Nice project + write-up from Jamie Bennett up on LinuxUK.
There are a wide range of solutions on the market today to help the avid gardener with their plants, some are low tech battery-less meters designed to give you a simple view of how your plant is doing, others are much more high-tech (and expensive). Two of the most important variables to monitor in a greenhouse are temperature and humidity, too hot and plants start to scorch and wilt, too cold and they can be damaged, and humidity is important for a whole host of plants you may find in a greenhouse.
At home I have a modest greenhouse, at 3 metres by 1.8 meters it has just enough room to start off delicate plants early and keep the family supplied throughout the season with chillies, tomatoes, cucumbers, strawberries, herbs, and even lemons. Although I can just barely make out the digital temperature gauge hanging on the inside of the greenhouse from my office window I thought is would be much better to have a more high-tech monitoring solution. After a bit of research I came across the Xiaomi Temperature and Humidity sensor which can be bought for as little as £9 from several Chinese online retailers. The sensor itself is a small, ZigBee-based device powered by a CR2032 button cell battery and capable of detecting temperatures from -20 to 60 celsius as well as humidity, perfect for this use case. The sensor does also need the Xiaomi gateway to work but I already had one of those. Another constraint is that any device you use to read the data needs to be within ZigBee range which, admittedly is 10s of meters, not a problem here.
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