Adafruit stocks a large number of Temperature/Humidity sensors, so you're probably wondering why another one? Well, this Adafruit HDC3021 Precision Temperature & Humidity Sensor breakout features the highest accuracy & precision one we've see so far!

It uses the TI HDC302x series chip with typical 0.5% accuracy for the RH sensor (with 0.19% long-term drift) and ±0.1°C typical accuracy for the temperature sensor. All at a very nice price.

This sensor has a true I2C interface for easy interfacing with only two wires (plus power and ground!). That makes it preferable to the janky DHT11/DHT22 sensors.

 The sensor is 5V safe, so you can use 1.8V to 5V power and logic levels without the need for shifters, so you can power and communicate with it using any microcontroller or microcomputer.

With 4 selectable I2C addresses, you can have multiple sensors on one bus - unlike many other fixed-address T/RH sensors.

We've written full-featured Arduino and CircuitPython / Python libraries, so whether you have an 8-bit microcontroller or a quad-core Raspberry Pi, you will be able to quickly get data from the sensor for your environmental monitoring needs. The libraries can setup the sensor rate/power, heater, alerts, offsets, and read data or NIST tracking ID.

Such a lovely chip - so we spun up a breakout board with the HDC3021 and some supporting circuitry such as pullup resistors and capacitors. To make things even easier, we've included SparkFun Qwiic compatible STEMMA QT connectors for the I2C bus so you don't even need to solder! QT Cable is not included, but we have a variety in the shop.

If you prefer working on a breadboard, each order comes with one fully assembled and tested PCB breakout and a small piece of header. You'll need to solder the header onto the PCB, but it's fairly easy and takes only a few minutes even for a beginner.

Please note: this sensor comes with a protective polyamide sticker on top, please remove before usage!

The default I2C address is 0x44.

Power Pins

• VIN - this is the power pin. To power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V micro like Arduino, use 5V.
• GND - common ground for power and logic.

I2C Logic Pins

• SCL - I2C clock pin, connect to your microcontroller's I2C clock line. This pin can use 3-5V logic, and there's a 10K pullup on this pin.
• SDA - I2C data pin, connect to your microcontroller's I2C data line. This pin can use 3-5V logic, and there's a 10K pullup on this pin.
• STEMMA QT - These connectors allow you to connect to dev boards with STEMMA QT (Qwiic) connectors or to other things with various associated accessories.

Other Pins

• RST - This is the reset pin. If this pin is pulled low, it will reset the sensor.
• ALERT - This is the interrupt pin. It is a push-pull output, meaning that you can monitor the state of this pin for interrupts that can be configured over I2C.

I2C Address Jumpers

On the back of the board are two address jumpers, labeled A0 and A1, between the STEMMA QT logo and cutouts for the HDC3021 sensor. These jumpers allow you to chain up to 4 of these boards on the same pair of I2C clock and data pins. To do so, you solder the jumpers "closed" by connecting the two pads.

The default I2C address is 0x44. The other address options can be calculated by “adding” the A0/A1 to the base of 0x44.

A0 sets the lowest bit with a value of 1 and A1 sets the next bit with a value of 2. The final address is 0x44 + A1 + A0 which would be 0x47.

If only A0 is soldered closed, the address is 0x44 + 1 = 0x45

If only A1 is soldered closed, the address is 0x44 + 2 = 0x46

The table below shows all possible addresses, and whether the pin(s) should be high (closed) or low (open).

Power LED and Jumper

• Power LED - In the upper left corner, above the STEMMA connector, on the front of the board, is the power LED, labeled on. It is a green LED.
• LED jumper - This jumper is located on the back of the board and is labeled LED on the board silk. Cut the trace on this jumper to cut power to the "on" LED.

It's easy to use the HDC3021 with Python or CircuitPython, and the Adafruit_CircuitPython_HDC302x module. This module allows you to easily write Python code to read temperature and humidity data from the sensor.

You can use this driver with any CircuitPython microcontroller board or with a computer that has GPIO and Python thanks to Adafruit_Blinka, our CircuitPython-for-Python compatibility library.

