MQ135.cpp

@@ -20,22 +20,12 @@ v1.0 - First release
 /*!
 @brief  Default constructor
 
-@param[in] pin      The analog input pin for the readout of the sensor
-@param[in] rload    Load resistance on board, in KOhms
-@param[in] rzero    Calibration sensor resistance at atmospheric CO2 level, in KOhms
-@param[in] atmoco2  Atmospheric CO2 level for calibration purposes, in PPM
-@param[in] vref     analogRead() reference voltage, in volts
-@param[in] vc       Test voltage applied to the A pins of MQ-135, in volts
+@param[in] pin  The analog input pin for the readout of the sensor
 */
 /**************************************************************************/
 
-MQ135::MQ135(uint8_t pin, float rload, float rzero, float atmoco2, float vref, float vc) {
+MQ135::MQ135(uint8_t pin) {
   _pin = pin;
-  _rload = rload;
-  _rzero = rzero;
-  _atmoco2 = atmoco2;
-  _vref = vref;
-  _vc = vc;
 }
 
 
@@ -50,12 +40,7 @@ MQ135::MQ135(uint8_t pin, float rload, float rzero, float atmoco2, float vref, f
 */
 /**************************************************************************/
 float MQ135::getCorrectionFactor(float t, float h) {
-  //return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
-
-  // This formula is reduced from temperature and humidity dependency graph,
-  // found in this datasheet:
-  // http://china-total.com/Product/meter/gas-sensor/MQ135.pdf
-  return (1.30732 - 0.0116044 * t) * (2.20591 - 0.296456 * log(h));
+  return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
 }
 
 /**************************************************************************/
@@ -67,13 +52,7 @@ float MQ135::getCorrectionFactor(float t, float h) {
 /**************************************************************************/
 float MQ135::getResistance() {
   int val = analogRead(_pin);
-  float r;
-
-  //r = ((1023./(float)val) - 1.)*_rload;
-  // or taking Vref and Vc into account:
-  r = ((1023. * _rload * _vc) / ((float)val * _vref)) - _rload;
-
-  return r;
+  return ((1023./(float)val) - 1.)*RLOAD;
 }
 
 /**************************************************************************/
@@ -99,7 +78,7 @@ float MQ135::getCorrectedResistance(float t, float h) {
 */
 /**************************************************************************/
 float MQ135::getPPM() {
-  return PARA * pow((getResistance()/_rzero), PARB);
+  return PARA * pow((getResistance()/RZERO), -PARB);
 }
 
 /**************************************************************************/
@@ -114,7 +93,7 @@ float MQ135::getPPM() {
 */
 /**************************************************************************/
 float MQ135::getCorrectedPPM(float t, float h) {
-  return PARA * pow((getCorrectedResistance(t, h)/_rzero), PARB);
+  return PARA * pow((getCorrectedResistance(t, h)/RZERO), -PARB);
 }
 
 /**************************************************************************/
@@ -125,7 +104,7 @@ float MQ135::getCorrectedPPM(float t, float h) {
 */
 /**************************************************************************/
 float MQ135::getRZero() {
-  return getResistance() * pow((_atmoco2/PARA), (1./-PARB));
+  return getResistance() * pow((ATMOCO2/PARA), (1./PARB));
 }
 
 /**************************************************************************/
@@ -140,5 +119,5 @@ float MQ135::getRZero() {
 */
 /**************************************************************************/
 float MQ135::getCorrectedRZero(float t, float h) {
-  return getCorrectedResistance(t, h) * pow((_atmoco2/PARA), (1./-PARB));
+  return getCorrectedResistance(t, h) * pow((ATMOCO2/PARA), (1./PARB));
 }

MQ135.h

@@ -21,52 +21,29 @@ v1.0 - First release
  #include "WProgram.h"
 #endif
 
-/// The load resistance on the board, in KOhms
-#define MQ135_RLOAD 10.0
-/// Calibration sensor resistance at atmospheric CO2 level, in KOhms
-#define MQ135_RZERO 76.63
+/// The load resistance on the board
+#define RLOAD 10.0
+/// Calibration resistance at atmospheric CO2 level
+#define RZERO 76.63
 /// Parameters for calculating ppm of CO2 from sensor resistance
-//#define PARA 116.6020682
-//#define PARB (-2.769034857)
-/// Correlation parameters from Davide Gironi <http://davidegironi.blogspot.ru/2014/01/cheap-co2-meter-using-mq135-sensor-with.html>
-#define PARA 56.0820
-#define PARB (-5.9603)
+#define PARA 116.6020682
+#define PARB 2.769034857
 
-/*
 /// Parameters to model temperature and humidity dependence
 #define CORA 0.00035
 #define CORB 0.02718
 #define CORC 1.39538
 #define CORD 0.0018
-*/
 
-/// Atmospheric CO2 level for calibration purposes, in PPM.
-/// See http://co2now.org/
-#define MQ135_ATMOCO2 397.16
-
-/// Test voltage applied to the A pins of MQ-135, in volts. Usually, it's equal to power supply voltage.
-#define MQ135_VC  5.0
-/// analogRead reference voltage, in volts. By default it's a power supply voltage on Arduino. Can be changed with analogReference().
-#define MQ135_VREF 5.0
+/// Atmospheric CO2 level for calibration purposes
+#define ATMOCO2 397.13
 
 class MQ135 {
  private:
   uint8_t _pin;
-  // Calibration sensor resistance at atmospheric CO2 level, in KOhms
-  float _rzero;
-  // Atmospheric CO2 level for calibration purposes, in PPM, see http://co2now.org/
-  float _atmoco2;
-  // analogRead reference voltage, in volts
-  float _vref;
-  // Load resistance on board, in KOhms
-  float _rload;
-  // Test voltage applied to the A pins of MQ-135, in volts
-  float _vc;
 
  public:
-  MQ135(uint8_t pin, float rload = MQ135_RLOAD, float rzero = MQ135_RZERO,
-          float atmoco2 = MQ135_ATMOCO2, float vref = MQ135_VREF,
-          float vc = MQ135_VC);
+  MQ135(uint8_t pin);
   float getCorrectionFactor(float t, float h);
   float getResistance();
   float getCorrectedResistance(float t, float h);

README.md

@@ -1,7 +1,4 @@
 MQ135
 =====
 
-Arduino library for the MQ135 Air Quality sensor. Read about using this library
-[here](https://hackaday.io/project/3475-sniffing-trinket/log/12363-mq135-arduino-library).
-A project using this library: [Sniffing
-Trinket](https://hackaday.io/project/3475-sniffing-trinket).
+Arduino library for the MQ135