The RG-11 Rain Sensor is about the size of a tennis ball, and is completely sealed. The RG-11 Rain Gauge senses water hitting its outside surface using beams of infrared light. It uses the same sensing principle used in millions of automotive rain sensing windshield wiper controls. Consequently, it is remarkably inexpensive. The RG-11 is optical-- not mechanical, chemical, or conductive. Consequently, it is far more rugged, sensitive and reliable than any other technology. The sensor is extremely sensitive, and virtually immune to false trips. Yet, it is completely unaffected by jostling and motion. There are no exposed conductors to corrode, and no openings for bugs to crawl into. There is no place for leaves or other debris to collect.