const int laserPin = 2;
int laser =0;
volatile byte counter = 0;
int COUNT1=0;
unsigned long elapsed;
unsigned long previous;
float velocity;

void Countfun()
{
  counter++;

   }

void setup() {

  Serial.begin (9600);
  previous = millis();

  pinMode (laserPin, INPUT);
}
void loop() {
  attachInterrupt(0, Countfun, RISING);
  COUNT1 =COUNT1+1;
  if (COUNT1==counter){
  Serial.println (counter);
  elapsed = millis() - previous;
  previous = millis();
  //Serial.println(elapsed);

  velocity = 30*1000.0/elapsed;
  velocity = velocity /1000.*3600.;
  Serial.print(velocity);
  Serial.println("  km/h");
  }
  COUNT1=counter;

  delay(100);
}