I would like to add a "rain detector" to my sprinkler system that would automatically disable the sprinklers. The system uses an old-style mechanical "dumb" timer. How about a simple circuit to do the job?
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You can buy a normally closed SPST switch containing a water absorbing material that expands when wet to open a microswitch. This is placed in between the common side of the valves and the return line. The switch can be adjusted to open after an appropriate amount of rain has fallen. They are sold by most sprinkler manufacturers in home improvement and hardware stores.