MQ-135 gas sensor, typically used for detecting Carbon Dioxide (CO2) PPM. More...

#include <MQ135.h>

Inheritance diagram for RumPi::MQ135:

Collaboration diagram for RumPi::MQ135:

Public Member Functions
	MQ135 ()
	Builds a default MQ-135 (carbon dioxide) sensor and initializes its datasheet values. More...

	MQ135 (IAnalogDigitalConverterComponent *analogToDigitalConverter, const unsigned int &analogToDigitalConverterIndex, const unsigned int &digitalOutputPinNumber)
	Builds an MQ-135 that reads through an ADC and also exposes a digital threshold pin. More...

	MQ135 (IAnalogDigitalConverterComponent *analogToDigitalConverter, const unsigned int &analogToDigitalConverterIndex)
	Builds an analog-only MQ-135 that reads through an ADC. More...

	MQ135 (const unsigned int &digitalOutputPinNumber)
	Builds a digital-only MQ-135 that reports only its threshold-output pin. More...

virtual	~MQ135 ()
	Destroys the sensor, turning it off first. More...

void	Initialize () override
	Initializes the MQ-135's datasheet values, its default temperature/humidity, and the pre-computed atmospheric CO2 constant used to correlate resistance to PPM. More...

double	ReadGasValue () const override
	Reads the current CO2 PPM for the sensor's raw reading (assuming only CO2 in the air). More...

double	CalculateR0 () override
	Gets the resistance R0 (RZero) of the sensor for calibration purposes. More...

double	CalculateResistance () const
	Gets the resistance of the sensor (its measurement value). More...

double	CalculatePPM () const
	Gets the PPM of CO2 sensed (assuming only CO2 in the air), from the sensor's resistance ratio. More...

double	CalculateCorrectedR0 (double temperatureInC, double humidityPercent)
	Gets the corrected resistance R0 of the sensor for calibration purposes, adjusted for temperature and humidity. More...

double	CalculateCorrectionFactor (double temperatureInC, double humidityPercent) const
	Gets the correction factor used to compensate the sensor's resistance for temperature and humidity. More...

double	CalculateCorrectedResistance (double temperatureInC, double humidityPercent) const
	Gets the sensor's resistance with the measurement value corrected for temperature and humidity. More...

double	CalculateCorrectedPPM (double temperatureInC, double humidityPercent) const
	Gets the PPM of CO2 sensed (assuming only CO2 in the air), corrected for temperature and humidity. More...

std::string	ToString () const override
	Serializes the MQ-135's gas name, raw reading, raw and corrected PPM, temperature, and humidity to a string. More...

double	GetCurrentTemperatureInC () const
	Gets the temperature (in Celsius) currently used to correct the sensor's readings. More...

double	GetCurrentHumidityPercent () const
	Gets the relative humidity (percent) currently used to correct the sensor's readings. More...

void	SetCurrentTemperatureInC (double currentTemperatureInC)
	Sets the temperature (in Celsius) used to correct the sensor's readings. More...

void	SetCurrentHumidityPercent (double currentHumidityPercent)
	Sets the relative humidity (percent) used to correct the sensor's readings. More...

Public Member Functions inherited from RumPi::GasSensor
virtual void	CalculateInitialR0 ()
	Calibrates the sensor: computes and stores its clean-air resistance (R0). More...

	GasSensor ()
	Builds a default gas sensor with zeroed calibration values and no ADC or digital pin. More...

	GasSensor (IAnalogDigitalConverterComponent *analogToDigitalConverter, const EComponentType &gasSensorType, const unsigned int &analogToDigitalConverterIndex, const unsigned int &digitalOutputPinNumber)
	Builds an analog gas sensor that reads through an ADC and also exposes a digital threshold pin. More...

	GasSensor (IAnalogDigitalConverterComponent *analogToDigitalConverter, const EComponentType &gasSensorType, const unsigned int &analogToDigitalConverterIndex)
	Builds an analog-only gas sensor that reads through an ADC (no digital threshold pin). More...

