volatile byte half_revolutions;
 unsigned int rpm;
 unsigned long timeold;
 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(0, magnet_detect, RISING);//Initialize the intterrupt pin (Arduino digital pin 2)
   half_revolutions = 0;
   rpm = 0;
   timeold = 0;
 }
 void loop()//Measure RPM
 {
   if (half_revolutions >= 20) { 
     rpm = 30*1000/(millis() - timeold)*half_revolutions;
     timeold = millis();
     half_revolutions = 0;
     
   }
   else if (half_revolutions <20){
    Serial.println("Not Detected");
   }
 }
 void magnet_detect()//This function is called whenever a magnet/interrupt is detected by the arduino
 {
   half_revolutions++;
   Serial.println("Detected");
 }