/*
 * Program to measure gas in ppm using MQ sensor
 * Program by: B.Aswinth Raj
 * Website: www.circuitdigest.com
 * Dated: 28-12-2017
 */
#define RL 47  //The value of resistor RL is 47K
#define m -0.263 //Enter calculated Slope 
#define b 0.42 //Enter calculated intercept
#define Ro 20 //Enter found Ro value
#define MQ_sensor A0 //Sensor is connected to A4
#include <LiquidCrystal.h> //Header file for LCD from https://www.arduino.cc/en/Reference/LiquidCrystal
const int rs = 8, en = 9, d4 = 10, d5 = 11, d6 = 12, d7 = 13; //Pins to which LCD is connected
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);
void setup() {
  lcd.begin(16, 2); //We are using a 16*2 LCD display
  lcd.print("NH3 in PPM"); //Display a intro message
  lcd.setCursor(0, 1);   // set the cursor to column 0, line 1
  lcd.print("-CircuitDigest"); //Display a intro message 
   delay(2000); //Wait for display to show info
   lcd.clear(); //Then clean it
}
void loop() {
  float VRL; //Voltage drop across the MQ sensor
  float Rs; //Sensor resistance at gas concentration 
  float ratio; //Define variable for ratio
  VRL = analogRead(MQ_sensor)*(5.0/1023.0); //Measure the voltage drop and convert to 0-5V
  Rs = ((5.0*RL)/VRL)-RL; //Use formula to get Rs value
  ratio = Rs/Ro;  // find ratio Rs/Ro
  float ppm = pow(10, ((log10(ratio)-b)/m)); //use formula to calculate ppm
  lcd.print("NH3 (ppm) = "); //Display a ammonia in ppm
  lcd.print(ppm);
  lcd.setCursor(0, 1);   // set the cursor to column 0, line 1
  lcd.print("Voltage = "); //Display a intro message 
  lcd.print(VRL);
   delay(200);
   lcd.clear(); //Then clean it
}