unsigned int t,revCount,rpm;
 
 void setup()
 {
    pinMode(3, INPUT_PULLUP);
    Serial.begin(9600);
 }

 void loop()
 {
    
    if(digitalRead(3)==0)
      revCount++;
    
    if(t>=1000)
    {
       t=0;
       rpm = revCount*60;

       Serial.println(rpm);
       revCount=0;
    }
    
    delay(1);
    t++;
 }