Night light

Course Introduction

This Arduino project uses a photoresistor (LDR) to detect ambient light levels and automatically control an LED.

When the environment gets dark, the LED turns on; when it’s bright, the LED turns off. The light threshold can be adjusted for sensitivity.

This simple system is ideal for automatic night lights or basic light-sensing applications.

Note

If this is your first time working with an Arduino project, we recommend downloading and reviewing the basic materials first.

1.1 Install Arduino IDE(Important)
1.2 Introduction of Arduino IDE

Wiring

Common Connections:

LED
Blue: Connect the LED anode to 9 on the Arduino, and the cathode to a 1kΩ resistor, then to the negative power bus on the breadboard.

Photoresistor
VCC: Connect to A0 on the Arduino.
GND: Connect to breadboard’s negative power bus.

Writing the Code

Note
You can copy this code into Arduino IDE.
Don’t forget to select the board(Arduino UNO R4 Minima/WIFI) and the correct port before clicking the Upload button.

// Define pins
const int sensorPin = A0;   // Photoresistor voltage divider output to A0
const int ledPin = 9;       // Blue LED connected to D9

// Sensitivity threshold (higher = darker, can be adjusted)
const int threshold = 500;  // Recommended initial value 500, adjust via Serial monitor

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);  // LED off by default
  Serial.begin(9600);         // For monitoring light level and threshold
}

void loop() {
  int sensorValue = analogRead(sensorPin);  // Range: 0~1023

  Serial.print("Light sensor: ");
  Serial.println(sensorValue);

  if(sensorValue < threshold) {
    // Environment is dark, turn on LED
    digitalWrite(ledPin, HIGH);
  } else {
    // Environment is bright, turn off LED
    digitalWrite(ledPin, LOW);
  }

  delay(100); // Slight delay to avoid Serial flooding
}