Abstract: A rain detector is provided for preventing operation of an irrigation system upon detection of an adjustably selected amount of rainfall. The rain detector is designed for use with an automatic irrigation control system of the type including a clock controller to electrically operate one or more normally closed valves for programmed supply of irrigation water to sprinklers individually or in selected groups. The rain detector comprises a switching circuit connected electrically in series with the sprinkler valves, with the switching circuit including spaced sensors extending for an adjustable depth into a collection tray exposed to rainfall. The switching circuit is normally closed in the absence of rainfall to permit normal programmed operation of the sprinkler valves.