Added external A2_D Support

examples/external_A2D/external_A2D.ino

@@ -0,0 +1,95 @@
+/*
+  MQUnifiedsensor Library - reading an MQ3
+
+  Demonstrates the use a MQ3 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 
+
+  Wiring:
+  https://github.com/miguel5612/MQSensorsLib_Docs/blob/master/static/img/MQ_Arduino.PNG
+  Please take care, arduino A0 pin represent the analog input configured on #define pin
+
+ 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 A0 //Analog input 0 -> This value can have any number, we recomend use A0
+#define type "MQ-3 With A2D External connected" //MQ3
+#define ADC_Bit_Resolution 10 // Resolution of your external A2D
+#define RatioMQ3CleanAir 60   //RS / R0 = 60 ppm 
+//#define calibration_button 13 //Pin to calibrate your sensor
+
+//Declare Sensor
+MQUnifiedsensor MQ3(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
+
+  //Set math model to calculate the PPM concentration and the value of constants
+  MQ3.setRegressionMethod(1); //_PPM =  a*ratio^b
+  MQ3.setA(4.8387); MQ3.setB(-2.68); // Configurate the ecuation values to get Benzene concentration
+  /*
+    Exponential regression:
+  Gas    | a      | b
+  LPG    | 44771  | -3.245
+  CH4    | 2*10^31| 19.01
+  CO     | 521853 | -3.821
+  Alcohol| 0.3934 | -1.504
+  Benzene| 4.8387 | -2.68
+  Hexane | 7585.3 | -2.849
+  */
+
+  
+  /*****************************  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 = 1; i<=10; i ++)
+  {
+    int yourA2DValue = random(0, 1024); // 10-bit emulation
+    MQ3.setADC(yourA2DValue);// Update data, the arduino will be read the voltage on the analog pin
+    calcR0 += MQ3.calibrate(RatioMQ3CleanAir);
+    Serial.print(".");
+  }
+  MQ3.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:
+    MQ3.setRL(10);
+  */
+
+  MQ3.serialDebug(true);
+}
+
+void loop() {
+  int yourA2DValue = random(0, 1024); // 10-bit emulation
+  MQ3.setADC(yourA2DValue); // Update data, the arduino will be read the voltage on the analog pin
+  MQ3.readSensor(); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
+  MQ3.serialDebug(); // Will print the table on the serial port
+  delay(500); //Sampling frequency
+}