Example of using Ping (ultrasonic sensor) with Arduino and three LEDs. See more at https://helgejohnsen.wordpress.com/2015/09/01/arduino-og-ping-ultrasonisk-sensor/

pingled.ino

const int pingPin = 7;

void setup() {
  Serial.begin(9600);
  pinMode(2, OUTPUT); // Led red
  pinMode(3, OUTPUT); // Led orange
  pinMode(4, OUTPUT); // Led green
}

void loop()
{
  long duration, cm;

  // Send pulse to ping
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW); // Low pulse to ensure a clean high pulse
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(pingPin, LOW);

  // Read pulse from ping
  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);

  // Convert the time into a distance
  cm = microsecondsToCentimeters(duration);

  // If very close (red LED)
  if (cm < 10) {
    digitalWrite(4,HIGH);
    digitalWrite(2,LOW);
    digitalWrite(3,LOW);
  }

  // If close (orange LED)
  if (cm > 10 && cm =< 25) {
    digitalWrite(3,HIGH);
    digitalWrite(2,LOW);
    digitalWrite(4,LOW);
  }

  // If far away (green LED)
  if (cm > 26) {
    digitalWrite(2,HIGH);
    digitalWrite(3,LOW);
    digitalWrite(4,LOW);
  }    

  // Print distance to serial
  Serial.print(cm);
  Serial.print("cm");
  Serial.println();

  delay(100);


}

long microsecondsToCentimeters(long microseconds)
{
  return microseconds / 29 / 2;
}