reading rpm signal from cpu fan

Ok I connected the fans ground to one of the arduinos ground pins and I am still getting noting but random rmp numbers. Did I connect the resistor wrong? Does the sketch look right?

This is how I have it wired currently.

680×554 118 KB

and the code

volatile byte NbTopsFan;
int hallsensor = 2;

void rpm()
{
 NbTopsFan++;
}


/***************************************/
void setup()
{
// pinMode(hallsensor, INPUT);
 Serial.begin(9600);
 attachInterrupt(0, rpm, RISING);
};

void loop ()
{
   NbTopsFan = 0;
   
   delay (1000);
   NbTopsFan = NbTopsFan * 30;
   Serial.print ("     ");
   Serial.print (NbTopsFan, DEC);
   Serial.print (" rpm");

   
};