Connecting a Photoresistor to an Arduino

A photoresistor is a resistor whose resistance decreases with increasing incident light intensity, using a photoresistor we can make arduino sense the intensity of light around it.

arduino photoresistor

Using the playground article as reference, resistor (100K) and photoresistor are connected in series. +5V goes to resistor, ground goes to photoresistor, the junction where the resistor and photoresistor meets goes to analog 0. Digital 13 is used for the LED.

In my room photoresistor reads around 80, when I put my thumb on it making it dark, it reads around 500. I wanted the LED to light up when its dark.

int lightPin = 0;  //define a pin for Photo resistor
int threshold = 250;

void setup(){
    Serial.begin(9600);  //Begin serial communcation
    pinMode(13, OUTPUT);


void loop(){

    if(analogRead(lightPin) > threshold ){    
        digitalWrite(13, HIGH);
        digitalWrite(13, LOW);


So I picked a number in between and used it as threshold, if the reading is above the threshold it turns the LED on when its below threshold it turns the LED off.