Rain sensor

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A rain sensor or rain switch is a switching device actuated by rainfall. There are two main types of rain sensors. The first is a water conservation device connected to an automatic irrigation system that causes the system to shut down in the event of rainfall. The second is a device used to protect the interior of an automobile from rain and to support the automatic mode of windscreen wipers.

[edit] Irrigation Sensors

Rain sensors for irrigation systems are available in both wireless and hard-wired versions, most employing hygroscopic disks that swell in the presence of rain and shrink back down again as they dry out - an electrical switch is in turn depressed or released by the hygroscopic disk stack. However, some electrical type sensors are also marketed that use tipping bucket or conductance type probes to measure rainfall. Wireless and wired versions both use similar mechanisms to temporarily suspend watering by the irrigation controller - specifically they are connected to the irrigation controller's sensor terminals, or are installed in series with the solenoid valve common circuit such that they prevent the opening of any valves when rain has been sensed.

Some irrigation rain sensors also contain a freeze sensor to keep the system from operating in freezing temperatures (typically freeze sensors are employed in regions where irrigation systems are not "blown-out" for the winter, yet there is sometimes a chance of overnight frost).

Some US states, such as Florida, New Jersey, Minnesota, and Connecticut mandate the use of a rain sensor in all new lawn sprinkler systems.

[edit] Automotive Sensors

In 1958, the Cadillac Motor Car Division of General Motors experimented with a water-sensitive switch that triggered various electric motors to close the convertible top and raise the open windows of a specially-built Eldorado Biarritz model, in case of rain.

The first such device appears to have been used by General Motors, for that same purpose, in a concept vehicle designated "Le Sabre" and built seven years earlier, in 1950-51.

Nowadays, rain sensor are integrated in a system to automatically start the windscreen wipers (intelligent windscreen wipers).

Most common rain sensor implementation is based on prinicple of Total internal reflection: an infrared light is beamed at a 45-degree angle into the windshield — if the glass is wet, less light makes it back to the sensor, and the wipers turn on.