Overview

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

TGS4161

MG811

Pinout

• 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

Calibration Theory

General information

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

Concentration calculation

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

Software calibration

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)

Hardware calibration

The module contains 2 potentiometers.

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

Usage

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.

CO2 concentration impact on humans

From here