Ultrasonic sensor

const int trig_pin = 11;
const int echo_pin = 12;

void setup()
{
  pinMode(trig_pin, OUTPUT);
  pinMode(echo_pin, INPUT);

  Serial.begin(115200);
}

void loop()
{
  digitalWrite(trig_pin, LOW);
  delayMicroseconds(2);
  digitalWrite(trig_pin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trig_pin, LOW);

  long duration = pulseIn(echo_pin, HIGH);
  long distance = (duration/2) / 29.1;

  Serial.print(distance);
  Serial.println("cm");  
}