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I wanted to be able to control the speed of the fans in my big NAS, Fred, individually. Even though the mainboard in use has five PWM fan connectors, the chipset can only control the speed of all fans together. There are probably good fan controllers commercially available that solve this problem better than I did. But they seemed overpriced and it seemed like a fun learning project for me.

The fan controller that I made uses an Arduino Nano clone that listens to it's serial port, waiting for a command to change the speed of a fan. When a command is recognised the continuous PWM signal for that fan is changed accordingly. It's possible to control up to six fans this way with an Arduino Nano. I'm only using three though since I only have three fan groups that need to be controlled separately.

The Arduino sketch/C code for the Arduino Nano that I used is as follows.

//fan speed sensor wire attached to digital pin 2 with a 10kohm pullup resistor
//fan PWM control wire attached directly to digital pin 9

#include <PWM.h> //include PWM library http://forum.arduino.cc/index.php?topic=117425.0

volatile int half_revolutions1; //allow half_revolutioins to be accesed in intterupt
volatile int half_revolutions2; //allow half_revolutioins to be accesed in intterupt
int rpm1; //set rpm as an integer
int rpm2; //set rpm as an integer
int pwm=255;
const byte numChars = 5;
char receivedChars[numChars];

boolean newData = false;

void setup()
{
  InitTimersSafe(); //not sure what this is for, but I think i need it for PWM control?
  bool success = SetPinFrequencySafe(9, 25000); //set frequency to 25kHz
  pwmWrite(9, 51); // 51=20% duty cycle, 255=100% duty cycle

  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  analogWrite(5, 170);
  analogWrite(6, 255);
  pinMode(2,INPUT_PULLUP); //set RPM pin to digital input
  pinMode(3,INPUT_PULLUP); //set RPM pin to digital input
  half_revolutions1 = 0;
  rpm1 = 0;
  half_revolutions2 = 0;
  rpm2 = 0;

  Serial.begin(9600);
}



void loop()
{
  sei(); //enable intterupts
  attachInterrupt(0, fan_rpm1, RISING); //record pulses as they rise
  delay(1000);
  detachInterrupt(0);
  attachInterrupt(1, fan_rpm2, RISING); //record pulses as they rise
  delay(1000);
  detachInterrupt(1);
  cli(); //disable intterupts

  rpm1 = (half_revolutions1/2)*60;

  Serial.print("1");
  Serial.println(rpm1);

  rpm2 = (half_revolutions2/2)*60;

  Serial.print("2");
  Serial.println(rpm2);

  rpm1 = 0;
  half_revolutions1 = 0;

  rpm2 = 0;
  half_revolutions2 = 0;

  pwm = 255;
  recvWithStartEndMarkers();
  processCommand();
}

void fan_rpm1()
{
  ++half_revolutions1; //increment before returning value
}


void fan_rpm2()
{
  ++half_revolutions2; //increment before returning value
}


void recvWithStartEndMarkers() {
    static boolean recvInProgress = false;
    static byte ndx = 0;
    char startMarker = 's';
    char endMarker = '\n';
    char rc;
 
    while (Serial.available() > 0 && newData == false) {
        rc = Serial.read();

        if (recvInProgress == true) {
            if (rc != endMarker) {
                receivedChars[ndx] = rc;
                ndx++;
                if (ndx >= numChars) {
                    ndx = numChars - 1;
                }
            }
            else {
                receivedChars[ndx] = '\0'; // terminate the string
                recvInProgress = false;
                ndx = 0;
                newData = true;
            }
        }

        else if (rc == startMarker) {
            recvInProgress = true;
        }
    }
}

void processCommand() {
    if (newData == true) {
        Serial.print("s");
        Serial.println(receivedChars);
        switch (receivedChars[0])
        {
            case '1':
                receivedChars[0] = '0';
                sscanf(receivedChars, "%d", &pwm);
                analogWrite(5, pwm);
                break;
            case '2':
                receivedChars[0] = '0';
                sscanf(receivedChars, "%d", &pwm);
                analogWrite(6, pwm);
                break;
            case '3':
                receivedChars[0] = '0';
                sscanf(receivedChars, "%d", &pwm);
                
                break;
//            default:
//                Serial.println("I don't know what that means.");
        }
        newData = false;
    }
}

Well, how should I put it? It works, usually.

(tbc?)

(tba:photos)