Library allows to read and calibrate modules based on CO2 sensor.

• VCC - stabilized voltage (5V for TGS4166 and 6V for MG811)
• GND - ground
• AOUT - link to an analog input of Arduino.
• DOUT - digital value indicates whether current concentration exceeds some thretshold value (can be adjusted by potentiometer)
• TCM - temperature compensation (I have no details)

Module wiring in Fritzing

Before calibration it is necessary to refresh the room. Then we treat concentration of co2 as 400 ppm.

Sensor provides a value (voltage between 0 and 5 V) which we need to convert to a concentration value by formula:

c=d^((a-v)/b)

where

• c - co2 concentration in ppm
• d - some constant. 400 in our case.
• v - input from AOUT in millivolts
• a - a parameter which we'll change during calibration. (Initially 1500)
• b - a constant. 600 in our case

In order to calibrate we should modify a value using this formula

a := v + b*(Ln(c1)/Ln(d))

where

c1 - is the current real concentration value. For example for well refreshed room it will be 400 ppm.

Actually we always take d = c1 in such case our calculation will be simpler and we do not need calculate Ln. Then actual adjustment formula is just

a := v + b

If we decide to change c1 then we should change b and d constants using such formulas

• d := c1
• b := 100*Ln(c1)

The module contains 2 potentiometers.

• Left - adjusts threshold of DOUT
• Rigth - hardware sensor calibration

Class constructor accepts one parameter - analog input of arduino. Specify the number of analog input linked to AOUT of the module (A0, A1, A2 etc)

The example of arduino sketch is here.

From here