I was looking into buying a commercial CO2 sensor (which typically costs over 200 CAD) and came across this site. After comparing products from different vendors, I decided to go with this one because:

1. Good design!
2. A lot of built-in functions.
3. Excellent documentation and flexibility for customization and coding.
4. Battery-powered!

As an embedded systems enthusiast, if you're tired of building your own, you should definitely own this one. It’s on par with any commercial option out there.

Bought for fun with the hope of integrating it with Home Assistant to control the mechanical ventilation.
Shipping was fast and the product securely packaged to avoid damage.

Basic setup was great with a clear 'quick start' guide. The magnetic charging cable is appreciated, makes charging even easier (if even if the battery life is a bit less than I was hoping for). The case is of a high quality and the e-ink display really looks amazing, gives it quite a premium look since the display seems to be of good quality (not much of the classic lingering pixels).

Temperature and humidity sensors seem to align well with the other sensors I have available. I have no way to check the accuracy of the CO2 sensor, but it seems to respond well to things like ventilation, candles, etc etc. It is remarkable to see how quickly CO2 levels can be influenced sometimes.

Integration in Home assistant is not really possible 'out of the box', there are some hints of MQTT in the source code but it is not natively that well supported and on battery mode the Wifi connection also isn't enabled. (it is somewhat easy to just hack the provided source code and the code is pretty easy to work with).
Ideally I'd be able to just make it connect like once an hour to the wifi to update home assistant, but would require quite a bit of tinkering I think, which I haven't gotten around to.

After I got scammed with a 6in1 CO2 HCHO TVOC unit XY-T02 that claims to measure Co2, but actually doesn't as it has just 1 analog sensor for TVOC that is used to determine a fake CO2 value, I did some research and bought the OpenCO2 sensor.

And I am not disappointed. The Co2 values are reasonable, the build quality is good and the epaper display is fantastic. While the XY-T02 just lasted 6h on it's internal battery as the TFt display eats a lot of battery, the OpenCo2 sensor now operates for 8h and hasn't lost 1% battery.

The epaper display is sharp and more convenient than TFT, despite being black and white.

The USB cable that came with it for charging has a magnetic connector to a USB adapter plugged in, which is smart. Way harder to rip out the USB-C jack when the magnetic adapter gives away first.

The Co2 readings are reasonable. When you exhale on it (breath has 5% Co2), the value explodes as it should be. The fake unit does that too, as it measures organic components in the air, and it assumes if the organic components go up, the CO2 is high too. Breath contains a lot of organic components. So the Co2 explodes too on the fake unit. BUT. The fake unit always shows 450.-500ppm CO2 when not being blown at. It's not going up in my small, unvenitilated chamber which isn't plausible. The OpenCO2 sensor actually started at 1100 in my small room, going down to 600 after ventilation. But it's astonishing how fast it went up again to 1300. it took just 4h with windows closed. Which is plausible.

When I put the openCO2 sensor outside, it went down to just 520ppm. I expected 420ppm, the global normal today after humanity burned all that fossile fuels, but I guess living in a city with a lot of cars near a road comes with higher CO2 ppm's.

Will take it on a trip into the Eifel to see what it will report in the woods. Should for shure show 420ppm there.

But again, very reasonable results.

The final test to check if it doesn't just esimate CO2 by using a organic components sensor is putting a perfume bottle next to it.

Perfume, unlike breath, does nothing to the CO2 concentration. While breath has organic components and very high CO2, perfume just releases a lot of organic components. So when I open up perfume next to the fake unit, it shows exploding HCHO TVOC and CO2 values, as they are all determined by 1 analog sensor ouput for organic.

When I open the perfume next to the OpenCo2 sensor, nothing happens, the CO2 stays constant. So it passed the fake Co2 by THOC-Sensor test masterfully.

The temperature value is very accurate, fits the precision thermometer value. +0,2°C, better than the +-0,8°C stated in the datasheet.

Humidity seems to be off the mark though. While no humidity sensor shows the same to another humidity sensor, as inaccuracy is about 3-5% on most units, the open CO2 is about 6-9% off on 2 of my other humiditysensors which are pretty close to each other
The sensor datasheet gives following performance figures:
15 °C – 35 °C, 20 %RH – 65 %RH ± 6 % RH
-10 °C – 60 °C, 0 %RH – 100 %RH ± 9 % RH
Currently showing 62%, the others 54% and 55%, at 24,8°C. So should be in the operating range of +-6% accuracy. But on the wider temperature range, could be as bad as +-9%:
The bresser Temp/hygrounits have +-5% on the whole range, so significantly better.

The reaction to breath is also unexpected. breath is very humid, but the humitity reading doesn't really change. 1% up is not what I would expect from breathing onto it.

So the humidity sensor is not the strength of this unit and could be way off.

But still happy. Really interesting how fast the air gets bad in my small room. Will shurely teach me to ventilate more often.

Cudos to the developer!
Update:
The Firmware is regularily updated and I got a lot of new functions:
5.2 ist really good, different fonts, inverse, History as graph and best of all: A webinterface with an excellent graph of the last 24h.
Really, really good. Very pleased with it.

I have so many air quality sensors my wife says i'm starting a collection. the OpenCO2 Sensor made by David is by far the most fun. i'm like a kid on christmas.

David was very helpful by answering all of my pre-sale questions and he also shipped the sensor very promptly.

It says 3d printed case, but wow it's the best 3d print i've seen. very durable, smooth, and nice quality.

This device's accuracy stands toe to toe with a similar unit that costs 2.5x's the price.

Thank you David! I sent another friend your way from NC. he's gonna love it!

Great sensor, fast shipping, very friendly and helpful contact. Interesting measurement results, especially in the bedroom.

What a genius little thing that is, that e-ink display is amazing, allows for super long battery life.
Super small, absolute bargain for that price and quality of sensor. Reacts quite fast on changing co2 levels.
Can only recommend, hope it will be back in store soon, will buy some for family members.
co2 values in rooms increase much faster then I thought.

Very nice product, very fast shipping, works right away. Would recommend it, might buy more. Suggestion: if you put the LED at the bottom, mid-frame towards the display instead of top left, it will signal the colour and illuminate the display at the same time for night use.

While a bit late to the party, we’re now measuring CO2 at our office thanks to this device. We noticed right away that the air was way worse than we guesstimated and therefore increased the frequency of ventilation. We hope to get MQTT or API functionality in the near future to be able to record the data and maybe trigger some automation, but thats not really needed. The device is reliable so far and very easy to use. Thanks!