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I am doing a project with Arduino to find CO2 in an environment. For this purpose, I bought a breakout board with an MG811 sensor on it. I bought it from eBay and it came with no datasheet. I searched on the internet and found the datasheet for the MG811.

With the help of the datasheet, I also found the formula to convert the input voltage to PPM. But later I realized that the device also has an amplifier on it, which is giving me higher voltages. I searched thoroughly and found the original device on Taobao.

Can someone please tell me how can I determine the gain so that I can convert the values I read to original values?

One more question. On the given web link, I also found that this device comes with a pin called TCM and with Google translate, I found that it shows temperature compensated output. Can you please explain me the use of this pin as well? If it is more advantageous for me, I'll use this instead of the normal analog output pin.

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  • The amplifier is there for a reason, why would you want to convert them back to original values? Can't you just work with the values the device is giving you?
    – Len
    Commented Sep 23, 2016 at 14:47
  • @Len what you said is true but I don't know the extremes of my values so that I can draw a new plot and find a new equation. The company who manufactured the device have tested it and gave a graph to me with which I can find proper equations. For this purpose I want to know the gain.
    – Gaurav Mishra
    Commented Sep 23, 2016 at 14:58
  • I assume the values are given in analogue values? If that is the case, you can just use a volt meter to detect which one is changing when the device is exposed to CO² (i.e. from your breath). If you know the gain, just read out the values of the gain and the one the devices gives to the Arduino. Get multiple readings of multiple values, then map the values the Arduino receives to the ones desired or similar to that of the datasheet.
    – Len
    Commented Sep 23, 2016 at 15:06
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    Measure the voltage before the amplifier. Measure the voltage after the amplifier. Divide the second value by the first. There's your gain.
    – Majenko
    Commented Sep 23, 2016 at 15:14
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    @Majenko do it twice to establish offset
    – Pete Kirkham
    Commented Jun 29, 2017 at 11:49

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The MG811 spec gives you the range of measurement, but without the values and connections of the surface mount op amp on the board the only way to determine the voltage output for each CO2 concentration is with a Voltmeter and an already calibrated CO2 PPM meter, which you could perhaps borrow from a lab or university. Once you have the CO2 values and Voltages that correspond, you can build a formula or function to convert the digital values to voltages and then to CO2 concentrations.

The other option is to trace the tracks on the PCB and create a schematic and then post the spec above and the schematic with resistor values included on the electronics Stack Exchange.

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