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#define PIN_SENSE 2 //where we connected the fan sense pin. Must be an interrupt capable pin (2 or 3 on Arduino Uno)
#define DEBOUNCE 0 //0 is fine for most fans, crappy fans may require 10 or 20 to filter out noise
#define FANSTUCK_THRESHOLD 500 //if no interrupts were received for 500ms, consider the fan as stuck and report 0 RPM
//Interrupt handler. Stores the timestamps of the last 2 interrupts and handles debouncing
unsigned long volatile ts1=0,ts2=0;
void tachISR() {
    unsigned long m=millis();
    if((m-ts2)>DEBOUNCE){
        ts1=ts2;
        ts2=m;
    }
}
//Calculates the RPM based on the timestamps of the last 2 interrupts. Can be called at any time.
unsigned long calcRPM(){
    if(millis()-ts2<FANSTUCK_THRESHOLD&&ts2!=0){
        return (60000/(ts2-ts1))/2;
    }else return 0;
}
void setup(){
    pinMode(PIN_SENSE,INPUT_PULLUP); //set the sense pin as input with pullup resistor
    attachInterrupt(digitalPinToInterrupt(PIN_SENSE),tachISR,FALLING); //set tachISR to be triggered when the signal on the sense pin goes low
    Serial.begin(9600); //enable serial so we can see the RPM in the serial monitor
}
void loop(){
    delay(100);
    Serial.print("RPM:");
    Serial.println(calcRPM());
}