Arduino Fan Control

fan_control.ino

#include <math.h>

/* PWM fan control for arduino */

const int rlyPin = 3; // digital 3 (optional relay)
const int pwmPin = 4; // digital 4 (connected to fan pwm pin)
const int ctrPin = A5; // analog 5 (connected to potentiometer measurement pin)
const int tempPin = A6; // analog 6 (connceted to temperature probe)
const int runCycles = 10000; // check new value every 10k cycles (2/sec)
const uint32_t cycleDuration = 50; // 50us for 20kHz

int rlyStatus = 0; // relay is off at start
uint32_t ctrPinVal = 0; // control pin value
uint32_t tempPinVal = 0; // temperature pin value
uint32_t cycleHighDuration = 0; // duration of the high phase of pwm cycle

double celcius(int RawADC) {
  double Temp;
  Temp = log(10000.0 * ((1024.0 / RawADC - 1))); 
  Temp = 1 / (0.001129148 + (0.000234125 + (0.0000000876741 * Temp * Temp)) * Temp);
  Temp = Temp - 273.15;            // Convert Kelvin to Celcius
   //Temp = (Temp * 9.0)/ 5.0 + 32.0; // Convert Celcius to Fahrenheit
   return Temp;
}

void setup() {
  pinMode(pwmPin, OUTPUT);
  pinMode(rlyPin, OUTPUT);
  pinMode(ctrPin, INPUT);
  //pinMode(tempPin, INPUT);
  //digitalWrite(rlyPin, HIGH);
  //Serial.begin(9600);
}

void loop() {
  // Measurement Cycle
  // check the value of the control pin to compute
  // to compute the cycle high phase duration
  {
    ctrPinVal = analogRead(ctrPin);
    cycleHighDuration = cycleDuration * ctrPinVal / 1024;
//    tempPinVal = analogRead(tempPin);
//    Serial.println(celcius(tempPinVal));
//    Serial.println(cycleHighDuration);
//    delay(1000);
  }

  if (cycleHighDuration < 2)
  {
    if (rlyStatus == 1)
    {
      // switch the relay to open state
      // if the potentiometer value is very low
      digitalWrite(rlyPin, LOW);
      rlyStatus = 0;
    }
  }
  else if (rlyStatus == 0)
  {
    digitalWrite(rlyPin, HIGH);
    rlyStatus = 1;
  }

  // Run Control Cycles
  if (rlyStatus == 1)
  {
    for (int i = 0; i < runCycles; i++)
    {
      // PWM high phase
      digitalWrite(pwmPin, HIGH);
      delayMicroseconds(cycleHighDuration);
      // PWM low phase  
      digitalWrite(pwmPin, LOW);
      delayMicroseconds(cycleDuration - cycleHighDuration);
    }
  }
  else
  {
    // wait if relay is open (fan off)
    delayMicroseconds(runCycles * cycleDuration);
  }
}