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Rain Drop Module
Designed by Mark Little
 
The Rain Drop module uses a pair of interleaved tracks as teh sensor to detect water on the plate. This water can come from either rain drops or condensation. As more water sits between the two tracks, the resistance of the sensor falls proportionally. The circuit board contains an adjustable trigger that will provide a digital output Do when the moisture reduces the resistance to a level where the alarm triggers.

The analogue output Ao will provide a signal whose output level is proportional to the amount of moisture bridging the tracks.
It should be noted that fresh water (rain) is less conductive that sea water, so if the sensor is located near the beach or similar, the levels will alter must faster if the moisture on the sensor comes from salt spray. It is also likely that the initial rain drops will end up being more conductive if the weather has been dry and the sensor is covered with dust, as the water will be more conductive when the dust dissolves.

This type of sensor can be used to determine "leaf wetness" by measuring the analogue output and correlating it to leaf wetness measured by a commercial instrument, or it can be used for uncalibrated measurements.

While the printed circuit board for most rain drop modules is water resistant and the tracks are plated to reduce the likelyhood or corrosion, the connection between the sensor and the wires is often a weak point from a corrosion perspective and the supplied wires are not usually UV resistant.

As a result, it is imperative that the connector is sealed with a weather resistant compound such as neutral cure Silastic™ to protect the connection from corrosion. It is also important that the connecting wires are protected from the Sun's UV radiation. Where the connecting wires enter the case with the electronics, it is also important that this entry is protected by Silastic.