CircuitPython Microcontroller Wiring

First wire up the sensor to your board exactly as follows. The following is the sensor wired to a Feather RP2040 using the STEMMA connector:

The following is the sensor wired to a Feather RP2040 using a solderless breadboard:

Python Computer Wiring

Since there are dozens of Linux computers/boards you can use, we will show wiring for Raspberry Pi. For other platforms, please visit the guide for CircuitPython on Linux to see whether your platform is supported.

Here's the Raspberry Pi wired with I2C using the STEMMA connector:

Here's the Raspberry Pi wired with I2C using a solderless breadboard:

Python Installation of HDC302x Library

You'll need to install the Adafruit_Blinka library that provides the CircuitPython support in Python. This may also require enabling I2C on your platform and verifying you are running Python 3. Since each platform is a little different, and Linux changes often, please visit the CircuitPython on Linux guide to get your computer ready!

Once that's done, from your command line run the following command:

• pip3 install adafruit-circuitpython-hdc302x

If your default Python is version 3 you may need to run 'pip' instead. Just make sure you aren't trying to use CircuitPython on Python 2.x, it isn't supported!

CircuitPython Usage

To use with CircuitPython, you need to first install the Adafruit_CircuitPython_HDC302x library, and its dependencies, into the lib folder on your CIRCUITPY drive. Then you need to update code.py with the example script.

Thankfully, we can do this in one go. In the example below, click the Download Project Bundle button below to download the necessary libraries and the code.py file in a zip file. Extract the contents of the zip file, and copy the entire lib folder and the code.py file to your CIRCUITPY drive.

Your CIRCUITPY/lib folder should contain the following folder and file:

• adafruit_bus_device/
• adafruit_hdc302x.mpy

Python Usage

Once you have the library pip3 installed on your computer, copy or download the following example to your computer, and run the following, replacing code.py with whatever you named the file:

python3 code.py

Example Code

If running CircuitPython: Once everything is saved to the CIRCUITPY drive, connect to the serial console to see the data printed out!

If running Python: The console output will appear wherever you are running Python.

# SPDX-FileCopyrightText: 2024 Liz Clark for Adafruit Industries
#
# SPDX-License-Identifier: MIT

"""HDC302x simple test"""

import time

import board

import adafruit_hdc302x

i2c = board.I2C()
sensor = adafruit_hdc302x.HDC302x(i2c)

while True:
    print(f"Temperature: {sensor.temperature:0.1f}°C")
    print(f"Relative Humidity: {sensor.relative_humidity:0.1f}%")
    print()
    time.sleep(2)

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# SPDX-FileCopyrightText: 2024 Liz Clark for Adafruit Industries
#
# SPDX-License-Identifier: MIT

"""HDC302x simple test"""

import time

import board

import adafruit_hdc302x

i2c = board.I2C()
sensor = adafruit_hdc302x.HDC302x(i2c)

while True:
    print(f"Temperature: {sensor.temperature:0.1f}°C")
    print(f"Relative Humidity: {sensor.relative_humidity:0.1f}%")
    print()
    time.sleep(2)

The HDC3021 is instantiated over I2C. Then in the loop, the temperature and humidity readings are printed to the serial monitor every two seconds.

Using the HDC3021 breakout with Arduino involves wiring up the breakout to your Arduino-compatible microcontroller, installing the Adafruit_HDC302x library, and running the provided example code.

Wiring

Wire as shown for a 5V board like an Uno. If you are using a 3V board, like an Adafruit Feather, wire the board's 3V pin to the breakout VIN.

Here is an Adafruit Metro wired up to the sensor using the STEMMA QT connector.

Here is an Adafruit Metro wired up using a solderless breadboard:

Library Installation

You can install the Adafruit_HDC302x library for Arduino using the Library Manager in the Arduino IDE.

Click the Manage Libraries ... menu item, search for Adafruit_HDC302x, and select the Adafruit HDC302x library:

If asked about dependencies, click "Install all".

If the "Dependencies" window does not come up, then you already have the dependencies installed.

