Testing the rain sensor

Connecting a Hydreon Infrared Rain Sensor to a ESP8266 (or Arduino or Raspberry Pi)

I’ve been working on a home-brew weather station and was looking into rain sensors when I discovered that you can get infrared (IR) rain detectors. A company in Minnesota sells one called the Hydreon RG-11. They shoot pulses of IR light around a plastic dome and monitor them on the other end. When rain hits the dome, the refraction changes and the pulses received are perturbed. This is nice because it’s very simple and has no moving parts. I figured I’d be able to find a way to read it into my weather station.

The RG-11 rain sensor with a NodeMCU for scale. (It’s actually bigger than I thought)

There are a variety of modes you can use. I wanted early indication of when it started raining (so I could get an alert from Home Assistant to close the windows). But I also wanted some measure of the actual rain intensity. Thus, I decided that a small “tipping bucket” mode would work best for me. In this mode, the sensor just closes a relay whenever it thinks a certain amount of rain has fallen. Here’s a quick demo of it clicking:

Short demo pouring water on the sensor and seeing it click

You can set the sensor to click at various sizes. Light, moderate, heavy, and violent rain rates are defined as follows:

Rain intensitymm/hrin/hr0.01″ clicks/hr0.001″ clicks/hr0.0001″ clicks/hr
Light<2.50.0989.898980
Moderate<7.60.3303003000
Heavy<502.0200200020000
Violent>50>2.0>200>2000>20000
How many clicks you can expect in different rain conditions with the various tipping bucket mode settings.

In testing, I got a zillion clicks out of my little squeeze bottle on the most sensitive setting, so I put it on 0.001″/click. But then I put it out in the misting rain of Seattle and it only clicked like once. So I did the math above and realized that in Seattle, the 0.0001″/click setting is probably the most useful one.

Top rainy days in Seattle, from Seattleweatherblog, data from Sea-Tac.

So, averaged over the rainiest day in Seattle history, I would see one click every 1.7 seconds. Surely there were periods of extra intensity, and the sensor will peak out clicking like crazy in those times. We will see if that becomes a problem.

Here are some basic counts that have been sent to my Home Assistant controller. I set it up to give a nice “drop” sound when it goes above zero to help me remember to close the window.

Readings from the sensor when there very low drizzle going on

I temporarily mounted it with non-waterproof, non-UV resistant cat5 cable and a indoor mounting bracket. Once COVID lets up (or I get a mail-order), I’ll replace these with waterproof stuff and stainless hardware.

 RG-11 rain sensor mounted temporarily on outside of house
Temporary non-waterproof mounting of my RG-11 rain sensor

Making a 12V to 3.3V voltage divider

The sensor needs 12V to operate, but ESP8266 and Raspberry Pis can only handle up to 3.3V on their GPIO digital input pins. I could run a separate 3.3V data line to the relay for signaling, but I decided to be more economical and just use the power line as the signal line as well. Besides, 12V signals will travel over longer distances on cables than 3.3V ones. Of course I have to step 12V down to 3.3V before going into the GPIO. Since the GPIO signal is very low current, this is appropriate.

UPDATE: I was convinced to just run 3.3V in this case rather than a voltage divider because if the ground fails or something else goes wrong, the ESP would see the full 12V and be fried, so it’s just not that safe. But the voltage divider info is still here…

We know the threshold for a HIGH signal in ESP8266 is about 2.5 V so we need a voltage divider that can go from 12 to somewhere between 2.5 V and 3.3 V. I dumped out my collection of resistors and made a spreadsheet with the options:

Which voltage divider to use to step 12V down to 3.3V?
My spreadsheet matrix of resistors showing the voltage I would get in a voltage divider starting from 12V. I had to dig out my dad’s old resistors from Radio Shack in the 1980s before I found a 3.3 kΩ 10kΩ combo that got it down into the 3.3V logic range.

Here’s a quick video showing the output pulses from the voltage divider on my scope:

I will be running outdoor CAT6 cable to it once it’s mounted. I will only need three of the cables: 12V in, ground, and 12V pulses out for signaling the tipping bucket.

The code

And that’s about it. To wire it up to the ESP8266 or Raspberry Pi you just use the same interrupt code that you’d use for, like, a PIR motion sensor or anything. In the code, you just count the number of pulses and divide by a running clock to get a read-out of precipitation per hour, or whatever.

Here are some code snippets to read the pulses in a ESP8266. I also have a software timer that sends reports over MQTT every 30 seconds and resets the count.

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