	GasSensor (const EComponentType &gasSensorType, const unsigned int &digitalOutputPinNumber)
	Builds a digital-only gas sensor that reports only its threshold-output pin (no ADC). More...

virtual	~GasSensor ()
	Destroys the gas sensor, turning it off first. More...

virtual void	TurnOn ()
	Turns the sensor on. More...

virtual void	TurnOff ()
	Turns the sensor off. More...

virtual void	Initialize ()
	Initializes the sensor's datasheet values. More...

int	ReadRawAnalogValue () const
	Reads the raw analog value from the sensor's ADC channel. More...

int	ReadDigitalValue ()
	Reads the sensor's digital threshold-output pin (1 when the configured gas threshold is exceeded). More...

virtual double	ReadGasValue () const
	Gets the most recently processed gas value. More...

virtual void	ProcessRawValues () override
	Reads the sensor and computes the current gas value (PPM) from the RS/R0 ratio using the datasheet's log-log curve. More...

virtual std::string	ToString () const override
	Serializes the sensor's current gas value (PPM), a rough percentage, and the raw reading to a string. More...

unsigned int	GetAnalogToDigitalConverterIndex () const
	Gets the ADC channel index this sensor reads from. More...

unsigned int	GetDigitalOutputPinNumber () const
	Gets the digital threshold-output pin (UINT_MAX if unused). More...

double	GetInitialValueR0 () const
	Gets the sensor's clean-air resistance (R0) from calibration. More...

double	GetResistorValueRL () const
	Gets the load resistor value (RL) from the sensor's datasheet. More...

double	GetCurrentValueRS () const
	Gets the sensor's current resistance (RS). More...

const char *	GetDetectibleConcentration () const
	Gets a human-readable detectable-concentration range for this sensor. More...

EComponentType	GetGasSensorType () const
	Gets this sensor's component type (which MQ sensor it is). More...

const char *	GetGasSensorTypeAsString () const
	Gets this sensor's type as a display string (e.g. More...

const char *	GetGasSensorTypeAsString (const EComponentType &gasSensorType) const
	Maps a gas-sensor component type to its display string (e.g. More...

IAnalogDigitalConverterComponent *	GetAnalogDigitalConverter () const
	Gets the ADC this sensor reads its analog value through. More...

const GasData &	GetGasData () const
	Gets the per-gas curve data (slope/intercept and concentration points) for this sensor. More...

Public Member Functions inherited from RumPi::BaseComponent
	BaseComponent ()
	Builds a base component with an unknown type and an empty name. More...

virtual	~BaseComponent ()
	Virtual destructor so deleting a component through a BaseComponent* also cleans up the concrete subclass. More...

virtual void	TurnOn ()=0
	Turns the component on (e.g. More...

virtual void	TurnOff ()=0
	Turns the component off. More...

virtual std::string	ToString () const =0
	Serializes the component's current readings to a string (usually JSON). More...

virtual void	ProcessRawValues ()=0
	Reads and processes the component's latest raw values (called once per poll). More...

virtual EComponentType	GetComponentType () const
	Gets the component's type. More...

const std::string &	GetName () const
	Gets the component's unique per-instance name. More...

void	SetName (const std::string &name)
	Sets the component's unique per-instance name. More...

Protected Attributes
double	myAtmosphericCO2PPMConstant
	Pre-calculated value that does not change; correlates CO2 and PPM based on the graph in the data sheet. More...

double	myCurrentTemperatureInC
	The temperature (in Celsius) used to correct PPM readings for the sensor's temperature dependence. More...

double	myCurrentHumidityPercent
	The relative humidity (percent) used to correct PPM readings for the sensor's humidity dependence. More...

Protected Attributes inherited from RumPi::GasSensor
unsigned int	myAnalogToDigitalConverterIndex
	The ADC channel index this sensor reads its analog value from. More...

unsigned int	myDigitalOutputPinNumber
	The digital output (threshold) pin, or UINT_MAX if unused. More...

double	myInitialValueR0
	The sensor's resistance in clean air (R0), from calibration. More...

double	myResistorValueRL
	The load resistor value (RL) from the sensor's datasheet. More...

double	myCurrentValueRS
	The sensor's current resistance (RS). More...

double	myCleanAirFactor
	The clean-air factor (RS/R0 in clean air) from the datasheet. More...

double	myRawGasSensorValue
	The most recent raw analog reading. More...

double	myCurrentGasValue
	The most recent computed gas value (PPM). More...

const char *	myDetectibleConcentration
	A human-readable detectable-concentration range for this sensor. More...

