.ino

// read RPM and calculate average every then readings.
const int numreadings = 10;
int readings[numreadings];
unsigned long average = 0;
int index = 0;
unsigned long total; 

volatile int rpmcount = 0;//see http://arduino.cc/en/Reference/Volatile 
unsigned long rpm = 0;
unsigned long lastmillis = 0;

void setup(){
 Serial.begin(9600); 
 attachInterrupt(0, rpm_fan, FALLING);
}

void loop(){
 
  
 if (millis() - lastmillis >= 1000){  /*Uptade every one second, this will be equal to reading frecuency (Hz).*/
 
 detachInterrupt(0);    //Disable interrupt when calculating
 total = 0;  
 readings[index] = rpmcount * 60;  /* Convert frecuency to RPM, note: this works for one interruption per full rotation. For two interrups per full rotation use rpmcount * 30.*/
 
 for (int x=0; x<=9; x++){
   total = total + readings[x];
 }
 
 average = total / numreadings;
 rpm = average;
 
 rpmcount = 0; // Restart the RPM counter
 index++;
 if(index >= numreadings){
  index=0; 
 } 
 
 
if (millis() > 11000){  // wait for RPMs average to get stable

 Serial.print(" RPM = ");
 Serial.println(rpm);
}
 
 lastmillis = millis(); // Uptade lasmillis
  attachInterrupt(0, rpm_fan, FALLING); //enable interrupt
  }
}


void rpm_fan(){ /* this code will be executed every time the interrupt 0 (pin2) gets low.*/
  rpmcount++;
}