MQ-7 Gas Sensor

MQ-7 Gas Sensor

Detector Alarm Stove Water Heater Boiler Room Garage Car Park Kitchen Generator Caravan Air Quality Measurement LM393

     The MQ-7 Sensor Module is a gas sensor board with analog and digital outputs that detects the carbon monoxide (CO) concentration in the air. The MQ-7 sensor on the board responds to CO in the 10ppm - 1000ppm range; as CO rises the sensor resistance drops, and the board turns this into a rising voltage on the AO pin. It runs on 5V and draws about 150mA including the heater.

     The LM393 comparator on the board compares the analog value with a threshold set by the blue potentiometer; when the threshold is exceeded the DO output goes LOW and the DO LED lights. This way a relay board or buzzer can be driven directly, even without a microcontroller. The power LED, the 4 pin (VCC, GND, DO, AO) 2.54mm header and the screw holes at the corners make it easy to fit the board into an enclosure.

     The sensor has a heater wire inside; it is normal for the sensor to get warm while running. After about 20 seconds of warm-up at each power-on the readings become stable. It is used to monitor CO rise around stoves, water heaters, boilers, garages and generators, to switch ventilation fans and to give warnings.

Key features:

     MQ-7 sensitive to carbon monoxide: The tin dioxide (SnO2) sensing element works in the 10ppm - 1000ppm CO range; the analog output voltage rises as CO increases.

     Dual analog and digital output: The AO pin gives a voltage that changes with concentration; the DO pin gives a TTL signal that goes LOW when the set threshold is exceeded.

     Adjustable alarm threshold: With the LM393 comparator and the potentiometer you set the CO level at which DO triggers; the DO LED shows the trigger.

     Ready board: The heater and load resistor circuit is on the board; it connects directly through the 4 pin header, no extra circuit is needed.

     Easy mounting: The screw holes at the corners fix it to an enclosure or panel; the power LED shows the board is powered.

Features:

• Product type: Carbon monoxide (CO) gas sensor module
• Sensor: MQ-7, tin dioxide (SnO2) sensing element
• Detection range: 10ppm - 1000ppm CO (module seller value; MQ-7 datasheet versions give 20ppm - 2000ppm or 10ppm - 10000ppm)
• Supply voltage: 5V DC
• Current consumption: about 150mA (including heater)
• Heater power: max 350mW in the datasheet (with 5V / 1.4V alternating heating); this board powers the heater continuously from 5V, so it uses more power
• Analog output (AO): voltage rising with CO; about 0.1V - 0.3V in clean air, about 4V at the highest concentration
• Digital output (DO): TTL, LOW when the threshold is exceeded (active low)
• Comparator: LM393, threshold set by potentiometer
• Indicators: Power LED and DO LED
• Connection: 4 pin 2.54mm male header (VCC, GND, DO, AO)
• Heater resistance: about 31Ω - 33Ω (at room temperature; depending on datasheet version)
• Sensing resistance (Rs): 2kΩ - 20kΩ
• Sensitivity: Rs (clean air) / Rs (100ppm CO) at least 5
• Warm-up time: at least 20sec at each power-on; at least 48 hours burn-in before first use (datasheet)
• Operating temperature: about -10°C - 50°C
• Material: Blue PCB, sensor cap with metal mesh
• Mounting holes: Screw holes at the board corners
• Dimensions: about 32mm x 20mm x 15mm
• Weight: about 8gr

Usage information:

• Connect VCC to 5V, GND to ground, AO to an analog input of the controller and DO to a digital input. On 3.3V boards connect both pins through a voltage divider, because AO can reach 4V and DO can reach 5V.
• To set the alarm threshold, warm up the sensor in clean air, then turn the potentiometer so that the DO LED stays off; when CO rises above this level DO goes LOW and the LED lights.
• The board powers the heater continuously from 5V; this is suitable for monitoring CO rise and relative change. The more selective measurement in the MQ-7 datasheet alternates the heater between 5V for 60sec and about 1.4V - 1.5V for 90sec; this needs separate control of the heater supply.
• Leave a new sensor powered for at least 48 hours before first use; evaluate readings after this period.

Areas of use:

•  CO monitoring in rooms with stoves and water heaters
•  CO warning for boilers and boiler rooms
•  Ventilation fan control in garages and indoor car parks
•  CO tracking around kitchens and cookers
•  Exhaust gas monitoring around generators
•  CO monitoring in caravan and boat cabins
•  Air quality measurement and logging stations
•  Alarm circuits driving a relay, buzzer or fan according to CO level
•  Gas detection experiments in school laboratories

Precautions:

•  This board is for experiments and monitoring and does not replace an approved (certified) carbon monoxide alarm. Use an approved CO detector where people live and sleep.
•  The sensor gets warm while running; this is normal, and a sensor that does not warm up is faulty. Do not touch the sensor cap while running and do not place it near flammable materials.
•  The sensor can also react to other gases such as hydrogen and alcohol vapour; a sudden rise is not always caused by CO.
•  Power the board with 5V; with a 3.3V supply the heater does not reach operating temperature and readings are unreliable.
•  Carbon monoxide is odourless and toxic; ventilate the room when testing the sensor with CO.

Package contents:

•  1 pc MQ-7 Sensor Module
•  1 pc MQ-7 gas sensor ( Assembled )
•  Note: Connection cables are not included in the package.