IAnalogDigitalConverterComponent *	myAnalogToDigitalConverter
	The ADC this sensor reads its analog value through. More...

GasData	myGasData
	Per-gas curve data (slope/intercept) used to convert the RS/R0 ratio to PPM. More...

Protected Attributes inherited from RumPi::BaseComponent
EComponentType	myComponentType
	The kind of component this is (set by each concrete subclass). More...

std::string	myName
	A unique per-instance name, assigned when the component is added to the manager. More...

Static Protected Attributes
static constexpr double	PARA = 116.6020682

static constexpr double	PARB = 2.769034857

static constexpr double	CORA = 0.00035

static constexpr double	CORB = 0.02718

static constexpr double	CORC = 1.39538

static constexpr double	CORD = 0.0018

static constexpr double	CORE = -0.003333333

static constexpr double	CORF = -0.001923077

static constexpr double	CORG = 1.130128205

static constexpr double	ATMOCO2 = 419.51

Additional Inherited Members
Static Public Member Functions inherited from RumPi::BaseComponent
static std::string	ComponentTypeToString (const EComponentType theComponentTypeToConvert)
	Maps an EComponentType to its string name. More...

static EComponentType	StringToComponentType (const std::string &theComponentTypeName)
	Maps a type name back to its EComponentType (the inverse of ComponentTypeToString), so the client can turn the type carried on the wire back into a value it can look an icon up with. More...

Static Public Attributes inherited from RumPi::GasSensor
static constexpr double	GAS_SENSOR_VOLTAGE = 5.0
	The supply voltage the gas sensor runs on. More...

static const int	CALIBRATION_SAMPLE_TIMES = 200
	The number of samples taken during the calibration phase. More...

static const int	CALIBRATION_SAMPLE_INTERVAL = 250
	Milliseconds between samples during the calibration phase. More...

static const int	READ_SAMPLE_INTERVAL = 250
	Milliseconds between samples during normal operation. More...

Protected Member Functions inherited from RumPi::GasSensor
virtual double	CalculateR0 ()
	Samples the sensor many times in clean air, averages the reading, converts it to resistance (RS), and divides by the clean-air factor to get R0. More...

virtual double	CalculateRS (const double &sensorValueInVolts) const
	Computes the sensor's resistance (RS) from its output voltage, using the load resistor RL and supply voltage. More...

virtual double	CalculateRSR0Ratio (const double &sensorValueInVolts) const
	Computes the RS/R0 ratio (used to look up PPM on the datasheet curve) from the sensor's output voltage. More...

double	ConvertRawSensorValueToVolts (const double &rawGasSensorValue) const
	Converts a raw analog reading to its equivalent voltage (using the sensor's supply voltage and the ADC's maximum value). More...

double	ConvertVoltsToRawSensorValue (const double &volts) const
	Converts a voltage back to its equivalent raw analog value (the inverse of ConvertRawSensorValueToVolts). More...

double	GetAnalogToDigitalConverterMaxRawValue () const
	Gets the ADC's full-scale raw count for scaling raw readings to volts (PCF8591 = 255 8-bit, MCP3008 = 1023 10-bit), falling back to the 10-bit convention when no ADC is wired. More...

double	GetAnalogToDigitalConverterVoltage () const
	Gets the ADC's reference voltage for scaling raw readings to volts (defined by the wired ADC, e.g. More...

Detailed Description

MQ-135 gas sensor, typically used for detecting Carbon Dioxide (CO2) PPM.

