From 6d1229977e2eceba220afb311ff4f2b524286682 Mon Sep 17 00:00:00 2001
From: miguel5612 <miguelangelcu@ufps.edu.co>
Date: Wed, 29 May 2019 20:58:22 -0500
Subject: [PATCH] fixed calibration

---
 src/MQUnifiedsensor.cpp | 27 ++++++++++++++++++++-------
 src/MQUnifiedsensor.h   | 11 ++++++++---
 2 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/src/MQUnifiedsensor.cpp b/src/MQUnifiedsensor.cpp
index 670406c..ca0868c 100644
--- a/src/MQUnifiedsensor.cpp
+++ b/src/MQUnifiedsensor.cpp
@@ -136,13 +136,26 @@ int MQUnifiedsensor::readPPM(int m, int b) {
   return floor(ppm);
 }
 double MQUnifiedsensor::calibrate() {
-    float sensor_volt; //Define variable for sensor voltage
-    float RS_air; //Define variable for sensor resistance
-    float R0; //Define variable for R0
-    float sensorValue; //Define variable for analog readings
-    sensor_volt = this->getVoltage(); //Convert average to voltage
-    R0 = sensor_volt / _ratioInCleanAir; //Calculate R0
-    return R0;
+  //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
+  /*
+  V = I x R 
+  VRL = [VC / (RS + RL)] x RL 
+  VRL = (VC x RL) / (RS + RL) 
+  Así que ahora resolvemos para RS: 
+  VRL x (RS + RL) = VC x RL
+  (VRL x RS) + (VRL x RL) = VC x RL 
+  (VRL x RS) = (VC x RL) - (VRL x RL)
+  RS = [(VC x RL) - (VRL x RL)] / VRL
+  RS = [(VC x RL) / VRL] - RL
+  */
+  float sensor_volt; //Define variable for sensor voltage
+  float RS_air; //Define variable for sensor resistance
+  float R0; //Define variable for R0
+  float sensorValue; //Define variable for analog readings
+  sensor_volt = this->getVoltage(); //Convert average to voltage
+  RS_air = ((VOLT_RESOLUTION*RLValue)/sensor_volt)-RLValue; //Calculate RS in fresh air 
+  R0 = RS_air/_ratioInCleanAir; //Calculate R0 
+  return R0;
 }
 double MQUnifiedsensor::getVoltage() {
   double avg = 0.0;
diff --git a/src/MQUnifiedsensor.h b/src/MQUnifiedsensor.h
index b034cf8..17380c3 100644
--- a/src/MQUnifiedsensor.h
+++ b/src/MQUnifiedsensor.h
@@ -4,9 +4,16 @@
 #include <Arduino.h>
 #include <stdint.h>
 
+
 //Count of posible lectures
 #define lecturesAvailable 19
 
+/************************Global vars************************************/
+int VOLT_RESOLUTION  = 5.0; // if 3.3v use 3.3
+int RLValue = 10; //Value in KiloOhms
+int  _ratioInCleanAir, _PPM;
+double RS_air,_R0, _m, _b, _ratio;
+
 /************************Hardware Related Macros************************************/
 
 //Index in the nameLecture vector
@@ -82,10 +89,8 @@ class MQUnifiedsensor
     
 
   private:
-    int _pin, _type, _PPM, _lecturePosInArray, _ratioInCleanAir;
-    double RS_air,_R0 = 11.820, _m, _b, _ratio;
+    int _pin, _type; 
     String _MQ[19], _nameLectureRequeired;   
-    int VOLT_RESOLUTION  = 5.0; // if 3.3v use 3.3
 };
 
 #endif //MQUnifiedsensor_H
\ No newline at end of file