Measuring Wind Speed with an Arduino / Anemometer

A while back, we got a set of weather sensors from Sparkfun. It includes a Anemometer (speed), Wind Vane (direction), and a Rain Gauge.

I used a hand held wind speed device to calibrate my Arduino code.

I’m using pinMode(2, INPUT_PULLUP); so no pull up or pull down resistor is needed.

I used a prototype dual 6 pin phone jack from IC Breakout to connect the weather sensors to the Arduino.

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For this sketch, I connected the two wires from the anemometer to pins 2 (interrupt 0) and ground.

Code:

 // diameter of anemometer
 float radius= 2.75; //inches from center pin to middle of cup

 float diameter = radius * 2; //inches from center pin to middle of cup
 float mph;
 
 // read RPM
 int half_revolutions = 0;
 int rpm = 0;
 unsigned long lastmillis = 0;
 
 void setup(){
 Serial.begin(9600); 
 pinMode(2, INPUT_PULLUP); 
 attachInterrupt(0, rpm_fan, FALLING);
 }
 
 void loop(){
 if (millis() - lastmillis == 1000){ //Update every one second, this will be equal to reading frequency (Hz).
 detachInterrupt(0);//Disable interrupt when calculating
 rpm = half_revolutions * 30; // Convert frequency to RPM, note: 60 works for one interruption per full rotation. For two interrupts 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.print(half_revolutions/2); //print revolutions per second or Hz. And print new line or enter. divide by 2 if 2 interrupts per revolution
 half_revolutions = 0; // Restart the RPM counter
 lastmillis = millis(); // Update lastmillis
 attachInterrupt(0, rpm_fan, FALLING); //enable interrupt
 mph = diameter * 3.14 * rpm * 60 / 63360;
 //mph = mph * 3.5; // calibration factor for anemometer accuracy, adjust as necessary

 Serial.print("t MPH=t"); //print the word "MPH".
 Serial.println(mph);
  }
 }
 // this code will be executed every time the interrupt 0 (pin2) gets low.
 void rpm_fan(){
  half_revolutions++;
 }

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