How to Controller LED using LDR sensor

 Fade an LED in and out

const int LED = 9; // the pin for the LED

int i = 0;         // We'll use this to count up and down

void setup() {

  pinMode(LED, OUTPUT); // tell Arduino LED is an output

}

void loop(){

  for (i = 0; i < 255; i++) { // loop from 0 to 254 (fade in)

    analogWrite(LED, i);      // set the LED brightness

    delay(10); // Wait 10ms because analogWrite

               // is instantaneous and we would

               // not see any change

  }

  for (i = 255; i > 0; i--) { // loop from 255 to 1 (fade out)

    analogWrite(LED, i); // set the LED brightness

    delay(10);           // Wait 10ms

  }

}

What is LDR ?

Light-dependent resistor (LDR)

As its name suggests, the light-dependent resistor (LDR) is

some sort of resistor that depends on light. In darkness, the

resistance of an LDR is quite high, but when you shine some

light at it, the resistance quickly drops and it becomes a reasonably good conductor of electricity. It is thus a kind of light activated switch.

Blink LED at a rate specified by the value of the analogue input from LDR sensor 

const int LED = 13; // the pin for the LED

int val = 0;   // variable used to store the value

   // coming from the sensor

void setup() {

  pinMode(LED, OUTPUT); // LED is as an OUTPUT

  // Note: Analogue pins are

  // automatically set as inputs

}

void loop() {

  val = analogRead(0); // read the value from

             // the sensor

  digitalWrite(LED, HIGH); // turn the LED on

  delay(val); // stop the program for

           // some time

  digitalWrite(LED, LOW); // turn the LED off

  delay(val); // stop the program for

          // some time

}

 Set the LED to a brightness specified by the value of the analogue input

const int LED = 9;  // the pin for the LED

int val = 0;   // variable used to store the value

               // coming from the sensor

void setup() {

  pinMode(LED, OUTPUT); // LED is as an OUTPUT

  // Note: Analogue pins are

  // automatically set as inputs

}

void loop() {

  val = analogRead(0); // read the value from

                       // the sensor

  analogWrite(LED, val/4); // turn the LED on at

                           // the brightness set

                           // by the sensor

  delay(10); // stop the program for

             // some time

}