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This Wiki is a place to keep notes about building this project.

Step 1 - read the blog. Please read the entire blog article before attempting to build this project. It can save a lot of frustration and clarify easier ways of doing things. Here is the link --> https://bitbanksoftware.blogspot.com/2023/04/building-pocket-co2-project.html

Test LED polarity with a partially dead coin cell battery Devices which use coin cell batteries generally stop working when the cell reaches 2.7V. This is still enough voltage to get most LEDs to light. I have a coin cell holder with loose black+red wires just for this purpose. If you don't have this, the diode checker on your average cheap digital multi-meter will work too.

You may not need to solder a programming header on the PCB The 3-pin programming header only needs to make contact for a couple of seconds to program the MCU. I have a cable with a male header and just hold it in place to program the device.

The pushbuttons I used are a great size, but not very sturdy. You can choose other buttons that will fit in the same space, but the "feel" of the buttons I used are quite good. They are also not very strongly held together and can melt or fall apart if stressed too much.

The SCD40 looks difficult to solder, but it's not so bad if you use very little solder on the pads. You can put plenty on the center ground pad for stability, but the actual used contacts have very small gaps, so less is more to not cause shorts under the sensor.

Double check the polarity of the battery cable.There is no official standard, but I used the convention used by Adafruit, Unexpected Maker and a few other 'Arduino' vendors.

I designed (badly) a case in Fusion360, but it doesn't fit very well. I'm not pleased with it, and didn't share it for this reason. You can let me know if I should share it anyway.