Optical Rain Sensor RG-11 Review

Dreaming about a little device capable of measuring rainfall or closing a skylight when it starts raining? Recently I found an amazing Optical Rain Gauge introduced by Hydreon Corporation. The RG11 Optical Rain Gauge is a novel, futuristic and relatively inexpensive optical rain sensor good for hobbyists and experimentalists, including those who need a sensor to trigger some electromechanical action when it starts to rain, as well as for more traditional weathermonitoring applications. The RG11 senses water hitting its outside surface using beams of infrared light. It uses the same sensing principle used in millions of automotive rain sensing windshield wiper controls, and is suitable for almost any application that requires a reliable and sensitive rain sensor, including condensation sensing, irrigation control, drop detection, and wiper control.

rg-11 rain dome

System Description

The sensor consists of a clear hemispherical dome Within which a set of four light beams are reflected and constantly monitored. The dome also covers a circular circuit board, both attached to a sturdy grey plastic mounting bracket. When rain or some other cause of water start to form on the outside of the dome, the beam reflection pattern changes in a subtle yet detailed way which can be translated by onboard circuitry and firmware into a carefully processed output signal. As the water droplets run off or evaporate or are replaced by fresh droplets so the reflection pattern changes and is continuously monitored, with outputs triggered accordingly. The sensor output is provided by contact closure of a builtin miniature relay with low-voltage contacts. The output can be connected to a low voltage circuit (such as a counter module) for further processing or, alternatively, used to control some external device directly.

The RG-11 sensor does need a source of external power for operation, although there is also a slightly less sensitive ‘micropower’ mode that can be set which should allow operation from a low voltage battery at a minimal current draw. Further, the sensor can be set to operate in six different regimes, as detailed below. Each regime corresponds to a different closure pattern of the relay contacts (number and timing of closure pulses) in response to the rain/water droplets sensed. These various modes of operation are programmed by the DIP switch positions to the left of the main RG11 circuit board (see the next figure).

programming dip switch

  • Mode 0 – Tipping Bucket Rain Gauge Mode: Tipping increments of 0.2, 0.01 and even 0.001mm and inch equivalents are all possible
  • Mode 1 – Is it raining: Sensor with variable sensitivity to rainfall rate
  • Mode 2 – Condensation sensor
  • Mode 3 – Wiper control
  • Mode 4 – Irrigation control
  • Mode 5 – Drop mode: Effectively a raw data mode where the contact closures pass through the drop detection signals and allow the raw drop data to be processed externally

You must set the DIP switches so that the RG-11 sensor behaves the right way for your application. Set the DIP switches for the application according to the instructions given in the RG-11 instruction manual. Generally, a few switch positions (5, 6, and 7) set the overall mode of operation, and others (1, 2, 3, 4) adjust the behavior within the modes. Also note that the relay output of the RG-11 is rated for 24V AC/DC, at 1 A. Thus, the RG-11 cannot drive a line voltage (120/240 VAC) load without an external relay. Similarly, RG-11 requires a low voltage AC/DC supply (9 – 30VDC or 9 – 26 VAC ) and never directly powered it from the 120/240 VAC grid.

Spark For DIYers

For a simple project (such as a rain alarm) you need only a few additional parts. Do make a 4-wire connection to the RG11 using the PWR1, PWR2, NO, COM connectors. For this type of application, a 12 Volt DC power supply can used. Connect the power supply wires to PWR1 – PWR2 screw terminals, and route the power to an external load (for example a piezo-sounder) via the relay contacts NO – COM.

rg-11 connections

In the figure, RG-11 is wired in “it’s raining mode”. Here, SW5 – in the DIP switch – sets the mode to “it’s raining”, and SW3 extends the monostable output for 15 mintues. With this configuration, RG-11 turns on the relay (and hence the connected load) to indicate that it is raining when the rainfall has reached a given intensity. To prevent the load from turning constantly on and off , the “monostable extend” option is enabled to hold the output in on state for 15 minutes after the rain has ceased.

Fortunately, the RG-11 can be powered by an external regulated 5V supply via its J2 connector. This can be very useful in microcontroller projects/applications. However, do not subject the RG-11 to over 5.5V or you could damage it. J2 is the “program/communication” connector reserved for programming, development, and testing of the RG-11. Pin 1 of J2 is the Ground (GND/0V) terminal, and pin 2 of J2 is the power (VCC /+5V) terminal.

rg-11 top circuit board

(6-pin J2 connector/header soldered at the right side of the RG-11 circuit board)

rg-11 bottom circuit board.png

(bottom side of the RG-11 circuit board)

No doubt, there may well be other applications for which the RG-11 sensor is suitable. The RG-11is a new idea-spark for us and we have almost certainly not yet identified all potential applications. Opening the sensor up gives you a better idea of how it works as you can see the infrared transmitters/receivers and how they sit inside the optics. Removing the circuit board and flipping it over presents the connections you need for whatever device you are controlling/recording, DIP switches to customize how you want the sensor operate and some communication pins. I think, this is the right time to get up with novel ideas for exploiting this marvel device!

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