//3 leds on pin 2, 4, 6
int a = 2;
int b = 4;
int c = 6;

//the ultrasonic sensors trig and echo are connected to pin 12 and 13
#define trigPin 12 
#define echoPin 13

//run only once at the beginning. Define pinMode-s depending on we gonna send or recive data from that
void setup() {
  //LEDs
    pinMode(a, OUTPUT);    
    pinMode(b, OUTPUT);  
    pinMode(c, OUTPUT);  
   //distance sensor
    pinMode(trigPin, OUTPUT);
    pinMode(echoPin, INPUT); 
}

// the loop routine runs over and over again forever:
void loop() {
  int duration, distance;
  //send the sound
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(1000); //wait a little bit
  digitalWrite(trigPin, LOW); //turn off the sound
  //perceive when it come back
  duration = pulseIn(echoPin, HIGH);
  //convert from time to distance (in cm)
  distance = (duration/2) / 29.1;
  //first turn off all the leds
  digitalWrite(a, LOW);
  digitalWrite(b, LOW);
  digitalWrite(c, LOW);
  if(distance<10) //if very close: 3 leds on
  {
    digitalWrite(a, HIGH);
    digitalWrite(b, HIGH);
    digitalWrite(c, HIGH);
  }
  else if(distance<20) //if close: 2 leds on
  {
    digitalWrite(a, HIGH);
    digitalWrite(b, HIGH);
  }
  else if(distance<30) //if not so far: only 1 led on
  {
     digitalWrite(a, HIGH); 
  }

}