Light Sensor + LED chaser. Read more here: http://alif.my/2013/10/project-3-light-sensor-led-chaser/

light-sensor-led-chaser

int photoRPin = 0;
int minLight;
int maxLight;
int lightLevel;
int adjustedLightLevel;

void setup(){
  
  //Setup the starting light level limits
 lightLevel=analogRead(photoRPin);
 minLight=lightLevel-20;
 maxLight=lightLevel;
  
  pinMode(12, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(10, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(8, OUTPUT);
}

void loop(){
  
  
  //auto-adjust the minimum and maximum limits in real time
 lightLevel=analogRead(photoRPin);
 
 Serial.println(lightLevel);
 
 if(lightLevel < 20) { playLED(); } else { stopLED(); }
 
 //slow down the transmission for effective Serial communication.
 delay(50);
  
  
}

void playLED() {
  digitalWrite(12,HIGH);
  delay(100);
  digitalWrite(12,LOW);
 delay(25);
  digitalWrite(11,HIGH);
  delay(100);
  digitalWrite(11,LOW);
 delay(25);
  digitalWrite(10,HIGH);
  delay(100);
  digitalWrite(10,LOW);
 delay(25);
  digitalWrite(9,HIGH);
  delay(100);
  digitalWrite(9,LOW);
 delay(25);
  digitalWrite(8, HIGH);
  delay(100);
  digitalWrite(8, LOW);
 delay(25);
}

void stopLED() {
 
  digitalWrite(12,LOW);
  digitalWrite(11,LOW);
  digitalWrite(10,LOW);
  digitalWrite(9,LOW);
  digitalWrite(8, LOW);
}