This is a simple-to-use Carbon Monoxide (CO) sensor, suitable for sensing CO concentrations in the air. The MQ-7 can detect CO-gas concentrations anywhere from 10 to 500ppm.
This sensor has a high sensitivity and fast response time. The sensor's output is an analog resistance. The drive circuit is very simple; all you need to do is power the heater coil with 5V, add a load resistance, and connect the output to an ADC.
This sensor comes in a package similar to our MQ-3 alcohol sensor, and can be used with the breakout board below.
Checking with a multimeter and it does not matter if it's A or B on any of the gas sensors that is connected with the breakout board. If you look at the datasheet, it shows that the pins for A1 and A2 or B1 and B2 are internally connected together, respectively. Also the application circuit shows that the polarity does not matter, just as long as the pins on each side align with the breakout board.
[Hazardous Gas Monitor w/ Particle Photon](https://learn.sparkfun.com/tutorials/hazardous-gas-monitor0
You would need to calibrate the MQ-7 gas sensor and generate an equation that converts the analog values to PPM. To calibrate, you need to use a sample of known concentration in ppm or a CO gas meter. By taking the data points from the known value, this will allow you to create a regression line that is calibrated with the gas sensor. Take a look at the datasheet for more detailed information in regards to calibration.
There are some articles about gas sensors that might be of some use:
Arduino: MQ Gas sensors [ http://playground.arduino.cc/Main/MQGasSensors]
Arduino CO Monitor Using MQ-7 Sensor [ http://www.instructables.com/id/Arduino-CO-Monitor-Using-MQ-7-Sensor/ ]
Air Quality Monitoring [ http://www.howmuchsnow.com/arduino/airquality/ ]*
*Note: The gas sensor in this tutorial is used to sense dust particles.
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