See the article here: https://www.avdweb.nl/arduino/measurement/rain-detector
My awning has an angle which is too shallow; rain doesn't run off the awning but pools on the awning and stretches the fabric. To prevent this, the awning has to be retracted when it begins to rain, but this is not possible when I'm not home. The rain sensor described here automatically retracts the awning when it begins to rain.
My motorized awning is radio controlled by Somfy. What I needed was a rain sensor that is radio controlled too, in order to avoid additional wiring. Also, the awning must automatically roll up when it gets dark since I do forget this frequently. Such a radio controlled rain sensor does not exist, therefore I have built one myself.