Update calculations, README and add Arduino example

MQ135.cpp

@@ -40,7 +40,15 @@ MQ135::MQ135(uint8_t pin) {
 */
 /**************************************************************************/
 float MQ135::getCorrectionFactor(float t, float h) {
-  return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
+    // Linearization of the temperature dependency curve under and above 20 degree C
+    // below 20degC: fact = a * t * t - b * t - (h - 33) * d
+    // above 20degC: fact = a * t + b * h + c
+    // this assumes a linear dependency on humidity
+    if(t < 20){
+        return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
+    } else {
+        return CORE * t + CORF * h + CORG;
+    }
 }
 
 /**************************************************************************/
@@ -52,7 +60,7 @@ float MQ135::getCorrectionFactor(float t, float h) {
 /**************************************************************************/
 float MQ135::getResistance() {
   int val = analogRead(_pin);
-  return ((1023./(float)val) * 5. - 1.)*RLOAD;
+  return ((1023./(float)val) - 1.)*RLOAD;
 }
 
 /**************************************************************************/

MQ135.h

@@ -34,6 +34,9 @@ v1.0 - First release
 #define CORB 0.02718
 #define CORC 1.39538
 #define CORD 0.0018
+#define CORE -0.003333333
+#define CORF -0.001923077
+#define CORG 1.130128205
 
 /// Atmospheric CO2 level for calibration purposes
 #define ATMOCO2 397.13

README.md

@@ -1,4 +1,26 @@
-MQ135
+MQ135 GAS SENSOR
 =====
 
-Arduino library for the MQ135
+Arduino library for the MQ135 gas sensor
+
+## Datasheet
+Can be found [here](https://www.olimex.com/Products/Components/Sensors/SNS-MQ135/resources/SNS-MQ135.pdf)
+
+## Application
+They are used in air quality control equipments for buildings/offices, are suitable for detecting of NH3, NOx, alcohol, Benzene, smoke, CO2, etc
+
+## Features
+This library has:
+ - Corrections for temperature and humidity
+ - Measurements:
+    - getResistance
+    - getCorrectedResistance
+    - getPPM
+    - getCorrectedPPM
+    - getRZero
+    - getCorrectedRZero
+
+More Info
+=====
+
+https://hackaday.io/project/3475-sniffing-trinket/log/12363-mq135-arduino-library

examples/MQ135/MQ135.pde

@@ -0,0 +1,43 @@
+#include "MQ135.h"
+
+// MQ135 gas sensor
+//
+// Datasheet can be found here: https://www.olimex.com/Products/Components/Sensors/SNS-MQ135/resources/SNS-MQ135.pdf
+//
+// Application
+// They are used in air quality control equipments for buildings/offices, are suitable for detecting of NH3, NOx, alcohol, Benzene, smoke, CO2, etc
+//
+// Original creator of this library: https://github.com/GeorgK/MQ135
+
+
+#define PIN_MQ135 A2
+MQ135 mq135_sensor = MQ135(PIN_MQ135);
+
+float temperature = 21.0; // assume current temperature. Recommended to measure with DHT22
+float humidity = 25.0; // assume current humidity. Recommended to measure with DHT22
+
+void setup() {
+  Serial.begin(9600);
+}
+
+void loop() {
+  float rzero = mq135_sensor.getRZero();
+  float correctedRZero = mq135_sensor.getCorrectedRZero(temperature, humidity);
+  float resistance = mq135_sensor.getResistance();
+  float ppm = mq135_sensor.getPPM();
+  float correctedPPM = mq135_sensor.getCorrectedPPM(temperature, humidity);
+
+  Serial.print("MQ135 RZero: ");
+  Serial.print(rzero);
+  Serial.print("\t Corrected RZero: ");
+  Serial.print(correctedRZero);
+  Serial.print("\t Resistance: ");
+  Serial.print(resistance);
+  Serial.print("\t PPM: ");
+  Serial.print(ppm);
+  Serial.print("\t Corrected PPM: ");
+  Serial.print(correctedPPM);
+  Serial.println("ppm");
+
+  delay(300);
+}