Example Code

#include <Adafruit_HDC302x.h>

Adafruit_HDC302x hdc = Adafruit_HDC302x();

void setup() {
  Serial.begin(115200);

  Serial.println("Adafruit HDC302x Simple Test");

  if (! hdc.begin(0x44, &Wire)) {
    Serial.println("Could not find sensor?");
    while (1);
  }
  delay(1000);
}

void loop() {
  double temp = 0.0;
  double RH = 0.0;

  hdc.readTemperatureHumidityOnDemand(temp, RH, TRIGGERMODE_LP0);

  Serial.print("Temperature: ");
  Serial.print(temp);
  Serial.println(" °C");

  Serial.print("Humidity: ");
  Serial.print(RH);
  Serial.println(" %");
  delay(2000);
  
}

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#include <Adafruit_HDC302x.h>

Adafruit_HDC302x hdc = Adafruit_HDC302x();

void setup() {
  Serial.begin(115200);

  Serial.println("Adafruit HDC302x Simple Test");

  if (! hdc.begin(0x44, &Wire)) {
    Serial.println("Could not find sensor?");
    while (1);
  }
  delay(1000);
}

void loop() {
  double temp = 0.0;
  double RH = 0.0;

  hdc.readTemperatureHumidityOnDemand(temp, RH, TRIGGERMODE_LP0);

  Serial.print("Temperature: ");
  Serial.print(temp);
  Serial.println(" °C");

  Serial.print("Humidity: ");
  Serial.print(RH);
  Serial.println(" %");
  delay(2000);
  
}

Upload the sketch to your board and open up the Serial Monitor (Tools -> Serial Monitor) at 115200 baud. After the HDC3021 is recognized over I2C, you'll see the temperature and humidity readings print to the Serial Monitor every two seconds.

What is WipperSnapper

WipperSnapper is a firmware designed to turn any WiFi-capable board into an Internet-of-Things device without programming a single line of code. WipperSnapper connects to Adafruit IO, a web platform designed (by Adafruit!) to display, respond, and interact with your project's data.

Simply load the WipperSnapper firmware onto your board, add credentials, and plug it into power. Your board will automatically register itself with your Adafruit IO account.

From there, you can add components to your board such as buttons, switches, potentiometers, sensors, and more! Components are dynamically added to hardware, so you can immediately start interacting, logging, and streaming the data your projects produce without writing code.

If you've never used WipperSnapper, click below to read through the quick start guide before continuing.

Wiring

First, wire up an HDC302x (HDC3021/HDC3022) to your board exactly as follows. Here is an example of the HDC3021 wired to an Adafruit ESP32 Feather V2 using I2C with a STEMMA QT cable (no soldering required)

Usage

Connect your board to Adafruit IO Wippersnapper and navigate to the WipperSnapper board list.

On this page, select the WipperSnapper board you're using to be brought to the board's interface page.

If you do not see your board listed here - you need to connect your board to Adafruit IO first.

Next, make sure the sensor is plugged into your board and click the I2C Scan button.

 You should see the HDC302x's default I2C address of 0x44 pop-up in the I2C scan list.

With the sensor detected in an I2C scan, you're ready to add the sensor to your board.

Click the New Component button or the + button to bring up the component picker.

Adafruit IO supports a large amount of components. To quickly find your sensor, type HDC302 into the search bar, then select the HDC302x component.

On the component configuration page, the HDC302x's sensor address should be listed along with the sensor's settings.

The Send Every option is specific to each sensor's measurements. This option will tell the Feather how often it should read from the HDC302x sensor and send the data to Adafruit IO. Measurements can range from every 30 seconds to every 24 hours.

For this example, set the Send Every interval to every 30 seconds.

Your device interface should now show the sensor components you created. After the interval you configured elapses, WipperSnapper will automatically read values from the sensor(s) and send them to Adafruit IO.

To view the data that has been logged from the sensor, click on the graph next to the sensor name.

Here you can see the feed history and edit things about the feed such as the name, privacy, webhooks associated with the feed and more. If you want to learn more about how feeds work, check out this page.

Files

• HDC302x Datasheet
• EagleCAD PCB Files on GitHub
• Fritzing object in the Adafruit Fritzing Library

Schematic and Fab Print