I remember making something very similar to this a very long time ago! (OK I’m old 
)
The simplest rain drop sensor is a small strip of ‘Veroboard’, or even two strips of un-insulated wire wrapped on a piece of wood works just as well for experimentation purposes. The conductors become a resistor when a droplet of water bridges the gap between them. It’s the effect you get when you drop a cellphone in a puddle, things short out.
So to make a sensor work, all you need is that basic device and some means to detect it,.. a simple op-amp would do that.