Monitor Air Quality with Arduino!

Introduction

In this beginner-friendly Arduino project, we’ll build a simple air quality monitor using the MQ135 gas sensor, the DHT11 temperature and humidity sensor, and a 16x2 LCD to display the live data. Whether you’re concerned about indoor air pollution or just curious, this setup is a great starting point.

MQ135

Components Needed

• Arduino Uno (or Nano)

• MQ135 Air Quality Sensor

• DHT11 Temperature and Humidity Sensor

• 16x2 LCD (with I2C module preferred)

• Breadboard + Jumper Wires

• USB cable for programming

Component	Arduino Pin
MQ135 VCC	5V
MQ135 GND	GND
MQ135 A0	A0
DHT11 VCC	5V
DHT11 GND	GND
DHT11 DATA	D2
LCD VCC (I2C)	5V
LCD GND (I2C)	GND
LCD SDA	A4
LCD SCL	A5

Arduino Code

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include <DHT.h>

// Setup LCD
LiquidCrystal_I2C lcd(0x27, 16, 2);

// Setup DHT11
#define DHTPIN 2
#define DHTTYPE DHT11

DHT dht(DHTPIN, DHTTYPE);

// MQ135 analog pin
const int mq135Pin = A0;

void setup() {
  Serial.begin(9600);
  dht.begin();
  lcd.begin();
  lcd.backlight();
}

void loop() {
  float temperature = dht.readTemperature();
  float humidity = dht.readHumidity();
  int airQuality = analogRead(mq135Pin);

  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("T:");
  lcd.print(temperature);
  lcd.print(" H:");
  lcd.print(humidity);

  lcd.setCursor(0, 1);
  lcd.print("Air:");
  lcd.print(airQuality);

  delay(2000);
}

What Do the Readings Mean?

• Air Quality Value: The MQ135 gives a general analog reading, higher values may indicate poor air quality (e.g., CO2, ammonia, benzene).

• Temperature & Humidity: These help correlate air quality with environmental conditions.

Can the MQ135 measure in PPM?

Yes, but it needs calibration. The sensor outputs an analog voltage, which you read via analogRead(). To convert this to PPM, you need to:

1. Determine the sensor resistance (Rs) at a given concentration.

2. Compare Rs to the baseline resistance (Ro) in clean air.

3. Use the logarithmic curve from the datasheet to estimate PPM.

   
   

The datasheet gives you:

• Rs/Ro curves for different gases (e.g., NH₃, CO₂, benzene)

• Rs is the sensor resistance in the target gas

• Ro is the resistance in clean air

The relationship is roughly:

log(Rs/Ro)vs.log(PPM)log(Rs/Ro) vs. log(PPM) log(Rs/Ro)vs.log(PPM)

So for each gas:

• You plot the measured Rs/Ro

• Use the relevant graph to find approximate PPM

Limitations

• You can’t get accurate readings without controlled gas samples for calibration.

• It detects multiple gases, so the PPM reading might not be specific (e.g., high CO₂ could be confused with high NH₃).

• It’s useful for relative changes in air quality, rather than precise readings.

As per the datasheet:

"When target pollution gas exists, the sensor’s conductivity gets higher along with the gas concentration rising. Users can convert the change of conductivity to correspond output signal of gas concentration"

Bonus Tips

• Place the MQ135 away from fans or vents for stable readings.

• For better accuracy, let the MQ135 warm up for a few minutes after powering on.

• Calibrate your sensor values using known good/bad air environments.

As from the community

This is what you could expect:

Air Condition	Analog Value (A0)	Rs/Ro Estimate	Notes
Clean outdoor air	150–200	~1.0	Baseline / calibrate Ro
Indoor average	250–350	0.5–0.8	Poorer air quality
Polluted area / VOCs	400–600+	<0.5	Very poor air
Close to alcohol/gas	700–900+	<< 0.4	Strong response

Here's a cheat sheet if you need it

Sensor Types and what they detect:

Sensor	Detects	Notes
MQ-2	Smoke, LPG, Butane, Propane, Methane, Alcohol, Hydrogen	General gas and smoke sensor, very common
MQ-3	Alcohol, Ethanol, Benzine	Breathalyzer projects, sensitive to alcohol vapors
MQ-4	Methane, Natural Gas	Used in methane/NG leak detectors
MQ-5	LPG, Natural Gas, Town Gas	Wider range gas detector than MQ-4
MQ-6	LPG, Butane, Propane	Good for cooking gas leak detection
MQ-7	Carbon Monoxide (CO)	Needs heating cycles for accurate CO readings
MQ-8	Hydrogen (H₂)	Useful for hydrogen leak detection
MQ-9	Carbon Monoxide (CO), Methane (CH₄), LPG	Dual gas detection, with heating cycles
MQ-135	Air Quality: CO₂, NH₃, Alcohol, Benzene, Smoke, VOCs	Good general indoor air quality sensor
MQ-136	Hydrogen Sulfide (H₂S)	Toxic gas monitoring
MQ-137	Ammonia (NH₃)	Focused on industrial ammonia detection
MQ-138	Benzene, Alcohol, Ammonia, VOCs	High VOC sensitivity, useful for pollution sensors

Maybe try the following as your own exercise and produce the code:

Have fun! Share your findings in the comments.