MQ-7 Carbon Monoxide Sensor

The MQ-7 Carbon Monoxide Sensor is a low-cost semiconductor gas sensor designed for detecting carbon monoxide (CO) in air. It is suitable for hobby electronics, Arduino projects, ESP32 projects, educational experiments, CO detector prototypes, and general gas-sensing demonstrations.

The sensor element uses tin dioxide (SnO2) as the gas-sensitive material. The sensor resistance changes when carbon monoxide is present, and this resistance change can be converted into a voltage signal using a simple external circuit with a load resistor.

Unlike many other MQ gas sensors, the MQ-7 Carbon Monoxide Sensor uses a cycling heater voltage. The heater is operated alternately at a high voltage and a low voltage. This heating cycle is important for proper carbon monoxide sensing and recovery, and should be considered when designing the circuit or writing microcontroller software.

Download MQ-7 Sensor Datasheet (PDF)

Typical applications:

• Arduino, ESP32, Raspberry Pi, and other microcontroller CO-sensing projects
• Carbon monoxide detector prototypes
• Educational gas sensor demonstrations
• Automotive air-quality and exhaust-related experiments
• DIY alarm and monitoring systems
• Analog sensor experiments and comparator-based alarm circuits
• Portable gas detector prototypes

Helpful information:

• The MQ-7 Carbon Monoxide Sensor is not a digital sensor. The sensing element changes resistance depending on carbon monoxide concentration.
• A load resistor is required to convert the sensor resistance change into a measurable voltage.
• The MQ-7 requires a timed heater cycle: typically 60 seconds at 5V followed by 90 seconds at 1.4V.
• For microcontroller projects, the heater cycle can be controlled with a transistor, MOSFET, or suitable driver circuit.
• The analog output should normally be evaluated at the correct point in the heater cycle, not randomly during warm-up or recovery.
• The sensor requires a long initial preheat period. Readings directly after power-up may drift and should not be treated as calibrated measurements.
• Temperature, humidity, oxygen concentration, airflow, sensor age, and previous gas exposure can affect readings.
• For accurate concentration readings, calibration with a known CO concentration is required.

Basic operating principle:

• The heater provides the required operating temperature for the sensing material.
• The sensor is alternated between a high heater voltage phase and a low heater voltage phase.
• The sensing resistance changes when carbon monoxide is present.
• The external load resistor converts the resistance change into a voltage.
• The output voltage can be connected to an analog input of a microcontroller or to a comparator circuit.

Recommended calibration approach:

• Connect the sensor to the final application circuit.
• Power the sensor and allow it to preheat for at least 48 hours before calibration.
• Use a known carbon monoxide concentration if accurate readings are required.
• The datasheet recommends calibration around 200ppm CO in air.
• A load resistor around 10kΩ is commonly recommended, with adjustment possible depending on the circuit and target alarm point.
• When setting an alarm threshold, consider the influence of temperature and humidity.

Recommended handling and operating precautions:

• Avoid exposure to silicone vapours, silicone adhesives, silicone rubber, putty, or plastics containing silicone. Silicone contamination can permanently reduce sensor sensitivity.
• Avoid highly corrosive gases such as H2S, SOx, Cl2, and HCl.
• Avoid contamination from alkali materials, alkali metal salts, salt spray, halogens, or similar harsh chemicals.
• Do not expose the sensor to water, condensation, freezing conditions, or direct liquid contact.
• Do not apply voltages above the rated values.
• Avoid long-term storage in polluted, humid, or high-temperature environments.
• Avoid strong vibration, shock, or dropping the sensor.
• Do not expose the sensor to very high gas concentrations for extended periods.

Construction details:

Important safety note:

This sensor is intended for electronics projects, prototyping, education, and experimental carbon monoxide detection. It is not a ready-made certified carbon monoxide alarm, workplace safety device, automotive safety device, or life-safety instrument. Carbon monoxide is dangerous and can be deadly. For home, vehicle, workplace, or code-required CO protection, use properly certified carbon monoxide alarm equipment.