Light-Activated Night Light Using Arduino – Beginner Project

Ever wanted a night light that just knows when to turn on? In this project, we’ll build a simple automatic night light using an Arduino and an LDR (light-dependent resistor). When it gets dark, the LED turns on. When it’s bright, the light stays off — no switches, no fuss.

This project is perfect for beginners who want to explore how sensors work with Arduino. You’ll learn how to read analog input, set a light threshold, and control an LED based on real-world conditions.

Let’s get started and make your lights a little smarter.

Parts Required

Arduino Uno Starter Kit: https://amzn.to/3GKRdQx
Bread Board: https://amzn.to/3GDwQoz
Jumper Wires : https://amzn.to/434rudw
5mm LEDs Assortment Pack : https://amzn.to/3Z4RAM4
Photoresistor : https://amzn.to/4k1jpNo
10k-ohm resistor :https://amzn.to/3SnW3Wn

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How It Works

This project uses a light sensor (LDR) to detect how bright or dark the environment is. The LDR is a special type of resistor that changes its resistance depending on how much light hits it:

In bright conditions, the resistance drops low.
In the dark, the resistance increases significantly.

We combine the LDR with a regular resistor to form a voltage divider. This setup sends a changing voltage signal to the Arduino’s analog input pin. When it’s bright, the voltage is higher; when it gets dark, the voltage drops.

The Arduino constantly reads this input value. If it detects that the room is dark (i.e., the value falls below a preset threshold), it sends a signal to turn on the LED. When the light comes back, the LED turns off again.

💡 Bonus tip: LDRs aren’t polarized — you can plug them in either direction, no problem.

This is the same principle used in automatic night-lights — simple, effective, and easy to build with just a few components.

Wiring the Circuit

Follow these steps to connect your components to the Arduino:

Let’s walk through how to wire the components for this light-activated night light:

Place the LDR on the breadboard, making sure each leg is on a separate row.
- Connect one leg to 5V on the Arduino.
- Connect the other leg to analog pin A0 — this is where we’ll measure the light level.
Add a 10kΩ resistor to form a voltage divider:
- Connect one end of the resistor to GND on the Arduino.
- Connect the other end to the same row as the LDR leg going to A0.

This setup creates a voltage divider that adjusts the voltage at pin A0 based on the surrounding light. The darker it gets, the lower the voltage at A0 — and that’s how we know when to turn on the LED.

Connect the LED:
- Insert the longer leg (anode) of the LED into digital pin 13.
- Insert the shorter leg (cathode) into GND.
- Note: Pin 13 on most Arduino boards already includes a current-limiting resistor, so you can safely connect the LED without an external resistor. (But you can always add a 220Ω if you want to be extra safe.)
Upload the code using the Arduino IDE. Once running, the LED will automatically turn on when it gets dark.

Code Breakdown: What’s Happening Here?

const int ldrPin = A0;
const int ledPin = 13;
const int threshold = 500;  // light level threshold to turn on the LED

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int lightLevel = analogRead(ldrPin);  // read value from 0–1023
  Serial.println(lightLevel);

  if (lightLevel < threshold) {
    digitalWrite(ledPin, HIGH);  // it's dark → turn on the LED
  } else {
    digitalWrite(ledPin, LOW);   // it's bright → turn off the LED
  }

  delay(200);
}

Link to Tinker CAD here

Let’s walk through what the sketch actually does under the hood:

First, we connect the photoresistor to analog pin A0, which we use as an input to detect how much light is hitting the sensor.
The LED is connected to digital pin 13, which acts as an output — the Arduino will send signals to this pin to turn the light on or off.

In the setup() function, we call Serial.begin(9600) to start serial communication. This allows the Arduino to send data to your computer, and if you open the Serial Monitor in the Arduino IDE, you’ll see real-time readings from the photoresistor.

Inside the loop()The Arduino runs continuously:

Reads the analog value from A0 — this gives us a number between 0 and 1023, depending on how bright it is.
That value is then divided by 4 (because analogWrite for PWM pins only accepts values between 0 and 255) — this scales the light level down to something the LED can use.
The LED receives this value as a PWM signal (a type of “fake analog”), which makes it appear dimmer or brighter depending on the surrounding light.

⚡ In short:
More light → lower LED brightness
Less light → LED gets brighter automatically

This setup creates a smooth, real-time response that mimics how night-lights work — fading in as the room darkens, and fading out when it’s bright again.