Arduino - Light Sensor

In this tutorial, we are going to learn:

If you are looking for a light sensor in the module form, please check out this Arduino - LDR Light Sensor Module tutorial.

Hardware Required

1×Arduino UNO or Genuino UNO
1×USB 2.0 cable type A/B
1×Light Sensor
1×10 kΩ resistor
1×Jumper Wires
1×(Optional) 9V Power Adapter for Arduino
1×(Recommended) Screw Terminal Block Shield for Arduino Uno
1×(Optional) Transparent Acrylic Enclosure For Arduino Uno

Or you can buy the following sensor kit:

1×DIYables Sensor Kit 30 types, 69 units
Please note: These are Amazon affiliate links. If you buy the components through these links, We will get a commission at no extra cost to you. We appreciate it.

About Light Sensor

The light sensor used in this tutorial is a photoresistor, which is also called light-dependent resistor or photocell.

It is used not only to detect light but also to measure the brightness/illuminance level of the ambient light.


A photoresistor has two pins. Since it is a kind of resistor, we do NOT need to distinguish these pins. They are symmetric.

Light Sensor Pinout

How It Works

The more light the photoresistor's face is exposed, the smaller its resistance is. Therefore, by measuring the photoresistor's resistance, we can know how bright the ambient light is.

How Light Sensor Works


The light sensor value only reflects the approximated trend of the intensity of light, it does NOT represent the exact luminous flux. Therefore, it should be used only in an application that does NOT require high accuracy.

Arduino - Light Sensor

Arduino Uno's pin A0 to A5 can work as the analog input. The analog input pin converts the voltage (between 0v and VCC) into integer values (between 0 and 1023), called ADC value or analog value.

By connecting a pin of the photoresistor to an analog input pin, we can read the analog value from the pin by using analogRead() function, and then we can know the light levels relatively.

Wiring Diagram

Arduino Light Sensor Wiring Diagram

This image is created using Fritzing. Click to enlarge image

Arduino Code

The below code reads the value from photocell and determine the light level qualitatively

/* * Created by * * This example code is in the public domain * * Tutorial page: */ void setup() { // initialize serial communication at 9600 bits per second: Serial.begin(9600); } void loop() { // reads the input on analog pin A0 (value between 0 and 1023) int analogValue = analogRead(A0); Serial.print("Analog reading: "); Serial.print(analogValue); // the raw analog reading // We'll have a few threshholds, qualitatively determined if (analogValue < 10) { Serial.println(" - Dark"); } else if (analogValue < 200) { Serial.println(" - Dim"); } else if (analogValue < 500) { Serial.println(" - Light"); } else if (analogValue < 800) { Serial.println(" - Bright"); } else { Serial.println(" - Very bright"); } delay(500); }

Quick Steps

  • Copy the above code and open with Arduino IDE
  • Click Upload button on Arduino IDE to upload code to Arduino
  • Open Serial Monitor
  • Radiates light to sensor
  • See the result on Serial Monitor:
Analog reading: 163 - Dim Analog reading: 152 - Dim Analog reading: 187 - Dim Analog reading: 188 - Dim Analog reading: 957 - Very bright Analog reading: 972 - Very bright Analog reading: 981 - Very bright
Autoscroll Show timestamp
Clear output
9600 baud  

Light Sensor and LED

  • The below code turns ON the LED when it is dark, otherwise turns OFF the LED
/* * Created by * * This example code is in the public domain * * Tutorial page: */ // constants won't change const int LIGHT_SENSOR_PIN = A0; // Arduino pin connected to light sensor's pin const int LED_PIN = 3; // Arduino pin connected to LED's pin const int ANALOG_THRESHOLD = 500; // variables will change: int analogValue; void setup() { pinMode(LED_PIN, OUTPUT); // set arduino pin to output mode } void loop() { analogValue = analogRead(LIGHT_SENSOR_PIN); // read the input on analog pin if(analogValue < ANALOG_THRESHOLD) digitalWrite(LED_PIN, HIGH); // turn on LED else digitalWrite(LED_PIN, LOW); // turn off LED }
  • Wring diagram for the above code:
Arduino Light Sensor LED Wiring Diagram

This image is created using Fritzing. Click to enlarge image

Video Tutorial

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Challenge Yourself

  • Automatically turn on the light when your room is dark. Hint: Refer to Arduino - Relay.

The Best Arduino Starter Kit