Building an Air-Quality Monitor

// Traffic-light air-quality monitor using an MQ-135 sensor.
// Shows RELATIVE air quality vs a clean-air baseline - not a calibrated ppm meter.
// MQ-135 analog out on A0; green LED pin 5, yellow pin 6, red pin 7.

const int MQ135_PIN = A0;
const int GREEN_PIN = 5;
const int YELLOW_PIN = 6;
const int RED_PIN = 7;

int cleanAir = 0;             // baseline, set at startup
const int MODERATE = 80;     // rise above baseline -> moderate
const int POOR = 200;        // rise above baseline -> poor

void setup() {
  pinMode(GREEN_PIN, OUTPUT);
  pinMode(YELLOW_PIN, OUTPUT);
  pinMode(RED_PIN, OUTPUT);
  Serial.begin(9600);

  Serial.println("Warming up MQ-135 (2 min) in clean air...");
  delay(120000);
  cleanAir = analogRead(MQ135_PIN);
  Serial.print("Clean-air baseline: ");
  Serial.println(cleanAir);
}

void loop() {
  int reading = analogRead(MQ135_PIN);
  int rise = reading - cleanAir;
  Serial.println(reading);

  digitalWrite(GREEN_PIN, LOW);
  digitalWrite(YELLOW_PIN, LOW);
  digitalWrite(RED_PIN, LOW);

  if (rise >= POOR) {
    digitalWrite(RED_PIN, HIGH);       // poor
  } else if (rise >= MODERATE) {
    digitalWrite(YELLOW_PIN, HIGH);    // moderate
  } else {
    digitalWrite(GREEN_PIN, HIGH);     // good
  }
  delay(500);
}