Most are tilting-buckets. A funnel fills one bucket until its weight tilts, it empties, and a magnet then passes by a reed switch. Then the other bucket begins to fill.
Rainwise and others make these. The math has to do with the diameter of the funnel and the tilting point weight - usually set to 0.01 inch of rain per tilt.
http://www.rainwise.com/products/detail … /index.php
I’ve bought sensor-only as above, not a full station.
Lots of under $100 weather stations, e.g., La Crosse, with rain, wind speed/direction, temperature, humidity, indoor/outdoor, all wireless, USB or wireless link to PC. I have two.