I have a friend who says that if you count the number of hours spent on the manufacture of this device, then it is cheaper to buy the most expensive one. Of course, I disagree with him, but I will not dwell on this provocative topic for a long time. Let me just say that in time, the development and manufacture of this device took about 30 hours. I used the parts that I had in stock, including the humidity sensor, only the battery was purchased. For me, such projects are needed in order to improve skills, learn something new, and get a device to my taste.
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Drilling of the board on a CNC machine, programs for drilling and milling are made in FlatCam from Gerber files generated by Eagle. The photomask was printed on a laser printer, soaked in acetone vapor for 20 minutes, after which it became black enough to make a printed circuit board using a photoresist.
In general, the topic of home-made is not very relevant now, but when you need to make one simple board, and you don't want to wait for a parcel from China, this skill helps a lot. I have worked out the method for years, I do it casually, doing other things in parallel, if there is interest, write in the comments, I will write a separate article on this topic.
Files with sources on GitHub