//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 (" rpmrn");
}