Arduino CO2 Sensor Controls PCM Fans in HRV

[AC_Hacker]

I've been doing more experimenting with the 'stalled fan' issue mentioned previously.

My current solution is to add an integer number to the PWM_Value, this way when the CO2 level drops, as when I'm away from the house, the fans will be kept just barely turning. In the unusual event that a gust of wind happened to be exactly enough to stop the fan, I wanted the fan to be able to restart, at a very low speed, even when CO2 values were at their minimum, which means that I need a PCM_value of 92 minimum, which translates to a minimum duty cycle of 36%.

The range of CO2 ppm that is conveyed by the sensor's analog value is:
0 volt = 0 ppm
4 volt = 2000 ppm

... and this will yield the following PWM_Value in my Teensy:
0 ppm = 0 PWM_Value
2000 ppm = 256 PWM_Value

So, I set up the ratio:

(300 ppm) / (2000 ppm) = (x PWM_Value) / (256 PWM_Value)

Therefore:

(x PWM_Value) = ((300 ppm) * (256 PWM_Value)) / (2000 ppm)

So: x PWM_Value = 53.6 (when ppm is 300)


But the fans have a minimum start-up threshold of 90 PCM_value, so:

90 - 38.4 = 53.6 (rounding up to 54)

I'll 'pad' my PCM_value by adding 54, as in the code below:

int CO2_ReadPin = 38;                    // initialize pin 38 for analog voltage in
int PWM_WritePin = 14;                   // initialize pin 14 for PWM
int CO2_Value;
int PWM_Value;
int DutyCycle;
int ppm;
 
void setup()   {                         // BEGIN SETUP
  pinMode(PWM_WritePin, OUTPUT);         // Make PWM_pin (AKA: pin 14) an output pin
  int CO2_Value = 0;                     // variable set to zero
  int PWM_Value = 0;                     // variable set to zero
  int DutyCycle = 0;                     // variable set to zero  
  int ppm = 0;                           // variable set to zero 
  Serial.begin(38400);                   // Initialize Serial Monitor
}                                        // END SETUP

void loop()               
{                                        // BEGIN LOOP
  CO2_Value = analogRead(CO2_ReadPin);   // read Teensy input pin 38
  unsigned int ppm = ((unsigned long)analogRead(CO2_ReadPin) * 2500)/1024;  // calc ppm
  Serial.print("CO2 level     = ");      // print ppm to serial monitor
  Serial.print(ppm); 
  Serial.println(" ppm");
  PWM_Value = CO2_Value/4;               // Scale CO2_Value (range = 1024) to PWM_Value (range = 256)
  analogWrite(PWM_WritePin, (PWM_Value + 54));  // Write PWM_Value to PWM_WritePin
  Serial.print("PWM_Value is  = ");      // print val to serial monitor
  Serial.println(PWM_Value + 54);
  DutyCycle = (100 * (PWM_Value + 54) / 256);   // Calculate DutyCycle
  Serial.print("PWM DutyCycle = ");      // Print DutyCycle to serial monitor
  Serial.print(DutyCycle);
  Serial.println("%");

  Serial.println();                      // print null line to serial monitor

  delay(1000);			         // delay 1 second before repeating loop
}                                        // END LOOP

So far, I have tested it out for several days, and no problem.

Best,

-AC