Smoke and Gas Detector

#include <LiquidCrystal.h>

//System state
const char onMessage[] = "Alarm on";
const char offMessage[] = "Alarm off";
const char activeMessage[][16] = {"Gas Detected", "Please evacuate","Fire Detected"};
const int OFF=0, ON=1, ACTIVE=2; 
int state = OFF;

//Pins
const int tempSensor = A2;
const int gasSensor = A3;
const int led = 5;
const int piezo = 3;
const int power = 2;

const int rs = 8, en = 9, d4=10, d5=11, d6=12, d7=13;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

int temperature;
int gas;
char degree = 176; //ASCII value of Degree
unsigned long time;

void setup()
 {
  Serial.begin(9600);               //
  delay(3000);                      //
  Serial.println("Start");          //
  
  pinMode(led, OUTPUT);
  pinMode(piezo, OUTPUT);
  
  pinMode(A3,INPUT);                //          
  pinMode(A2,INPUT);                //
  
  lcd.begin(16, 2);
  setOff(); 
 }

void loop()
{
  int on = digitalRead(power);
  int Gas_state = analogRead(A3);   //

  int tmp = analogRead(A0);//Reading data from the sensor.This voltage is stored as a 10bit number.
  float voltage = (tmp * 5.0)/1024;//(5*temp)/1024 is to convert the 10 bit number to a voltage reading.
  float milliVolt = voltage * 1000;//This is multiplied by 1000 to convert it to millivolt.
  float tmpCel =  (milliVolt-500)/10 ;//For TMP36 sensor. Range(−40°C to +125°C)
  float tmpFer = (((tmpCel*9)/5)+32);//used to convert Celsius -> Fahrenheit

  if(on && (state == OFF)){
    setOn();
    Serial.println("Gas Leaked at your premises");
  }else if(!on && (state != OFF)){
    setOff();
  }

if(state == ON) {
  readSensors();
  if(gas > 60) {
    setActive();
  }else if(temperature > 100){
    setActive1();
  }
    
}else if(state == ACTIVE) {
  int ledState = digitalRead(led);
  digitalWrite(led, !ledState);
  if(ledState == HIGH) {
    delay(50);
  }else {
    delay(20);
  }
 }
  
  time = millis();
  Serial.print("Time: ");
  Serial.println(time);
  
  /*Serial.print("10bit number(0-1023):  ");
  Serial.println(tmp);
  
  Serial.print("voltage: ");
  Serial.print(voltage);
  Serial.println("V");
  
  Serial.print("millivolt: ");
  Serial.print(milliVolt);
  Serial.println("mV");*/
  
  Serial.print("Celsius: ");
  Serial.print(tmpCel);
  Serial.println(degree);
  
  Serial.print("Fahrenheit: ");
  Serial.println(tmpFer);
  Serial.println("");
  
  delay(1000);
}
 
void setOff() {
 state = OFF;
 digitalWrite(led, LOW);
 noTone(piezo);
 lcd.clear();
 lcd.setCursor(4, 0);
 lcd.print(offMessage);  
}

void setOn() {
 state = ON;
 digitalWrite(led, HIGH);
 lcd.clear();
 lcd.setCursor(4, 0);
 lcd.print(onMessage);  
}

void setActive(){
 state = ACTIVE;
 tone(piezo, 750);
 lcd.clear();
 lcd.print(activeMessage[0]);
 lcd.setCursor(0, 1);
 lcd.print(activeMessage[1]);
}

void setActive1(){
 state = ACTIVE;
 tone(piezo, 750);
 lcd.clear();
 lcd.print(activeMessage[2]);
 lcd.setCursor(0, 1);
 lcd.print(activeMessage[1]);
}

void readSensors(){
  int t = analogRead(tempSensor); 
  temperature = map(t, 20, 358, -40, 125);
  int q = analogRead(gasSensor);
  gas = map(q, 157, 600, 0, 100);
 }