#define PIN_TRIG 16
#define PIN_ECHO 15

long duration, cm;

void setup() {

   // Initialize communication on the serial port

   Serial.begin(9600);
   //Define inputs and outputs
   pinMode(PIN_TRIG, OUTPUT);
   pinMode(PIN_ECHO, INPUT);
} 

void loop() {

   // First, we generate a short pulse with a duration of 2-5 microseconds.

   digitalWrite(PIN_TRIG, LOW);
   delayMicroseconds(5);
   digitalWrite(PIN_TRIG, HIGH);

   // With the signal high, wait about 10 microseconds. At this point, the sensor will send signals at a frequency of 40 kHz.
   delayMicroseconds(10);
   digitalWrite(PIN_TRIG, LOW);

   // The delay time of the acoustic signal on the echo sounder.
   duration = pulseIn(PIN_ECHO, HIGH);

   // Now it remains to convert time to distance
   cm = (duration / 2) / 29.1;
  
  if (cm > 400) {
    Serial.println("Distance was measured incorrectly");
  } else {
    Serial.print("Distance to object: ");
    Serial.print(cm);
    Serial.println("cm.");
  }

   // Delay between measurements for the sketch to work correctly
   delay(250);
}