This library implements an MQ7 class with the following features:
- device calibration
- read carbon monoxide parts per million (ppm)
After downloading the library, load the MQ7_Example.ino example sketch onto your device, with
- analog out connected to A2
- GND connected to GND
- Vin connected to 5V
Upload the sketch, and then monitor serial. The example sketch will calibrate the device over 15 seconds, and then print the carbon monoxide ppm once every second to the serial console.
In the MQ7 Data sheet is provided the typical behaviour of the device. The carbon monoxide ppm is a function of the surface resistance divided by a reference resistance, following the general exponential equation
NB: the plot in the data sheet has reversed axes.
The reference resistance is defined by the constraint
Using this point, and PPM(0.4) ~ 400, one can calculate the coefficients of fit with
or
which are defined in MQ7.h. The library then calibrates R0 by continuously recalculating the value every second for CALIBRATION_SECONDS (TODO: until delta based condition met). As such, this calibration function must be called in setup() to avoid faulty data.
The R0 is calculated by assuming clean air (approx 10 ppm of carbon monoxide) -- it is therefore best to perform this calibration outside. You can read in the value calculated for R0 using the getR0 function.
Alternatively, as the data sheet recommends, the device should be calibrated after ambient on-time of 48 hours -- thus, it would be recommended for accurate use, to leave the device on for some hours in clean air, and then call calibrate() and getR0 to read in an accurate calibration value (though in practice I do not know if this mitigates errors enough to be worth the effort).
In MQ7.h, uncomment the line
#define _R0 0.025 and put in your R0 value. Doing this will override the calibrate() method to do nothing, so there would be no need to call it under these conditions.