Original algorithm for CO2 detection on the MQ-135 is from G. Krocker (Mad Frog Labs), ViliusKraujutis, and NuclearPhoenixx. Source: https://www.arduino.cc/reference/en/libraries/mq135/

Author: Eddie O'Hagan

Date: 8/13/2026

Constructor & Destructor Documentation

◆ MQ135() [1/4]

RumPi::MQ135::MQ135 ( )

Builds a default MQ-135 (carbon dioxide) sensor and initializes its datasheet values.

Author: Eddie O'Hagan

Date: 8/13/2026

MQ135 carbonDioxideSensor;

RumPi::MQ135::MQ135

MQ135()

Builds a default MQ-135 (carbon dioxide) sensor and initializes its datasheet values.

Definition: MQ135.cpp:15

◆ MQ135() [2/4]

RumPi::MQ135::MQ135	(	IAnalogDigitalConverterComponent *	analogToDigitalConverter,
		const unsigned int &	analogToDigitalConverterIndex,
		const unsigned int &	digitalOutputPinNumber
	)

Builds an MQ-135 that reads through an ADC and also exposes a digital threshold pin.

Author: Eddie O'Hagan

Date: 8/13/2026

MQ135 carbonDioxideSensor(adc, 0, PIN_GPIO_27);

RumPi::PIN_GPIO_27

constexpr unsigned int PIN_GPIO_27

BCM GPIO pin 27.

Definition: Common.h:53

Parameters

[in]	analogToDigitalConverter	The ADC the sensor reads through.
[in]	analogToDigitalConverterIndex	The ADC channel index.
[in]	digitalOutputPinNumber	The digital threshold-output pin.

◆ MQ135() [3/4]

RumPi::MQ135::MQ135	(	IAnalogDigitalConverterComponent *	analogToDigitalConverter,
		const unsigned int &	analogToDigitalConverterIndex
	)

Builds an analog-only MQ-135 that reads through an ADC.

Author: Eddie O'Hagan

Date: 8/13/2026

MQ135 carbonDioxideSensor(adc, 0);

Parameters

[in]	analogToDigitalConverter	The ADC the sensor reads through.
[in]	analogToDigitalConverterIndex	The ADC channel index.

◆ MQ135() [4/4]

RumPi::MQ135::MQ135 ( const unsigned int & digitalOutputPinNumber )

Builds a digital-only MQ-135 that reports only its threshold-output pin.

Author: Eddie O'Hagan

Date: 8/13/2026

MQ135 carbonDioxideSensor(PIN_GPIO_27);

Parameters

[in] digitalOutputPinNumber The digital threshold-output pin.

◆ ~MQ135()

RumPi::MQ135::~MQ135 ( )

virtual

Destroys the sensor, turning it off first.

Author: Eddie O'Hagan

Date: 8/13/2026

MQ135* sensor = new MQ135();

delete sensor;

Member Function Documentation

◆ CalculateCorrectedPPM()

double RumPi::MQ135::CalculateCorrectedPPM	(	double	temperatureInC,
		double	humidityPercent
	)		const

Gets the PPM of CO2 sensed (assuming only CO2 in the air), corrected for temperature and humidity.

Author: Eddie O'Hagan

Date: 8/13/2026

double ppm = carbonDioxideSensor.CalculateCorrectedPPM(21.0, 25.0);

Parameters

[in]	temperatureInC	The current temperature, in Celsius.
[in]	humidityPercent	The current relative humidity, as a percent.

Returns: The temperature/humidity-corrected CO2 concentration, in PPM.

◆ CalculateCorrectedR0()

double RumPi::MQ135::CalculateCorrectedR0	(	double	temperatureInC,
		double	humidityPercent
	)

Gets the corrected resistance R0 of the sensor for calibration purposes, adjusted for temperature and humidity.

Author: Eddie O'Hagan

Date: 8/13/2026

double correctedR0 = carbonDioxideSensor.CalculateCorrectedR0(21.0, 25.0);

Parameters

[in]	temperatureInC	The current temperature, in Celsius.
[in]	humidityPercent	The current relative humidity, as a percent.

Returns: The corrected calibration resistance R0, in kOhm.

◆ CalculateCorrectedResistance()

double RumPi::MQ135::CalculateCorrectedResistance	(	double	temperatureInC,
		double	humidityPercent
	)		const

Gets the sensor's resistance with the measurement value corrected for temperature and humidity.

