MQ-135 Air Quality Sensor Module - Ammonia, Benzene, Smoke & CO2 Detector

Specifications

• Target Gases: Ammonia, Sulfide, Benzene, Smoke, CO2 (Relative).

• Detection Range:

  • Ammonia: 10ppm – 300ppm.

  • Benzene: 10ppm – 1000ppm.

  • Alcohol: 10ppm – 300ppm.

• Operating Voltage: 5V DC ±0.1.

• Heater Consumption: ~150mA (750mW).

• Output Format:

  • Analogue (A0): Voltage increases as air quality worsens.

  • Digital (D0): Logic High/Low trigger based on adjustable threshold.

• Preheat Time: 24-48 hours (for precise calibration), 5 mins (for functional testing).

• Dimensions: 32mm x 20mm x 22mm.

Compatibility & Platform Support

• Arduino (Uno/Nano/Mega): Directly Compatible. The 5V logic matches perfectly.

• ESP32 / ESP8266: ⚠️ Caution Required. The Analog output can reach up to 4V in very polluted air. You MUST use a voltage divider (e.g., 10kΩ & 20kΩ resistors) to drop the signal below 3.3V to avoid damaging the ESP32 ADC.

• Raspberry Pi: ⚠️ Requires ADC. The Pi cannot read analogue voltages. Use an external ADC chip (MCP3008) or the Digital (D0) pin for a simple "Bad Air" alarm.

Key Applications & Projects

• Desktop Air Quality Monitor: Combine with an LCD and a DHT11 temperature sensor to display a full "Room Health" score.

• Smart Ventilation System: Automatically turn on an exhaust fan when the air quality dips below a certain threshold.

• Intelligent Bathroom Fan: Detects ammonia/bad odours and triggers ventilation.

• Pollution Logger: Connect to an ESP32 and log AQI data to the cloud (Thingspeak/Blynk) to track daily trends.

Project Evolution

• The Reality Check: The MQ-135 is great for detecting changes in air quality, but it is not a calibrated scientific instrument.

• The Upgrade: If you need precise PM2.5 (Dust) readings, upgrade to a laser sensor like the SDS011. If you need precise CO2 PPM for greenhouse work, upgrade to an NDIR sensor like the MH-Z19.

• The "Smart" Move: Use the MQ-135 as a general "Trigger" to wake up more expensive, power-hungry sensors only when the air gets dirty.

Virtual Technician: Wiring & Safety ⚠️

• VCC: Connect to 5V (Stable source required).

• GND: Connect to Common Ground.

• D0: Connect to Digital Pin (for simple limit detection).

• A0: Connect to Analog Pin (for AQI measurement).

• Burn-In Period: When you first power it, the readings will be high and erratic. This is normal. The sensor needs to run for at least 24 hours to "burn off" oxidation and stabilize.

Accessories

• Included in the Package: 1 x MQ-135 Air Quality Sensor Module

• Required Accessories (Not Included):

  • Controller: Arduino Uno or ESP32 (with voltage divider).

  • Power: 5V 1A Adapter (USB power is okay, but wall power is better for long-term stability).

  • Wires: Female-to-Male Jumper Wires.

  • Display: 0.96" OLED or 16x2 LCD (to show the air quality values).

Frequently Asked Questions (FAQ)

• Can this sensor measure CO2 in PPM accurately? Not accurately. It detects CO2 along with other gases. For precise CO2 measurement (e.g., for plants), use an NDIR sensor like MH-Z19.
• What does the output voltage mean? Higher Voltage = More Pollutants. It's a relative scale. You need to calibrate it against "fresh air" to define what "clean" looks like.
• Why is the sensor getting hot? It contains a heater coil that must reach ~40°C to function. This is perfectly normal.
• How long does it take to warm up? For a quick demo: 5 minutes. For a reliable project: 24 hours of continuous power.
• Does it detect dust (PM2.5)? No. It detects gases. It cannot "see" dust particles. Use a PM sensor (like SDS011) for dust.
• Can I use it outdoors? Yes, but protect it from water and direct wind. Temperature and humidity changes will affect the readings.
• How do I calibrate it? Take the sensor outside in fresh air. Measure the resistance (Ro). Use this as your baseline in the code.
• Can I use a 9V battery? No. 9V will destroy the heater. Use a 5V regulator.
• Why is the LED always ON? The sensitivity potentiometer is set too high. Turn it counter-clockwise until the LED turns off.
• Is it sensitive to perfume/cologne? Yes! It is very sensitive to alcohol and volatile organic compounds (VOCs) found in perfumes.
• Can I use it with NodeMCU? Yes, but use a voltage divider (10k/20k) on the A0 pin because NodeMCU typically only handles up to 3.3V (or 1V on some models).
• What is the lifespan? Approx 4-5 years in clean environments.
• Does humidity affect the reading? Yes, high humidity can change the sensor's conductivity. Advanced codes use temperature/humidity compensation formulas.
• Can it detect mold? It can detect the VOCs (smells) released by mold, but not the mold spores themselves.
• What is the "Zero" value? There is no absolute zero. There is always some resistance. You measure relative to the "Fresh Air" baseline.
• Is it safe to leave on 24/7? Yes, it is designed for continuous monitoring.
• Can I use it to detect natural gas leaks? It will react, but MQ-4 is much better and safer for that specific purpose.
• Why do readings jump when I breathe on it? Your breath contains CO2 and humidity, both of which trigger the sensor.
• What library should I use? The "MQ135" library by GeorgK is the standard choice for Arduino users.
• Is this sensor RoHS compliant? Most hobby modules are, but check the specific batch details if required for industrial use.