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Reading Fan RPM

Connect the fan like this:

The hall effect sensor pin goes to pin 2, or interrupt 0. The LED and 10k resistor needed to be there for this to work. If your fan takes a lot of current, you might need to use an external power source.

Simple Circuit Option:
If the internal pull-up resistor for pin 2 (interrupt 0) is enabled by adding the line:
digitalWrite(2, HIGH);
to setup(), then the hall effect sensor can be connected directly to pin 2 without requiring an external 10k pull-up resistor and LED. The LED is actually only serving as a visual indicator in the original circuit and is not mandatory in for either.

Ze code:


 volatile byte half_revolutions;

 unsigned int rpm;

 unsigned long timeold;

 void setup()
   attachInterrupt(0, rpm_fun, RISING);

   half_revolutions = 0;
   rpm = 0;
   timeold = 0;

 void loop()
   if (half_revolutions >= 20) { 
     //Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     rpm = 30*1000/(millis() - timeold)*half_revolutions;
     timeold = millis();
     half_revolutions = 0;

 void rpm_fun()
   //Each rotation, this interrupt function is run twice


Plot of RPM over time:

(right click -> "view image" to see the larger picture)

by zitron

Edited by Elimelec Lopez - April 25th 2013

 // read RPM

 int half_revolutions = 0;

 int rpm = 0;

 unsigned long lastmillis = 0;

 void setup(){
 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

 rpm = half_revolutions * 60; // Convert frecuency to RPM, note: this works for one interruption per full rotation. For two interrups per full rotation use half_revolutions * 30.

 Serial.print("RPM =\t"); //print the word "RPM" and tab.
 Serial.print(rpm); // print the rpm value.
 Serial.print("\t Hz=\t"); //print the word "Hz".
 Serial.println(half_revolutions); //print revolutions per second or Hz. And print new line or enter.

 half_revolutions = 0; // Restart the RPM counter
 lastmillis = millis(); // Uptade lasmillis
 attachInterrupt(0, rpm_fan, FALLING); //enable interrupt

 // this code will be executed every time the interrupt 0 (pin2) gets low.

 void rpm_fan(){