Author: Eddie O'Hagan

Date: 8/13/2026

double corrected = carbonDioxideSensor.CalculateCorrectedResistance(21.0, 25.0);

Parameters

[in]	temperatureInC	The current temperature, in Celsius.
[in]	humidityPercent	The current relative humidity, as a percent.

Returns: The corrected sensor resistance, in kOhm.

◆ CalculateCorrectionFactor()

double RumPi::MQ135::CalculateCorrectionFactor	(	double	temperatureInC,
		double	humidityPercent
	)		const

Gets the correction factor used to compensate the sensor's resistance for temperature and humidity.

The temperature dependency curve is linearized under and above 20 degrees C, assuming a linear dependency on humidity: below 20C fact = a*t*t - b*t + c - (h - 33)*d; above 20C fact = e*t + f*h + g.

Author: Eddie O'Hagan

Date: 8/13/2026

double factor = carbonDioxideSensor.CalculateCorrectionFactor(21.0, 25.0);

Parameters

[in]	temperatureInC	The current temperature, in Celsius.
[in]	humidityPercent	The current relative humidity, as a percent.

Returns: The temperature/humidity correction factor.

◆ CalculatePPM()

double RumPi::MQ135::CalculatePPM ( ) const

Gets the PPM of CO2 sensed (assuming only CO2 in the air), from the sensor's resistance ratio.

Author: Eddie O'Hagan

Date: 8/13/2026

double ppm = carbonDioxideSensor.CalculatePPM();

Returns: The CO2 concentration, in PPM.

◆ CalculateR0()

double RumPi::MQ135::CalculateR0 ( )

overridevirtual

Gets the resistance R0 (RZero) of the sensor for calibration purposes.

Author: Eddie O'Hagan

Date: 8/13/2026

double r0 = carbonDioxideSensor.CalculateR0();

Returns: The calibration resistance R0, in kOhm.

Reimplemented from RumPi::GasSensor.

◆ CalculateResistance()

double RumPi::MQ135::CalculateResistance ( ) const

Gets the resistance of the sensor (its measurement value).

Author: Eddie O'Hagan

Date: 8/13/2026

double resistance = carbonDioxideSensor.CalculateResistance();

Returns: The sensor resistance, in kOhm.

◆ GetCurrentHumidityPercent()

double RumPi::MQ135::GetCurrentHumidityPercent ( ) const

Gets the relative humidity (percent) currently used to correct the sensor's readings.

Author: Eddie O'Hagan

Date: 8/13/2026

double humidity = carbonDioxideSensor.GetCurrentHumidityPercent();

Returns: The current relative humidity, as a percent.

◆ GetCurrentTemperatureInC()

double RumPi::MQ135::GetCurrentTemperatureInC ( ) const

Gets the temperature (in Celsius) currently used to correct the sensor's readings.

Author: Eddie O'Hagan

Date: 8/13/2026

double temperature = carbonDioxideSensor.GetCurrentTemperatureInC();

Returns: The current temperature, in Celsius.

◆ Initialize()

void RumPi::MQ135::Initialize ( )

overridevirtual

Initializes the MQ-135's datasheet values, its default temperature/humidity, and the pre-computed atmospheric CO2 constant used to correlate resistance to PPM.

Author: Eddie O'Hagan

Date: 8/13/2026

//Called by every MQ135 constructor.

Reimplemented from RumPi::GasSensor.

◆ ReadGasValue()

double RumPi::MQ135::ReadGasValue ( ) const

overridevirtual

Reads the current CO2 PPM for the sensor's raw reading (assuming only CO2 in the air).

Author: Eddie O'Hagan

Date: 8/13/2026

double ppm = carbonDioxideSensor.ReadGasValue();

Returns: The current CO2 concentration, in PPM.

Reimplemented from RumPi::GasSensor.

◆ SetCurrentHumidityPercent()

void RumPi::MQ135::SetCurrentHumidityPercent ( double currentHumidityPercent )

Sets the relative humidity (percent) used to correct the sensor's readings.

