Xiaomi packed a sensor into a box for just a few euros that elsewhere costs about five times as much

Cheap Xiaomi thermometers and hygrometers are at home for almost everyone who has ever played around with Mi Home. The new Mijia Smart Thermometer and Hygrometer 3 Pro adds a third piece of data to temperature and humidity that you almost never see in this price range—the concentration of carbon dioxide.

A traffic light that tells you when to ventilate

The new model measures CO₂ continuously, and its level is displayed via a three-color indicator — green, yellow, red. So you don’t have to decipher ppm numbers; just look at the color.

Why does this matter? In a closed bedroom overnight, the CO₂ concentration commonly rises to a level at which you wake up tired in the morning and struggle to focus during the day. You won’t notice it yourself — carbon dioxide has no smell, and the only way to know is to measure it.

So what sensor is actually inside?

You need to be precise here, because “CO₂ measurement” can mean all sorts of things with cheap devices. Based on available information, it uses a low-power Sensirion sensor that works on the principle of thermal conductivity — meaning it really measures carbon dioxide, not just an estimate calculated from volatile substances, like some of the cheaper “eCO₂” sensors do.

On the other hand, thermal conductivity is a cheaper approach than infrared NDIR sensors you’ll find in professional meters that cost hundreds of euros. It’s enough for guidance like “you should ventilate,” but don’t expect exact lab figures. Still, for under fifteen euros, it’s an unusual set of features.

Bigger display and a year on one battery

The display has grown to 3.6 inches and will show the time, date, day of the week, temperature, humidity, and CO₂ all at once. It’s a standard LCD panel, so it doesn’t glow and uses almost no energy — with a single CR2450 coin battery, the device should last for about a year.

You can attach it magnetically to a wall or simply place it on a shelf. It connects to the ecosystem via Bluetooth Mesh, so to send data to the Xiaomi Home app you need a gateway—without it, it will only show values on the display. With the gateway, you can set up automations, for example turning on an air purifier when values cross a set threshold.

When and for how much

The device is currently running in crowdfunding on Xiaomi Youpin and costs 114 yuan, which is roughly 14 euros. For now, it’s exclusively for the Chinese market, and Xiaomi hasn’t said anything about EU availability.

For this category, though, it’s not bad news—small Mijia sensors usually reach European warehouses quite reliably and typically take a few months. As soon as it appears with verified sellers at a reasonable price, we’ll let you know.

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