MQ-135 Gas Sensor Guide

Practical Answers Hub Verdict

MQ-135 specifications at a glance

Exact values can vary between the bare sensor, the module circuit and different manufacturers. Always check the markings and circuit on your own board.

What is an MQ-135?

What can it detect?

The MQ-135 responds broadly to several gases and vapours. A high reading does not identify which substance caused it.

MQ-135 setup and real-world test video

Watch the complete practical guide covering Arduino wiring, protected ESP32 wiring, warm-up behaviour, calibration basics and real-world testing with breath, IPA, perfume, smoke and outdoor air.

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Beginner Arduino code: read the raw analogue value

const int MQ135_PIN = A0;

void setup() {
  Serial.begin(9600);
  Serial.println("MQ-135 raw reading");
}

void loop() {
  int rawValue = analogRead(MQ135_PIN);

  Serial.print("ADC: ");
  Serial.println(rawValue);

  delay(1000);
}

On a typical Arduino Uno, the result is between 0 and 1023. A changing number means the module is responding, but it does not yet tell you the identity or concentration of a gas.

Beginner ESP32 code: protected analogue input

const int MQ135_PIN = 34;   // Example for classic ESP32 only

void setup() {
  Serial.begin(115200);

  analogReadResolution(12);
  analogSetPinAttenuation(MQ135_PIN, ADC_11db);

  Serial.println("MQ-135 protected ESP32 reading");
}

void loop() {
  int rawValue = analogRead(MQ135_PIN);

  Serial.print("ADC: ");
  Serial.println(rawValue);

  delay(1000);
}

This example assumes AO is connected through the resistor divider described above. Do not use the code as permission to connect a 5 V signal directly.

Advanced calculation: sensor resistance Rs

The common module forms a voltage divider using the sensing resistance and a load resistance. If you know the load resistance and the voltage at AO, you can estimate Rs:

Where:

• Rs is the sensor resistance.
• RL is the module’s load resistance.
• Vc is the sensing circuit supply.
• Vout is the measured analogue output before any external divider.

Do not assume every breakout board uses the same RL. Measure or inspect the resistor on your actual module. When using the ESP32 divider, reconstruct the original AO voltage before applying the equation.

Warm-up, burn-in and calibration

For a simple beginner project, record a stable baseline in the same room and observe relative changes. For advanced concentration estimates, control temperature, humidity, airflow, reference gas and exposure time.

Planned Practical Answers Hub test suite

We will log one reading per second and compare the sensor’s baseline, peak response, recovery time and repeatability. These tests demonstrate response, not certified gas concentration.

Common problems

Practical Answers Hub Tested

The example numbers below are placeholders. Replace them with your own measured results before publication.

Replace these values with your own data before presenting them as PAH test results.

How does it compare?

Downloads

References and further reading

• Winsen MQ135 semiconductor gas sensor manual
• Espressif ESP32 Series Datasheet
• Espressif ESP32 ADC documentation