Rain Sensor Tutorial for Arduino, ESP8266 and ESP32 (2024)

Rain Sensors also called raindrop sensors are very handy sensors that are used in a variety of use cases. Alone a rain sensor can only detect if it is raining and how strong it rains but in combination with other electrical devices you can build useful applications.

For me the most useful application is to detect an open window when it starts to rain because I am often not sure if I closed a window after I left the house.

The rain sensor consists of 2 components, which we consider in more detail below:

• Rain drop module to detect if it is raining or not
• Control board to process the data from the rain drop module

The schematic illustration shows the difference between a variety of states for the rain board. With no rain there is no connection between the two copper tracks and the resistance is infinite.

During light rain there are some water drops that land on the surface of the rain board and connect the two conductor tracks. Because water is conductive, the resistance of the rain board decreases resulting in a medium resistance. The same happened when I touched the rain sensor, because my skin is also conductive, connecting both copper tracks.

If there is heavy rain, more and more rain drops falling on the board and further decrease the resistance until the resistance is 0Ω and the circuit is shorted.

Summarizing the rain board: The rain board is a rain depended resistor that has a high resistance when there is no rain and a low resistance when rain drops are on the circuit board. The operating principle is the same as that of a photoresistor.

Now we have to make sure that we can measure the change of the resistance. If your read my resistor tutorial, you know that this can be done with a voltage divider.

The control board consists of two input pins and four output pins. The input pins are connected to the rain board and the output pins to your favorite microcontroller, for example an Arduino Uno or an ESP32 NodeMCU.

On the control board you find multiple resistors that also functions are the voltage divider to provide an analog signal for the rain intensity. Therefore as input we get a resistance from the rain board and the control board converts this resistance into a voltage drop between the analog pin and ground. The microcontroller uses the internal analog to digital converter (ADC) to convert the voltage from the analog pin to a digital value between 0 and 1023 that can be printed to the serial output in your Arduino IDE.

The following pictures show the wiring between the rain drop module and the control board as well as the connection between the control board and different Arduino boards. Because the rain board only connects a resistance to the control board, switching the connection pins does not have any influence.

The control board is powered from the 5V pin of the Arduino microcontroller board. For the analog and digital connection, you can use any digital I/O and analog pin on your board, but if the connection differs from the one in the pictures, make sure that you define the correct pins in the following sections with the program code.

If you are missing an Arduino microcontroller board, please write a comment in the section below this article and I will add more fritzing pictures of the wiring of the rain sensor. You can also enlarge the wiring pictures by clicking on them.

The wiring between different ESP8266 boards and the rain sensor module is shown in the following pictures. For the power supply, I take the VIN and 5V pin from the USB converter, but you can also use the 3.3V pin instead because the control board operates also with a voltage of 3.3V.

The connection between the rain drop module and the control board can be switched in polarity without influencing the functionality, because the rain drop module is nothing else than a resistor for the whole circuit.

If you are missing an ESP8266 microcontroller board, please write a comment in the section below and I will add more fritzing pictures of the wiring of the rain sensor. You can also enlarge the wiring pictures by clicking on them.

The wiring between the rain sensor and the ESP32 microcontroller board is shown in the following picture. Because the ESP32 has multiple analog to digital converter, you can choose several pins for the analog and digital input of the ESP32. I recommend to download myMicrocontroller Datasheet eBook with detailed pinouts that shows every analog and digital pin of multiple Arduino, ESP8266 and ESP32 microcontroller boards.

You can switch the connection between the rain drop module and the control board without influencing the functionality, because the rain drop module behaves like a resistor for the whole circuit.

If you are missing an ESP32 microcontroller board, please write a comment in the section below and I will add more fritzing pictures of the wiring of the rain sensor. You can also enlarge the wiring pictures by clicking on them.

In this example we can use the wiring from the previous chapter and want to print the analog and digital sensor values to the serial monitor. Additionally we use the serial plotter of the Arduino IDE to show the time-series chart of the analog output.

The following sections shows the Arduino program code for this example.

The first step of the Arduino script is to define the analog and the digital pins that connect the microcontroller and the control board. In my case I use the pin A1 for the analog connection and D2 for the digital connection of the Arduino microcontroller as you see in the fritzing sketch of the Arduino Uno.

For the ESP8266 NodeMCU and WeMos D1 Mini and also for the ESP32, the definition of the analog and digital pins differ. They are included in the script but commented. If you use an ESP8266 or ESP32 microcontroller uncomment the lines of code and comment the definition of the Arduino pins.

In the setup function I set the baud rate of the serial connection to 9600 that have to match to the serial output setting of the Arduino IDE to print the values. Also I have to set the digital pin as input because I want to read values at these pin.

In the loop function I create the variable rainAnalogVal as integer and read the analog value from the analog pin with the build in function analogRead(). Then I read also the digital value, with the build in function digitalRead() and store the digital value in the variable rainDigitalVal that has the following values:

• = 0, if it is raining
• = 1, if it is not raining

To view the time series chart of the analog rain sensor we print the analog and digital value to the serial output and with a short delay of 200 milliseconds we pause the script at the end before the loop is executed again.

The following two picture show the serial monitor and the serial plotter of the Arduino IDE. You see clearly that the sensor value drops when I touch the rain board with my dry or wet finger. You can click on each image to enlarge it.

Due to the current flow and the contact with water, the rain sensor will build rust from electrolysis that damages the sensor and makes the sensor inaccurate. How strong the electrolysis will be depends on three different factors:

1. the quality of the rain board,
2. the amount of water on the rain board
3. how often and how much current is passed through electrodes of the rain board

To increase the lifetime of the rain sensor we reduce the operating voltage from 5V to 3.3V and also only provide voltage to the sensor when we want to read the sensor values with the microcontroller. Therefore we change the wiring and do not longer connect the 5V pin of the Arduino microcontroller or the VIN pin of the ESP8266 and ESP32 based microcontroller to the control board but we connect the microcontroller via the 3.3V pin.

Moreover we do not connect the Arduino, ESP8266 or ESP32 microcontroller direct to the rain sensor, but we use a MOSFET to disconnect the rain sensor completely from the circuit when we do not need to read the analog sensor value. I use the IRLZ44N MOSFET because this logic level MOSFET responds also for low voltages between 2.5V or 5V.

The following picture shows the new wiring with an Arduino Uno as example.

From the fritzing picture you see also that I use an 1MΩ resistor to protect the gate to source from electrostatic discharge.

With this change we reduce the operation voltage to 3.3V and with the digital pin we can control when the digital pin is set HIGH to to turn on the rain sensor and when to set the pin LOW to shut down the sensor.

One downside is that the internal analog to digital converter (ADC) of the microcontroller has 5V as reference voltage and now not the full value range is used for the analog signal. You see the reduced maximum analog value in the last picture of this article.

I also build the digital value based on the analog value in the Arduino script and not by the analog pin of the control board. Therefore I do not have to connect the digital pins.

The following lines show the new script and the changes that we have to make.

The first change we make is to define two new variables:

• sensorPower defines the new digital pin that connects the control board to the microcontroller.
• bisRaining is a boolean value that shows if it is raining or not.

The variable for the digital value is deleted in the define block at the beginning of the script and also in the setup function.

In the loop function we turn on the control board by setting the pin of the sensorPower HIGH and let the power settle before we read the analog value. The boolean value is built by a simple if else function. In my example I choose 100 as the threshold but you can choose a different value if you want. After the analog and digital values are calculated we turn off the rain sensor by setting the power pin LOW.

The following picture shows the output of the Arduino script with a maximum analog value of 131 instead of 1023.