//project done by www.theorycircuit.com
//code by Crenn from http://thebestcasescenario.com  thank you!
//Varibles used for calculations
int NbTopsFan; int Calc;
 
//The pin location of the sensor
int hallsensor = 2; typedef struct{
 
//Defines the structure for multiple fans and 
//their dividers 
char fantype;
unsigned int fandiv; }fanspec;
 
//Definitions of the fans
//This is the varible used to select the fan and it's divider,
//set 1 for unipole hall effect sensor
//and 2 for bipole hall effect sensor
fanspec fanspace[3]={{0,1},{1,2},{2,8}}; char fan = 1;
 
void rpm ()
//This is the function that the interupt calls
{ NbTopsFan++; }
 
//This is the setup function where the serial port is initialised,
//and the interrupt is attached
void setup()
{ pinMode(hallsensor, INPUT);
Serial.begin(9600);
attachInterrupt(0, rpm, RISING); }
 
void loop ()
//Set NbTops to 0 ready for calculations
{ NbTopsFan = 0;
 
//Enables interrupts
sei();
 
//Wait 1 second
delay (1000);
 
//Disable interrupts
cli();
 
//Times NbTopsFan (which is apprioxiamately the fequency the fan 
//is spinning at) by 60 seconds before dividing by the fan's divider
Calc = ((NbTopsFan * 60)/fanspace[fan].fandiv);
 
//Prints the number calculated above
Serial.print (Calc, DEC);
 
//Prints " rpm" and a new line
Serial.print (" rpm\r\n");
}