Home-Built Electronic Rain Fall Sensor


Introduction

After a having a rain sensor on the weather pod on the roof for a few years I realized that dirt and contamination is an issue with these mechanical sensors.  After a few months of use, dirt would foul up the mechanism, and it would stop working.  As a result, I removed the rain sensor from the rooftop, and decided to move it to ground level, for easier accessibility and maintenance.


The original weather pod on the roof.  The funnel is part of the rain gauge.  Since this
picture was shot, the rain sensor has been removed from the roof.

After I had run the cabling to the new sensor location and tested the sensor, I recognized a design flaw I have ignored for a while.  The old sensor used a tipping spoon to count rain fall.  However, if the rain fall is heavy, the pouring rain prevents the tipped spoon from returning to the "fill" position, and the sensor will stop working.  After some research on the web, I decided to build a new rain sensor using a pair of tipping buckets.  My research also led me to Fascinating Electronic's website, from whom I derived the design of the original tipping spoon rain gauge.  To my surprise, they have also modified their rain gauge to a tipping dual bucket design.


The new rain sensor (right) installed on the pergola in the back yard, next to the
low-tech one for comparison/calibration.  The new sensor can be removed by
unplugging its connector and lifting it from its mount without using any tools.

Design


Diagram of the rain sensor.  Note that the funnel is not shown.  Rain enters the left triangular
bucket filling it until the center of gravity of the whole assembly moves over the pivot point.
The dual buckets then tip, exposing the right bucket for rain collection, and emptying the left one.

The body of the rain sensor is a 2" Schedule 40 PVC "Tee" with a 5" diameter funnel attached to it.  Inside the body is the dual bucket made from aluminum, and an inductive proximity sensor to detect the state of the assembly.  I had several of these sensors in my parts bin, and they have three connections: power, ground, and sensor output.  This type of sensor has proven itself with years of service on the old rain gauge on the roof.  The cable is attached to the Power Monitor Node, and software has been added to increment a counter on every transition (high or low going) of the sensor's output line.  As an alternative to the inductive sensor, you could use many other types of sensors such as a magnet with a small reed switch, or an optical sensor.


The dual bucket collector made from aluminum sheeting.

The critical element in this assembly is the dual-bucket collector.  This is made from aluminum flashing (used in trimming aluminum siding) and bent to the correct shape.  The bottom and sides are one piece, and the divider is slid into a slot cut by tin snips.  The divider is then bent at right angles to form the tabs that stick down.  Two holes are drilled into these tabs as the hinge points.  Epoxy is then used to cement the divider into place, and to provide a waterproof seal for each bucket.  If filled to capacity, each bucket holds about 0.8 cubic inches of water.  I don't expect it to fill completely as at some point the weight will be sufficient to tip the buckets over.


The various parts of the rain gauge.  It takes only a few minutes to go from this stage to complete
assembly and installed for service.  Construction of the various parts took less than one day.

Holes are drilled for the hinge (a thick copper wire) to be slid into the body, and for the mounting of the sensor.  Assembly is straightforward as the design allows easy disassembly for cleaning and repair.


End view of the rain sensor.  Note the inductive sensor on the bottom, and the short pieces of
hose centering the dual bucket collector under the opening of the funnel.  When the buckets tip
toward this end of the Tee, the sensor is contacted.  Note that the copper wire does not touch
the bottom of the rain collector, but is held by the two tabs.  This low surface area is meant to
reduce friction and improve the accuracy of the sensor.

If you need to mount your rain sensor on the top of a post, you can use a PVC "cross" instead of a "Tee".  The additional opening on the bottom will allow an adapter to the post of your choice.

Calibration

As mentioned above, the diameter of the funnel is 5".  This is a surface area of 19.63 in2.  One inch of rainfall is then of course 19.63 in3.  This corresponds to about 322 mL.  When this amount of liquid is slowly poured into the funnel, the rain collector tips 52 times.  This results in a resolution of slightly better than 0.02" of rainfall.  It is quite satisfying to hear the "click" as it tips, and to see the little quantity of water spew from the side ports of the Tee.  Even at the torrential rain rate of 5" an hour, it will take a leisurely 13 seconds between tips.

Long term update

  • 23 July 2003 - Rain Gauge placed into service.  A short rainfall that night shows good agreement with the rain collector from Brookstone.
  • 29 July 2003 - With the help of the Brookstone rain gauge, the amount of rain per tip is adjusted to 0.0152".  This is thus the resolution and sensitivity of the electronic rain gauge featured above.
  • 30 July 2003 - Updated the Home Control Program with an irrigation control feature:
    • Every day at midnight, the day's rainfall is added to an accumulated amount.
    • This amount is decremented by 0.1" per day.
    • If the accumulated amount is greater than zero, irrigation is inhibited for that day.
    This rule will prevent irrigation if more than about three quarters of an inch of rain falls in one week.


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