MQ-135 Air Quality Sensor

#define VERSION "MQ135_Air_Quality_2025_0916"

/*
   IMPORTANT - read project description at
   https://github.com/Bobbo117/MQ135-Air-Quality-Sensor

  This software incorporates the MQ135 library to do perform two functions:
  1. When the #define CALIBRATE statement is active, calibarate the MQ135 sensor 
  2. When the #define CALIBRATE statement is inactive, read the MQ135 sensor and report the CO2 ppm data on the following platforms:
  
    computer monitor via USB connection
    OLED display via I2C connection

*/    
//////////////////////////////////////////////////////////////  
//*******         Compile-time Options           ***********//
//        (Disable unwanted options with leading //)
//////////////////////////////////////////////////////////////

#define CALIBRATE_         // Comment out if not in calibration mode

//////////////////////////////////////////////////////////////  
//*******         Compile-time Parameters           ***********//
// IMPORTANT - read project description at
// https://github.com/Bobbo117/MQ135-Air-Quality-Sensor
//////////////////////////////////////////////////////////////

#define PIN_MQ135 ADC3    // ADC3 pin on the CubeCell Plus, which connects to the analog output of the sesor. 
#define RZERO 310.0       // Measured resistance from calibration run
#define RLOAD 74.2        // Measured resistance Kohms pin module A0 to GND with power off

//////////////////////////////////////////////////////////////  
//*******         Libraries          ***********//
// Use the Arduino Library Manager to install libraries for selected compile-time options 
// 
//////////////////////////////////////////////////////////////

//***Sensor library
#include "MQ135.h"        // <-- from Arduino Library Manager
MQ135 gasSensor(PIN_MQ135, RZERO, RLOAD);

#include <HT_SH1107Wire.h>      // CubeCell Plus display library
SH1107Wire  boardDisplay(0x3c, 500000, SDA, SCL, GEOMETRY_128_64, GPIO10); // addr, freq, sda, scl, resolution, rst

//***Variable Definitions

  unsigned long reportPeriod_msec = 15000;        // Report period in msec
  float ppmArray[8];                             // array used to store and calculate rolling average of ppm data
  int ppmArrayLength = 8;                        // array length 
  int validArrayData = 0;                        // number of entries in the array which are valid data for use in rolling average
  
//   ***********************
//   **  setup()  **
//   ***********************

void setup() {
  Serial.begin(115200);
  ppmArray[0]=1;                                      // ppmArray[0] is index pointing to first data location

  pinMode(Vext, OUTPUT);    // Sensor power
  digitalWrite(Vext, LOW);  // Activate Vext (we're really writing to Vext_Ctrl here)
  pinMode(ADC3, INPUT);     // MQ-135 anlaog input

  boardDisplay.init();
  boardDisplay.setFont(ArialMT_Plain_10);
  boardDisplay.screenRotate(ANGLE_180_DEGREE);
    
  boardDisplay.clear();
  boardDisplay.setFont(ArialMT_Plain_10);
  boardDisplay.setTextAlignment(TEXT_ALIGN_LEFT);
  boardDisplay.drawString(5,0,"MQ-135 Sensor");
  boardDisplay.display();
  delay(1000);
}
 
//   ***********************
//   **  loop()  **
//   ***********************

void loop() {            
  //***print to Serial Monitor
  #ifdef CALIBRATE_
    //****Read sensor resistance
    float value = gasSensor.getRZero();
    Serial.print("R Zero Ohms: ");  
    Serial.println(value);
  #else
     //****Read air quality sensor
    float value = gasSensor.getPPM();
    Serial.print("Air Quality: "); 
    Serial.println(value);
  #endif

  //***Store in array
  ppmArray[int(ppmArray[0])]=value;
  //Serial.print(ppmArray[0]); Serial.print(": ");Serial.println(ppmArray[int(ppmArray[0])]);
  ppmArray[0]++;
  if (int(ppmArray[0]) > ppmArrayLength-1) {
    ppmArray[0]=1;
  }  
  if (validArrayData < ppmArrayLength-1) {
    validArrayData ++;
  }
  
  //***Compute rolling average
  float total = 0;
  for (int i=1; i< validArrayData+1; i++) {
    total = total + ppmArray[i];   
  }
  float avg = total/(validArrayData);   
  Serial.print("Rolling Average ");Serial.print(validArrayData); Serial.print(": ");Serial.println(avg);
  
  //****Display AQI value on OLED
  boardDisplay.clear();
  boardDisplay.setFont(ArialMT_Plain_10);
  boardDisplay.setTextAlignment(TEXT_ALIGN_LEFT);
  #ifdef CALIBRATE_
    boardDisplay.drawString(5,10,"Calibration");

    boardDisplay.setTextAlignment(TEXT_ALIGN_RIGHT);
    boardDisplay.drawString(45,20,"R0");
    boardDisplay.drawString(45,30,"Count");
    boardDisplay.drawString(45,40,"Average");

    boardDisplay.setTextAlignment(TEXT_ALIGN_LEFT);
    boardDisplay.drawString(55,20,String(value) + "kOhms");
    boardDisplay.drawString(55,30,String(validArrayData));
    boardDisplay.drawString(55,40,String(avg) + "kOhms");
  #else
    boardDisplay.drawString(5,10,"Air Quality Index");

    boardDisplay.setTextAlignment(TEXT_ALIGN_RIGHT);
    boardDisplay.drawString(45,20,"R0");
    boardDisplay.drawString(45,30,"Count");
    boardDisplay.drawString(45,40,"Average");

    boardDisplay.setTextAlignment(TEXT_ALIGN_LEFT);
    boardDisplay.drawString(55,20,String(int(value)) + "ppm");
    boardDisplay.drawString(55,30,String(validArrayData));
    boardDisplay.drawString(55,40,String(int(avg)) + "ppm");
  #endif   
  boardDisplay.display();
  
  //****Pause to throttle data rate.
  Serial.print(reportPeriod_msec/1000);Serial.println(" second pause..");Serial.println("");
  delay(reportPeriod_msec);
}