Hi,
Simple way to detect rain…
Design a circuit to bias transisitor as a switch. Once its done, cut the wire giving signal to base of transistir and keep two open ends very close to each other and put at the desired location (Please mount sensor properly). When there will be rain, it will come in contact with the broken wire and will provide a conductive path as rain water is a conductive material of water.
In this way you can easily sense the rain.