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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.


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

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Michael Mesloh
Michael Mesloh
3 years ago

Hello Steve Spence , I am concerned about the value 63360. Where does it come from? Is this a costant for the anemometer? Can you explain this value?
I know that for the anemometer shown in your picture above, a wind speed of 1.492 MPH (2.4 km/h) causes the switch to close once per second.

Brian Jenkins
Brian Jenkins
2 years ago
Reply to  Michael Mesloh

Hey there! Steve no longer maintains this blog. As far as your question goes, he wrote the code, but maybe when I get a chance I’ll take a peek at it…

martin creek
7 years ago

I think the first line of code incorrect. From centre pin to middle of the cup would be radius. Diameter would be twice that eg float diameter should be 5.5 inches.

Indrajeet hujare
5 years ago

Is it a perfectly working code ?

Steve Spence
5 years ago

assuming diameter is 5.5", would the following be correct?

rpm * 60 * diam * 3.14 / 12 / 5280

Steve Spence
5 years ago

mph = diam * 3.14 * rpm * 60 / 63360 should be correct. I will correct the code.


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