Added lineal vs exponential example

examples/LinearVsExponential/LinearVsExponential.ino

@@ -0,0 +1,102 @@
+/*
+  MQUnifiedsensor Library - reading an MQ4
+
+  Demonstrates the use a MQ4 sensor.
+  Library originally added 01 may 2019
+  by Miguel A Califa, Yersson Carrillo, Ghiordy Contreras, Mario Rodriguez
+ 
+  Added example
+  modified 23 May 2019
+  by Miguel Califa 
+
+  Updated library usage
+  modified 26 March 2020
+  by Miguel Califa 
+
+ This example code is in the public domain.
+
+*/
+
+//Include the library
+#include <MQUnifiedsensor.h>
+
+//Definitions
+#define placa "Arduino UNO"
+#define Voltage_Resolution 5
+#define pin A4 //Analog input 4 of your arduino
+#define type "MQ-4" //MQ4
+#define ADC_Bit_Resolution 10 // For arduino UNO/MEGA/NANO
+#define RatioMQ4CleanAir 4.4  //RS / R0 = 4.4 ppm 
+//#define calibration_button 13 //Pin to calibrate your sensor
+
+//Declare Sensor
+MQUnifiedsensor MQ4(placa, Voltage_Resolution, ADC_Bit_Resolution, pin, type);
+
+void setup() {
+  //Init the serial port communication - to debug the library
+  Serial.begin(9600); //Init serial port
+
+ 
+  
+  
+  /*****************************  MQ CAlibration ********************************************/ 
+  // Explanation: 
+  // In this routine the sensor will measure the resistance of the sensor supposing before was pre-heated
+  // and now is on clean air (Calibration conditions), and it will setup R0 value.
+  // We recomend execute this routine only on setup or on the laboratory and save on the eeprom of your arduino
+  // This routine not need to execute to every restart, you can load your R0 if you know the value
+  // Acknowledgements: https://jayconsystems.com/blog/understanding-a-gas-sensor
+  Serial.print("Calibrating please wait.");
+  float calcR0 = 0;
+  for(int i = 0; i<=10; i ++)
+  {
+    MQ4.update(); // Update data, the arduino will be read the voltage on the analog pin
+    calcR0 += MQ4.calibrate(RatioMQ4CleanAir);
+    Serial.print(".");
+  }
+  MQ4.setR0(calcR0/10);
+  Serial.println("  done!.");
+  
+  if(isinf(calcR0)) {Serial.println("Warning: Conection issue founded, R0 is infite (Open circuit detected) please check your wiring and supply"); while(1);}
+  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);}
+  /*****************************  MQ CAlibration ********************************************/ 
+
+  /* 
+    //If the RL value is different from 10K please assign your RL value with the following method:
+    MQ4.setRL(10);
+  */
+
+  /*****************************  MQ Init ********************************************/ 
+  //Input: setup flag, if this function are on setup will print the headers (Optional - Default value: False)
+  //Output: print on serial port the information about sensor and sensor readings
+  //Remarks: Configure the pin of arduino as input.
+  /************************************************************************************/ 
+  MQ4.init(); 
+
+  Serial.println("** Lectures from MQ-4**********");
+  Serial.println("|      LPG (Linear EQ)        |         LPG (Exponential EQ)      |");  
+}
+
+void loop() {
+  MQ4.update(); // Update data, the arduino will be read the voltage on the analog pin
+  
+  //https://jayconsystems.com/blog/understanding-a-gas-sensor 
+  //Set math model to calculate the PPM concentration and the value of constants
+  MQ4.setRegressionMethod(0); //_PPM =  pow(10, (log10(ratio)-b)/a)
+  MQ4.setA(-0.318); MQ4.setB(1.133); // A -> Slope, B -> Intersect with X - Axis
+  float LPG1 = MQ4.readSensor(); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
+  
+  //Set math model to calculate the PPM concentration and the value of constants
+  MQ4.setRegressionMethod(1); //_PPM =  a*ratio^b
+  MQ4.setA(1012.7); MQ4.setB(-2.786); // Configurate the ecuation values to get CH4 concentration
+  float LPG2 = MQ4.readSensor(); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
+
+  //  exposure to 2000 ppm of LPG gas is immediately dangerous to life and health. In this section
+  if(LPG1>=2000 || LPG2>=2000) Serial.println("Warning - Very high concentrations detected!");
+
+  Serial.print("|              "); Serial.print(LPG1);
+  Serial.print("            |           "); Serial.print(LPG2);
+  Serial.println("          |");
+
+  delay(500); //Sampling frequency
+}