).
4. Turn the potentiometer with a small screwdriver until the digital output toggles.
5. The threshold is now set for that specific light level.

3 Project Ideas

ESP32 / ESP8266 Compatibility

The LM393 light sensor works identically with ESP32 and ESP8266. Just change the pin numbers:

• ESP32: AO → GPIO 34 (ADC1_CH6), DO → GPIO 4. Use analogRead() returns 0-4095 (12-bit ADC).
• ESP8266 (NodeMCU): AO → A0, DO → D1. Note: ESP8266 ADC is only 0-1V, use voltage divider.
• Raspberry Pi Pico: AO → GP26 (ADC0), DO → GP15. Use MicroPython or CircuitPython.

Frequently Asked Questions