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You are here: Home > Sprinkler School™ > Parts and Components > Sensors > Installing a rain sensor

A rain sensor determines whether or not enough rainfall has occurred in order to skip an irrigation cycle. The electrical connection between the sensor and your sprinkler system controller is interrupted when a certain amount of rain triggers the sensor. The sensor breaks the electrical connection so that electricity cannot flow to either the sprinkler valves or to a pump start (if your system is on a well without a pressure tank).

Types of rain sensors:

  1. One type breaks the connection by weighing the water in a rainfall collection cup. The problem with this type is that leaves, sticks, or the occasional lizard will find its way into the collection cup and therefore turning off the system.
  2. The next type uses electrodes to determine how much water is in the collection cup. This kind of rain sensor device also has a system problem of collecting things other than rainfall in the collection cup.
  3. The most common type of rain sensor used by professionals is the expansion disk device. This kind of sensor does not use a collection cup; instead rainfall causes cork disks to expand. This device uses a pressure switch to break the electrical connection. These can be adjusted in increments of 1/4" to the desired rain fall setting. This adjustment is usually set to turn off the sprinklers after 1/2" of rain has fallen.

No matter the type of rain sensor type that you choose, all rain sensors offer many advantages over not adding this detector to your automatic sprinkler system. Some of the immediate and long term benefits include:

  • Saves Money. Whether you pay for city water or spend electricity running a pump, the money you save over time will more than pay for the rain sensor.
  • Extends Sprinkler System Life Span. Irrigation systems are made up of moving parts. If the system parts are utilized less frequently (during the rainy season), they last longer.
  • Protects Water Resources. By limiting the over use of your sprinkler system, rain sensors reduce excess water run-off that carries fertilizers and pest control chemicals into our shared water supply.
  • Conserves Water. Less water is wasted when less supplemental water is needed for your turf and in your garden


  1. The most important aspect of installing a rain sensor is where to place it. It should be installed in an area that is unobstructed by trees, roof over hangs, or anything else that might block rain from getting to the sensor. If it is a wired sensor, placement is generally near the sprinkler controller. The wires should be connected inside the controller's valve wiring panel. This allows for easier electrical trouble-shooting of the system as the sensor can be easily disconnected.
  2. If your controller has rain sensor terminals, connect the two sensor wires to the two terminals (usually marked RS.) If not, disconnect the common wire from your controller. Next connect one of the two sensor wires to the common wire using a small wire nut. Then connect the other sensor wire to the common terminal.

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In the last few years, FM wireless rain sensors have become very popular. Although they are more expensive than the wired devices, the ease of installation and increased placement options offset the cost. Most wireless units come with by-pass switches built into the device. Some digital controllers also offer by-pass options for both wired devices and wireless rain sensors.

So the next time you see a sprinkler system running in the rain, you now know it doesn't have to be that way. Rain sensors are affordable solutions to conserving our water, protecting our water resources, and saving you money.



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