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Notes
1.
Confirm the polarity and the voltage of the power supply before powering ON the sensor.
2.
The analog output of this module is non-linear. When connect to microcontroller, it should be connected to ADC interface of the microcontroller.
3.
The relationship between the resistance ratio and the concentration is a logarithmic relationship. For more info on typical sensitivity curve, check
MQ135 Probe on internet.
Typical Sensitivity Curve
The ordinate in the figure is the resistance ratio (Rs/Ro) of the sensor, and the abscissa is the gas concentration
Rs: The resistance value of the sensor in different concentrations of gas
R0: The resistance value of the sensor in 100 ppm ammonia gas
The calculation of the resistance ratio according to the output voltage is as follows:
The sensor is energized in relatively clean air for more than half an hour and after measure the output voltage value of Aout.
VCC is the power supply voltage, 5 VDC
RL is the load resistance
VRL = Aout = Vair value
When the sensor detects gas, the RS value can be calculated after measuring the value of Aout.
The VRL value at this time is the Aout value after gas is detected. It is different from the value of the above formula. The VCC value and the RL value
are the same as above.
The R0 and RS values are obtained, and the specific concentration value of the gas is converted by the RS / R0 value of the Sensitivity Characteristic
Curve.
3
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