It's easy to use the Si7021 sensor with Python or CircuitPython and the Adafruit CircuitPython SI7021 module.  This module allows you to easily write Python code that reads the humidity and temperature, pressurefrom the sensor.

You can use this sensor 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 a SI7021 to your board exactly as shown on the previous pages for Arduino. Here's an example of wiring a Feather M0 or M4 to the sensor with I2C:

  • Board 3V to sensor VIN (red wire on STEMMA QT version)
  • Board GND to sensor GND (black wire on STEMMA QT version)
  • Board SCL to sensor SCL (yellow wire on STEMMA QT version)
  • Board SDA to sensor SDA (blue wire on STEMMA QT version)

Python Computer Wiring

The Si7021 can potentially clock stretch the I2C signal, which can cause issues on Raspberry Pi's. See here for suggested fix:

Since there's 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:

  • Pi 3V3 to sensor VIN (red wire on STEMMA QT version)
  • Pi GND to sensor GND (black wire on STEMMA QT version)
  • Pi SCL to sensor SCL (yellow wire on STEMMA QT version)
  • Pi SDA to sensor SDA (blue wire on STEMMA QT version)

CircuitPython Installation of Si7021 Library

You'll need to install the Adafruit CircuitPython SI7021 library on your CircuitPython board.

First make sure you are running the latest version of Adafruit CircuitPython for your board.

Next you'll need to install the necessary libraries to use the hardware--carefully follow the steps to find and install these libraries from Adafruit's CircuitPython library bundle.  For example the Circuit Playground Express guide has a great page on how to install the library bundle for both express and non-express boards.

Load the following libraries into your lib folder on your CIRCUITPY drive:

  • adafruit_si7021/
  • adafruit_bus_device/

Before continuing make sure your board's lib folder or root filesystem has the adafruit_si7021/, and adafruit_bus_device/ folders copied over.

Next connect to the board's serial REPL so you are at the CircuitPython >>> prompt.

Python Installation of Si7021 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:

  • sudo pip3 install adafruit-circuitpython-si7021

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 & Python Usage

To demonstrate the usage of the sensor we'll initialize it and read the temperature and humidity from the board's Python REPL.  First run the following code to import the necessary modules and initialize the I2C bus:

import board
import adafruit_si7021
sensor = adafruit_si7021.SI7021(board.I2C())

Now you're ready to read values from the sensor using any of these properties:

  • temperature - The sensor temperature in degrees Celsius.
  • relative_humidity - The percent humidity as a value from 0 to 100%.
print('Temperature: {} degrees C'.format(sensor.temperature))
print('Humidity: {}%'.format(sensor.relative_humidity))

That's all there is to using the Si7021 sensor with CircuitPython!

Full Example Code

# SPDX-FileCopyrightText: 2021 ladyada for Adafruit Industries
# SPDX-License-Identifier: MIT

Initializes the sensor, gets and prints readings every two seconds.
import time
import board
import adafruit_si7021

# Create library object using our Bus I2C port
i2c = board.I2C()  # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C()  # For using the built-in STEMMA QT connector on a microcontroller
sensor = adafruit_si7021.SI7021(i2c)

# If you'd like to use the heater, you can uncomment the code below
# and pick a heater level that works for your purposes
# sensor.heater_enable = True
# sensor.heater_level = 0  # Use any level from 0 to 15 inclusive

while True:
    print("\nTemperature: %0.1f C" % sensor.temperature)
    print("Humidity: %0.1f %%" % sensor.relative_humidity)

This guide was first published on Sep 22, 2016. It was last updated on Jul 22, 2024.

This page (Python & CircuitPython) was last updated on Jul 22, 2024.

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