File content as of revision 0:a43405a3d273:

/**************************************************************************/
/*!
@file     MQ135.cpp
@author   G.Krocker (Mad Frog Labs)
@license  GNU GPLv3

First version of an Arduino Library for the MQ135 gas sensor
TODO: Review the correction factor calculation. This currently relies on
the datasheet but the information there seems to be wrong.

@section  HISTORY

v1.0 - First release
*/
/**************************************************************************/

#include "MQ135.h"

/**************************************************************************/
/*!
@brief  Default constructor

@param[in] pin  The analog input pin for the readout of the sensor
*/
/**************************************************************************/
MQ135::MQ135(PinName pin) {
    _pin = pin;
}

MQ135::~MQ135() {
}

/**************************************************************************/
/*!
@brief  Get the correction factor to correct for temperature and humidity

@param[in] t  The ambient air temperature
@param[in] h  The relative humidity

@return The calculated correction factor
*/
/**************************************************************************/
float MQ135::getCorrectionFactor(float t, float h) {
  return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
}

/**************************************************************************/
/*!
@brief  Get the resistance of the sensor, ie. the measurement value

@return The sensor resistance in kOhm
*/
/**************************************************************************/
float MQ135::getResistance() {
//  AnalogIn ain(_pin);
  AnalogIn ain(PA_0);
  int val = ain.read();
  return ((1023./(float)val) * 5. - 1.)*RLOAD;
}

/**************************************************************************/
/*!
@brief  Get the resistance of the sensor, ie. the measurement value corrected
        for temp/hum

@param[in] t  The ambient air temperature
@param[in] h  The relative humidity

@return The corrected sensor resistance kOhm
*/
/**************************************************************************/
float MQ135::getCorrectedResistance(float t, float h) {
  return getResistance()/getCorrectionFactor(t, h);
}

/**************************************************************************/
/*!
@brief  Get the ppm of CO2 sensed (assuming only CO2 in the air)

@return The ppm of CO2 in the air
*/
/**************************************************************************/
float MQ135::getPPM() {
  return PARA * pow((getResistance()/RZERO), -PARB);
}

/**************************************************************************/
/*!
@brief  Get the ppm of CO2 sensed (assuming only CO2 in the air), corrected
        for temp/hum

@param[in] t  The ambient air temperature
@param[in] h  The relative humidity

@return The ppm of CO2 in the air
*/
/**************************************************************************/
float MQ135::getCorrectedPPM(float t, float h) {
  return PARA * pow((getCorrectedResistance(t, h)/RZERO), -PARB);
}

/**************************************************************************/
/*!
@brief  Get the resistance RZero of the sensor for calibration purposes

@return The sensor resistance RZero in kOhm
*/
/**************************************************************************/
float MQ135::getRZero() {
  return getResistance() * pow((ATMOCO2/PARA), (1./PARB));
}

/**************************************************************************/
/*!
@brief  Get the corrected resistance RZero of the sensor for calibration
        purposes

@param[in] t  The ambient air temperature
@param[in] h  The relative humidity

@return The corrected sensor resistance RZero in kOhm
*/
/**************************************************************************/
float MQ135::getCorrectedRZero(float t, float h) {
  return getCorrectedResistance(t, h) * pow((ATMOCO2/PARA), (1./PARB));
}