I must admit, I like those Sonoff SNZB-04. They cost close to nothing, they are very small and simple. I have seen a bunch of rain sensor projets using Aqara sensors but those are very bulky.
I decided to see how well a home brew version could work and the form factor is super sweet.
It is worth noticing that such a project could be extended to a lot of sensors as long as you can translate what you measure to a on/off contact.
It would be trivial to make a fire (not smoke) detector for instance, using lead or tin as “fuse”.
I tinned only half of the “rain sensor” (which is simply a dump piece of pcb, w/o any electronic). To see what ages best under the elements.
The sensor is tiltable which phycically adjust the “persistence” of the rain. If you mount it vertical, the rain drops slide and vanish quicker, at the risk of reporting no rain while it is still raining. A flatter mount will “see” rain longer. I am still experimenting to see the best angle.
I made a 3D printed enclosure with no screws to avoid rust. The PETG used for the print also does not disturb the RF signals.