const byte trig = 12;
const byte echo = 11;

const int numReadings = 25;

long readings[numReadings];      // the readings from the analog input
long readIndex = 0;              // the index of the current reading
long total = 0;                  // the running total
float average = 0;

int duration;
float distance;

void setup() {
  pinMode(trig, OUTPUT);
  pinMode(echo, INPUT);

  Serial.begin(9600);
  for (int thisReading = 0; thisReading < numReadings; thisReading++) {
    readings[thisReading] = 0;
  }
}

void loop() {
  digitalWrite(trig, LOW);
  delayMicroseconds(2);

  digitalWrite(trig, HIGH);
  delayMicroseconds(10);
  digitalWrite(trig, LOW);

  duration = pulseIn(echo, HIGH);

  total = total - readings[readIndex];
  readings[readIndex] = duration;
  total = total + readings[readIndex];
  readIndex = readIndex + 1;
  average = total / numReadings;

  distance = average*0.034 / 2.00;

  if (readIndex >= numReadings) {
    readIndex = 0;
  }

  delay(10);
}