const int TRIG= PB3;
const int ECHO = PB4;

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT_PULLDOWN);
  attachInterrupt(ECHO, interrupt, CHANGE);
  Serial.begin(9600);
}

long time1 = 0;
long time2 = 0;
bool state = 0;

void loop() {
  if(micros() - time1 > 1000000) //1 másodpercenként uj mérést kezd
  {
    digitalWrite(TRIG,HIGH);
    digitalWrite(TRIG,LOW);
    time1 = micros();
  }
}

void interrupt() {
  if(state)
  {
    time2=micros();
  }
  else
  {
    Serial.println((float)(micros() - time2)/1000000.0 * 170.0);
  }
  state = !state;
}