I have read a pretty good article about optical rain sensors:
http://www.yoctopuce.com/EN/article/how-to-build-a...

But to do this, it requires too many hard to find components.

So I took advantage of the broken optical mouse to make my own optical rain sensor

Supplies

Step 1: Principle and Experiment

Theory:

Light will enter the horizontal line. It is fully reflective when it meets an inclined surface (intersection between solids and gases).

When I put a drop of water on this point. It will no longer be fully reflective

As shown in the 2nd picture

Teardown:

We will need to get the prisms and LEDs in the optical mouse

As shown in the 1sti picture

Experiment:

I stick the light sensor to the lower part where the light will reach. Attach the LED bulb at the top

As shown in the 3rd picture

Use light sensor DO pin to connect to MCU (arduino, raspberrypi, esp8266 ...)

As in the picture I linked it to Esp8266 and brought it to my home assistant. This type of sensor will not rust like conventional sensors.

You can see more in my video in Vietnamese

or my website