#define maxLed 13
#define minLed 12
#define trigPin 11
#define echoPin 10

int maximumRange = 200;
int minimumRange = 0;
int oneThird = (maximumRange - minimumRange) / 3;
int twoThird = 2 * oneThird;
int maxLedState;
int minLedState;

void setup() {
  Serial.begin (9600);
  pinMode(maxLed, OUTPUT);
  pinMode(minLed, OUTPUT);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  long duration;
  long distance;

  digitalWrite(trigPin, LOW);  // Added this line
  delayMicroseconds(2); // Added this line
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10); // Added this line
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = (duration/2) / 29.1;
  
  if ((distance <= minimumRange) || (distance >= maximumRange))
   { // out of range
      maxLedState = LOW;
      minLedState = LOW;
      Serial.println("Out of range");
   } // out of range
  else
   { // in range
     Serial.print(distance);
     Serial.println(" cm");
     if (oneThird > distance)
      { // within first 1/3 of range
        maxLedState = LOW;
        minLedState = HIGH;
      } // within first 1/3 of range
    else
     if (twoThird > distance)
      { // within middle of range
        maxLedState = HIGH;
        minLedState = HIGH;
      } // within middle of range
    else
      { // at last 1/3 of range
        maxLedState = HIGH;
        minLedState = LOW;
      } // at last 1/3 of range
   } // in range
  digitalWrite(minLed,maxLedState);
  digitalWrite(maxLed,minLedState);

  delay(500); // wait a bit
}