Now we have everything we need to know to start building our project, let's start by writing the code:

int sensorPin = A0;            // select the input pin for the ldr 
unsigned int sensorValue = 0;  // variable to store the value coming from the ldr 
 
void setup() 
{ 
  pinMode(13, OUTPUT); 
  //Start Serial port 
  Serial.begin(9600);        // start serial for output - for testing 
} 
 
void loop() 
{ 
  // read the value from the ldr: 
  sensorValue = analogRead(sensorPin);      
  if(sensorValue<400) 
       digitalWrite(13, HIGH);   // set the LED on 
  else digitalWrite(13, LOW);   // set the LED off 
   
} 

We start by storing the pin numbers in variables, so that it doesn't become confusing, and then initialize our LED pins for output.

The code consists of a if statement that checks the value of sensorValue. If the values stored in it is less than 400, meaning that...