Fix formula in getResistance(): 5.0 is unnecessary here

MQ135.cpp

@@ -20,12 +20,22 @@ v1.0 - First release
 /*!
 @brief  Default constructor
 
-@param[in] pin  The analog input pin for the readout of the sensor
+@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
 */
 /**************************************************************************/
 
-MQ135::MQ135(uint8_t pin) {
+MQ135::MQ135(uint8_t pin, float rload, float rzero, float atmoco2, float vref, float vc) {
   _pin = pin;
+  _rload = rload;
+  _rzero = rzero;
+  _atmoco2 = atmoco2;
+  _vref = vref;
+  _vc = vc;
 }
 
 
@@ -40,7 +50,12 @@ MQ135::MQ135(uint8_t pin) {
 */
 /**************************************************************************/
 float MQ135::getCorrectionFactor(float t, float h) {
-  return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
+  //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));
 }
 
 /**************************************************************************/
@@ -52,7 +67,13 @@ float MQ135::getCorrectionFactor(float t, float h) {
 /**************************************************************************/
 float MQ135::getResistance() {
   int val = analogRead(_pin);
-  return ((1023./(float)val) * 5. - 1.)*RLOAD;
+  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;
 }
 
 /**************************************************************************/
@@ -78,7 +99,7 @@ float MQ135::getCorrectedResistance(float t, float h) {
 */
 /**************************************************************************/
 float MQ135::getPPM() {
-  return PARA * pow((getResistance()/RZERO), -PARB);
+  return PARA * pow((getResistance()/_rzero), PARB);
 }
 
 /**************************************************************************/
@@ -93,7 +114,7 @@ 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);
 }
 
 /**************************************************************************/
@@ -104,7 +125,7 @@ float MQ135::getCorrectedPPM(float t, float h) {
 */
 /**************************************************************************/
 float MQ135::getRZero() {
-  return getResistance() * pow((ATMOCO2/PARA), (1./PARB));
+  return getResistance() * pow((_atmoco2/PARA), (1./-PARB));
 }
 
 /**************************************************************************/
@@ -119,5 +140,5 @@ 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

@@ -21,29 +21,52 @@ v1.0 - First release
  #include "WProgram.h"
 #endif
 
-/// The load resistance on the board
-#define RLOAD 10.0
-/// Calibration resistance at atmospheric CO2 level
-#define RZERO 76.63
+/// 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
 /// Parameters for calculating ppm of CO2 from sensor resistance
-#define PARA 116.6020682
-#define PARB 2.769034857
+//#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)
 
+/*
 /// 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
 
-/// Atmospheric CO2 level for calibration purposes
-#define ATMOCO2 397.13
+/// 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
 
 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);
+  MQ135(uint8_t pin, float rload = MQ135_RLOAD, float rzero = MQ135_RZERO,
+          float atmoco2 = MQ135_ATMOCO2, float vref = MQ135_VREF,
+          float vc = MQ135_VC);
   float getCorrectionFactor(float t, float h);
   float getResistance();
   float getCorrectedResistance(float t, float h);

README.md

@@ -1,4 +1,7 @@
 MQ135
 =====
 
-Arduino library for the 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).