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Following comment in #31 we added example
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/*
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MQUnifiedsensor Library - reading an MQ135
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Demonstrates the use a MQ135 sensor.
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Library originally added 01 may 2019
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by Miguel A Califa, Yersson Carrillo, Ghiordy Contreras, Mario Rodriguez
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Added example
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modified 23 May 2019
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by Miguel Califa
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Updated library usage
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modified 26 March 2020
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by Miguel Califa
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Wiring:
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https://github.com/miguel5612/MQSensorsLib_Docs/blob/master/static/img/MQ_Arduino.PNG
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Please take care, arduino A0 pin represent the analog input configured on #define pin
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This example code is in the public domain.
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*/
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//Include the library
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#include <MQUnifiedsensor.h>
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#include <Adafruit_Sensor.h>
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#include <DHT.h>
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#include <DHT_U.h>
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//Definitions
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#define placa "Arduino UNO"
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#define Voltage_Resolution 5
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#define pin A0 //Analog input 0 of your arduino
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#define type "MQ-135" //MQ135
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#define ADC_Bit_Resolution 10 // For arduino UNO/MEGA/NANO
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#define RatioMQ135CleanAir 3.6//RS / R0 = 3.6 ppm
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//#define calibration_button 13 //Pin to calibrate your sensor
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#define MQ135_DEFAULTPPM 399 //default ppm of CO2 for calibration
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#define MQ135_DEFAULTRO 68550 //default Ro for MQ135_DEFAULTPPM ppm of CO2
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#define MQ135_SCALINGFACTOR 116.6020682 //CO2 gas value
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#define MQ135_EXPONENT -2.769034857 //CO2 gas value
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#define MQ135_MAXRSRO 2.428 //for CO2
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#define MQ135_MINRSRO 0.358 //for CO2
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/// Parameters for calculating ppm of CO2 from sensor resistance
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#define PARA 116.6020682
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#define PARB 2.769034857
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/// Parameters to model temperature and humidity dependence
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#define CORA 0.00035
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#define CORB 0.02718
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#define CORC 1.39538
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#define CORD 0.0018
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#define DHTPIN 2 // Digital pin connected to the DHT sensor
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// Feather HUZZAH ESP8266 note: use pins 3, 4, 5, 12, 13 or 14 --
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// Pin 15 can work but DHT must be disconnected during program upload.
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// Uncomment the type of sensor in use:
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//#define DHTTYPE DHT11 // DHT 11
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#define DHTTYPE DHT22 // DHT 22 (AM2302)
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//#define DHTTYPE DHT21 // DHT 21 (AM2301)
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// See guide for details on sensor wiring and usage:
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// https://learn.adafruit.com/dht/overview
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DHT_Unified dht(DHTPIN, DHTTYPE);
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uint32_t delayMS;
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//Declare Sensor
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MQUnifiedsensor MQ135(placa, Voltage_Resolution, ADC_Bit_Resolution, pin, type);
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void setup() {
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//Init the serial port communication - to debug the library
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Serial.begin(9600); //Init serial port
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dht.begin();
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sensor_t sensor;
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//Set math model to calculate the PPM concentration and the value of constants
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MQ135.setRegressionMethod(1); //_PPM = a*ratio^b
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MQ135.setA(102.2); MQ135.setB(-2.473); // Configurate the ecuation values to get NH4 concentration
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/*
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Exponential regression:
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GAS | a | b
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CO | 605.18 | -3.937
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Alcohol | 77.255 | -3.18
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CO2 | 110.47 | -2.862
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Tolueno | 44.947 | -3.445
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NH4 | 102.2 | -2.473
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Acetona | 34.668 | -3.369
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*/
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/***************************** MQ Init ********************************************/
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//Remarks: Configure the pin of arduino as input.
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/************************************************************************************/
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MQ135.init();
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/*
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//If the RL value is different from 10K please assign your RL value with the following method:
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MQ135.setRL(10);
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*/
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/***************************** MQ CAlibration ********************************************/
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// Explanation:
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// In this routine the sensor will measure the resistance of the sensor supposing before was pre-heated
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// and now is on clean air (Calibration conditions), and it will setup R0 value.
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// We recomend execute this routine only on setup or on the laboratory and save on the eeprom of your arduino
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// This routine not need to execute to every restart, you can load your R0 if you know the value
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// Acknowledgements: https://jayconsystems.com/blog/understanding-a-gas-sensor
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Serial.print("Calibrating please wait.");
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float calcR0 = 0;
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for(int i = 1; i<=10; i ++)
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{
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MQ135.update(); // Update data, the arduino will be read the voltage on the analog pin
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calcR0 += MQ135.calibrate(RatioMQ135CleanAir);
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Serial.print(".");
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}
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MQ135.setR0(calcR0/10);
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Serial.println(" done!.");
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if(isinf(calcR0)) {Serial.println("Warning: Conection issue founded, R0 is infite (Open circuit detected) please check your wiring and supply"); while(1);}
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if(calcR0 == 0){Serial.println("Warning: Conection issue founded, R0 is zero (Analog pin with short circuit to ground) please check your wiring and supply"); while(1);}
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/***************************** MQ CAlibration ********************************************/
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MQ135.serialDebug(true);
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// Set delay between sensor readings based on sensor details.
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delayMS = sensor.min_delay / 1000;
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}
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void loop() {
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delay(delayMS); //Sampling frequency
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// if you want to apply corelation factor, you will add in this program the temperature and humidity sensor
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sensors_event_t event;
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dht.temperature().getEvent(&event);
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float cFactor = 0;
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if (!isnan(event.temperature) && !isnan(event.relative_humidity)) cFactor = getCorrectionFactor(event.temperature, event.relative_humidity);
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Serial.print("Correction Factor: "); Serial.println(cFactor);
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MQ135.update(); // Update data, the arduino will be read the voltage on the analog pin
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MQ135.readSensor(false, cFactor); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
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MQ135.serialDebug(); // Will print the table on the serial port
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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 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 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 getCorrectedResistance(long resvalue, float t, float h) {
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return resvalue/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), 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 getCorrectedPPM(long resvalue,float t, float h, long ro) {
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return PARA * pow((getCorrectedResistance(resvalue, t, h)/ro), -PARB);
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}
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