Author: Eddie O'Hagan

Date: 8/13/2026

carbonDioxideSensor.SetCurrentHumidityPercent(40.0);

Parameters

[in] currentHumidityPercent The relative humidity to use, as a percent.

◆ SetCurrentTemperatureInC()

void RumPi::MQ135::SetCurrentTemperatureInC ( double currentTemperatureInC )

Sets the temperature (in Celsius) used to correct the sensor's readings.

Author: Eddie O'Hagan

Date: 8/13/2026

carbonDioxideSensor.SetCurrentTemperatureInC(23.5);

Parameters

[in] currentTemperatureInC The temperature to use, in Celsius.

◆ ToString()

std::string RumPi::MQ135::ToString ( ) const

overridevirtual

Serializes the MQ-135's gas name, raw reading, raw and corrected PPM, temperature, and humidity to a string.

Author: Eddie O'Hagan

Date: 8/13/2026

std::string data = carbonDioxideSensor.ToString();

Returns: A multi-line string describing the sensor's reading.

Reimplemented from RumPi::GasSensor.

Member Data Documentation

◆ ATMOCO2

constexpr double RumPi::MQ135::ATMOCO2 = 419.51

staticconstexprprotected

◆ CORA

constexpr double RumPi::MQ135::CORA = 0.00035

staticconstexprprotected

◆ CORB

constexpr double RumPi::MQ135::CORB = 0.02718

staticconstexprprotected

◆ CORC

constexpr double RumPi::MQ135::CORC = 1.39538

staticconstexprprotected

◆ CORD

constexpr double RumPi::MQ135::CORD = 0.0018

staticconstexprprotected

◆ CORE

constexpr double RumPi::MQ135::CORE = -0.003333333

staticconstexprprotected

◆ CORF

constexpr double RumPi::MQ135::CORF = -0.001923077

staticconstexprprotected

◆ CORG

constexpr double RumPi::MQ135::CORG = 1.130128205

staticconstexprprotected

◆ myAtmosphericCO2PPMConstant

double RumPi::MQ135::myAtmosphericCO2PPMConstant

protected

Pre-calculated value that does not change; correlates CO2 and PPM based on the graph in the data sheet.

◆ myCurrentHumidityPercent

double RumPi::MQ135::myCurrentHumidityPercent

protected

The relative humidity (percent) used to correct PPM readings for the sensor's humidity dependence.

◆ myCurrentTemperatureInC

double RumPi::MQ135::myCurrentTemperatureInC

protected

The temperature (in Celsius) used to correct PPM readings for the sensor's temperature dependence.

◆ PARA

constexpr double RumPi::MQ135::PARA = 116.6020682

staticconstexprprotected

◆ PARB

constexpr double RumPi::MQ135::PARB = 2.769034857

staticconstexprprotected

The documentation for this class was generated from the following files:

Source/Headers/RumPi/Components/MQ135.h
Source/Implementations/RumPi/Components/MQ135.cpp

Public Member Functions

Protected Attributes

Static Protected Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ MQ135() [1/4]

◆ MQ135() [2/4]

◆ MQ135() [3/4]

◆ MQ135() [4/4]

◆ ~MQ135()

Member Function Documentation

◆ CalculateCorrectedPPM()

◆ CalculateCorrectedR0()

◆ CalculateCorrectedResistance()

◆ CalculateCorrectionFactor()

◆ CalculatePPM()

◆ CalculateR0()

◆ CalculateResistance()

◆ GetCurrentHumidityPercent()

◆ GetCurrentTemperatureInC()

◆ Initialize()

◆ ReadGasValue()

◆ SetCurrentHumidityPercent()

◆ SetCurrentTemperatureInC()

◆ ToString()

Member Data Documentation

◆ ATMOCO2

◆ CORA

◆ CORB

◆ CORC

◆ CORD

◆ CORE

◆ CORF

◆ CORG

◆ myAtmosphericCO2PPMConstant

◆ myCurrentHumidityPercent

◆ myCurrentTemperatureInC

◆ PARA

◆ PARB