DIY Connected Rain Gauge

Building a low-cost rain gauge connected to Home Assistant. A project I’ve had in place since 2021.

Introduction

With the goal of optimizing my garden watering, I wanted to know the amount of rainfall on my garden. Indeed, weather reports are not necessarily accurate depending on your distance from the sensors.

I looked at what was available commercially and found the Netatmo Weather Station. It seemed perfect as it includes all the sensors one could want: an indoor temperature sensor, an outdoor one, a rain gauge, and even an anemometer. But this station is particularly expensive (around €200 for the base station, €80 for the rain gauge extension, €100 for the anemometer module).

I therefore decided to turn to less expensive solutions, and after several searches, I came across this ingenious solution. There are several versions of this tutorial online, some involving soldering. Here I present the simple solution that I implemented at home.

Materials

• This rain gauge model
• An Aqara door sensor

That’s all you need to complete this project ;-)

Assembly

First, remove the rain gauge cover by pressing the two tabs at the base.

Next, unclip the cover containing the magnetic sensor in the plastic column and remove the small board. You can cut the wire to remove the assembly, as we won’t be reusing any of it.

Now for the door sensor. Disassemble the door sensor by starting with removing the cover. There is a notch to access the battery.

Finally, to keep everything in position, I suggest simply stuffing the cavity with cardboard. Moisture is not supposed to enter this part. Close the column cover, then put the rain gauge cover back on.

Home Assistant

If you use Zigbee2mqtt, I recommend checking the “Retain” box in the parameters of the newly added Aqara sensor. Take the opportunity to rename your sensor, for example to “rain_gauge”.

Creating Virtual Entities

We will create a counter that will increment each time the rain gauge’s tipping bucket activates the door sensor. Go to the Home Assistant settings, then “Devices & Services” and finally to the “Helpers” tab.

Create a counter type entry, and name it for example “rain_gauge_tips”.

Creating Automations

For the counter to increment, you need to create an automation that will activate with each tip of the rain gauge. In the Home Assistant settings, go to the “Automations & Scenes” tab, then to the “Automations” tab. Create a new automation from scratch. In the “When” block, add a trigger, choose “Entity”, then “State”. Select the “rain_gauge” entity (its name and type should be similar to binary_sensor.rain_gauge). In the “And if” block, don’t add anything. In the “Then do” block, search for “counter” and select “Counter: Increment”. Then click on the “Choose entity” button in the “Targets” line. Select the newly created “rain_gauge_tips” counter entity. Save the automation under a descriptive name, for example “Rain gauge counting”.

Now create a second automation following the same procedure. In the “When” block, add a “Time & Location” trigger, then choose “Time”, and finally set it to midnight.

Now we have a rain gauge tip counter that increments with each tip and resets at midnight. But how do we convert this to rainfall amount?

Interpreting Pulses

Let’s do some math (nothing too difficult, I assure you). The collection surface of our rain gauge is 5 cm by 11 cm, giving a total surface area of:

\[S = 5 \text{ cm} \times 11 \text{ cm} = 55 \text{ cm}^2\]

Relationship Between Collection Surface and Water Volume

When it rains 1 mm, it means the water height is 0.1 cm over the entire surface. The volume of water collected is therefore:

\[V = S \times h = 55 \text{ cm}^2 \times 0.1 \text{ cm} = 5.5 \text{ cm}^3 = 5.5 \text{ mL}\]

Thus, 1 mm of rain corresponds to 5.5 mL of water in our collector.

Calibration of Pulses

If we pour 10 mL of water into the collector, we can observe that the sensor will generate 6 pulses, which gives:

\[\frac{10 \text{ mL}}{6 \text{ pulses}} = 1.67 \text{ mL/pulse}\]

Conversion Factors

To convert pulses to rainfall height, we calculate:

\[\text{Pulses per mm} = \frac{5.5 \text{ mL}}{1.67 \text{ mL/pulse}} = 3.3 \text{ pulses/mm}\]

Conversely, to convert pulses to rainfall height:

\[\text{mm per pulse} = \frac{1}{3.3} = 0.30303 \text{ mm/pulse}\]

This conversion factor (0.30303 mm/pulse) is the value we will use in our code to transform the counted pulses into precipitation measurement.

Creating the Sensor

I suggest creating a new entity using the Home Assistant template integration.

In your Home Assistant yaml configuration file, add the following code:

- platform: template
  sensors:
    todays_rainfall:
      friendly_name: Today's Rain
      unit_of_measurement: mm
      value_template: >-
        {% set count = states('counter.rain_gauge_tips') | int %}
        {% set mm_per_pulse = 0.30303 %}
        {% set mm = count * mm_per_pulse %}
        {{ mm|round(1, 'floor') }}        
      availability_template: >
        {% if is_state("counter.rain_gauge_tips", "unavailable") %}
          false
        {% else %}
          true
        {% endif %}        

We now have an entity called sensor.todays_rainfall that contains the value of the amount of rain that has fallen during the day.

Weekly Display

Here’s the code for a card to display the evolution of the amount of rain that has fallen during the week:

type: custom:mini-graph-card
icon: mdi:weather-rainy
aggregate_func: max
hours_to_show: 168
group_by: date
show:
  graph: bar
entities:
  - entity: sensor.todays_rainfall
    color: aqua

Note the dependency on the mini-graph-card. Remember to add it to your Home Assistant beforehand.

Conclusion

I hope this tutorial has been useful. I’ve tried to gather here the information I found at the time to offer a simple and economical solution. Feel free to recreate it at your home, and remember to place the rain gauge flat towards a clear area of sky.