float rev = 0;
int time;
int old_time = 0;
int rpm;

void isr(){
  rev++;
}

void setup() {
  Serial.begin(9600);
  attachInterrupt(1, isr, RISING);    //  0 for 2 no. pin // 1 for 3 no. pin
}

void loop() {
  delay(1000);
  detachInterrupt(0);
  time = millis() - old_time;
  rpm = (rev/time) * 60000/3;
  old_time = millis();
  rev = 0;

  Serial.print("RPM: ");
  Serial.println(rpm);
  attachInterrupt(1, isr, RISING);

}