140 lines
4.4 KiB
C++
Executable File
140 lines
4.4 KiB
C++
Executable File
/**************************************************************************/
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/*!
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@file MQ135.cpp
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@author G.Krocker (Mad Frog Labs)
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@license GNU GPLv3
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First version of an Arduino Library for the MQ135 gas sensor
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TODO: Review the correction factor calculation. This currently relies on
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the datasheet but the information there seems to be wrong.
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@section HISTORY
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v1.0 - First release
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*/
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/**************************************************************************/
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#include "MQ135.h"
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/**************************************************************************/
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/*!
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@brief Default constructor
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@param[in] pin The analog input pin for the readout of the sensor
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@param[in] rload Load resistance on board, in KOhms
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@param[in] rzero Calibration sensor resistance at atmospheric CO2 level, in KOhms
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@param[in] atmoco2 Atmospheric CO2 level for calibration purposes, in PPM
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@param[in] vref analogRead() reference voltage, in volts
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@param[in] vc Test voltage applied to the A pins of MQ-135, in volts
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*/
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/**************************************************************************/
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MQ135::MQ135(uint8_t pin, float rload, float rzero, float atmoco2, float vref, float vc) {
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_pin = pin;
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_rload = rload;
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_rzero = rzero;
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_atmoco2 = atmoco2;
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_vref = vref;
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_vc = vc;
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}
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/**************************************************************************/
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/*!
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@brief Get the correction factor to correct for temperature and humidity
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@param[in] t The ambient air temperature
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@param[in] h The relative humidity
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@return The calculated correction factor
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*/
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/**************************************************************************/
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float MQ135::getCorrectionFactor(float t, float h) {
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return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
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}
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/**************************************************************************/
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/*!
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@brief Get the resistance of the sensor, ie. the measurement value
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@return The sensor resistance in kOhm
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*/
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/**************************************************************************/
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float MQ135::getResistance() {
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int val = analogRead(_pin);
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float r;
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//r = ((1023./(float)val) - 1.)*_rload;
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// or taking Vref and Vc into account:
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r = ((1023. * _rload * _vc) / ((float)val * _vref)) - _rload;
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return r;
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}
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/**************************************************************************/
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/*!
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@brief Get the resistance of the sensor, ie. the measurement value corrected
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for temp/hum
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@param[in] t The ambient air temperature
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@param[in] h The relative humidity
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@return The corrected sensor resistance kOhm
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*/
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/**************************************************************************/
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float MQ135::getCorrectedResistance(float t, float h) {
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return getResistance()/getCorrectionFactor(t, h);
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}
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/**************************************************************************/
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/*!
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@brief Get the ppm of CO2 sensed (assuming only CO2 in the air)
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@return The ppm of CO2 in the air
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*/
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/**************************************************************************/
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float MQ135::getPPM() {
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return PARA * pow((getResistance()/_rzero), -PARB);
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}
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/**************************************************************************/
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/*!
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@brief Get the ppm of CO2 sensed (assuming only CO2 in the air), corrected
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for temp/hum
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@param[in] t The ambient air temperature
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@param[in] h The relative humidity
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@return The ppm of CO2 in the air
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*/
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/**************************************************************************/
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float MQ135::getCorrectedPPM(float t, float h) {
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return PARA * pow((getCorrectedResistance(t, h)/_rzero), -PARB);
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}
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/**************************************************************************/
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/*!
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@brief Get the resistance RZero of the sensor for calibration purposes
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@return The sensor resistance RZero in kOhm
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*/
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/**************************************************************************/
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float MQ135::getRZero() {
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return getResistance() * pow((_atmoco2/PARA), (1./PARB));
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}
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/**************************************************************************/
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/*!
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@brief Get the corrected resistance RZero of the sensor for calibration
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purposes
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@param[in] t The ambient air temperature
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@param[in] h The relative humidity
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@return The corrected sensor resistance RZero in kOhm
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*/
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/**************************************************************************/
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float MQ135::getCorrectedRZero(float t, float h) {
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return getCorrectedResistance(t, h) * pow((_atmoco2/PARA), (1./PARB));
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}
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