Overview

It's summer and you're sweating and your hair's all frizzy and all you really want to know is why the weatherman said this morning that today's relative humidity would max out at a perfectly reasonable 42% when it feels more like 77%. Enter the Si7021 Temperature + Humidity Sensor - the best way to prove the weatherman wrong!

This lovely sensor for Silicon labs has ± 3% relative humidity measurements with a range of 0–80% RH, and ±0.4 °C temperature accuracy at a range of -10 to +85 °C. Great for all of your environmental sensing projects. It uses I2C for data transfer so it works with a wide range of microcontrollers.

We put this nice sensor on a breakout board with a 3.3V regulator and level shifting so you can use it safely with 3.3V or 5V power & logic. There's a PTFE filter to keep the sensor clean, that's the white flat thing on top. Also comes with some pin header. Some light soldering is required to attach the header but it's easy to do.

Pinouts

The Si7021 is a I2C sensor. That means it uses the two I2C data/clock wires available on most microcontrollers, and can share those pins with other sensors as long as they don't have an address collision. For future reference, the I2C address is 0x40 and you can't change it!

Power Pins:

  • Vin - this is the power pin. Since the chip uses 3 VDC, we have included a voltage regulator on board that will take 3-5VDC and safely convert it down. 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
  • 3v3 - this is the 3.3V output from the voltage regulator, you can grab up to 100mA from this if you like
  • GND - common ground for power and logic

I2C Logic pins:

  • SCL - I2C clock pin, connect to your microcontrollers I2C clock line.
  • SDA - I2C data pin, connect to your microcontrollers I2C data line.

Assembly

The photos below show a an HTUD21 sensor rather than the Si7021 but the soldering procedure is identical!

Prepare the header strip:

Cut the strip to length if necessary. It will be easier to solder if you insert it into a breadboard - long pins down

Add the breakout board:

Place the breakout board over the pins so that the short pins poke through the breakout pads

And Solder!

Be sure to solder all pins for reliable electrical contact.

(For tips on soldering, be sure to check out our Guide to Excellent Soldering).

You're done! Check your solder joints visually and continue onto the next steps

Arduino Code

You can easily wire this breakout to any microcontroller, we'll be using an Arduino. For another kind of microcontroller, just make sure it has I2C, then port the code - its pretty simple stuff!

  • Connect Vin to the power supply, 3-5V is fine. Use the same voltage that the microcontroller logic is based off of. For most Arduinos, that is 5V
  • Connect GND to common power/data ground
  • Connect the SCL pin to the I2C clock SCL pin on your Arduino. On an UNO & '328 based Arduino, this is also known as A5, on a Mega it is also known as digital 21 and on a Leonardo/Micro, digital 3
  • Connect the SDA pin to the I2C data SDA pin on your Arduino. On an UNO & '328 based Arduino, this is also known as A4, on a Mega it is also known as digital 20 and on a Leonardo/Micro, digital 2

The Si7021 has a default I2C address of 0x40 and cannot be changed!

Install Adafruit_Si7021 library

To begin reading sensor data, you will need to install the Adafruit_Si7021 library (code on our github repository). It is available from the Arduino library manager so we recommend using that.

From the IDE open up the library manager...

And type in adafruit si7021 to locate the library. Click Install

Also install the Adafruit Unified Sensor library in the same way:

We also have a great tutorial on Arduino library installation at:
http://learn.adafruit.com/adafruit-all-about-arduino-libraries-install-use

Load Demo

Open up File->Examples->Adafruit_Si7021->si7021 and upload to your Arduino wired up to the sensor

Thats it! Now open up the serial terminal window at 115200 speed to begin the test.

You can try breathing on the sensor to increase the humidity. The sensor reacts very fast!

Library Reference

The library we have is simple and easy to use

You can create the Adafruit_Si7021 object with:

Download: file
Adafruit_Si7021 sensor = Adafruit_Si7021();

There are no pins to set since you must use the I2C bus!

Then initialize the sensor with:

Download: file
sensor.begin();

this function returns True if the sensor was found and responded correctly and False if it was not found

Once initialized, you can query the temperature in °C with

Download: file
sensor.readTemperature()

Which will return floating point (decimal + fractional) temperature. You can convert to Fahrenheit by multiplying by 1.8 and adding 32 as you have learned in grade school!

Reading the humidity is equally simple. Call

Download: file
sensor.readHumidity()

to read the humidity also as a floating point value between 0 and 100 (this reads % humidity)

This sensor also has a serial number which you might find handy! Call sensor.readSerialNumber() to read out the 8 bytes of unique ID. Then you can access them from sensor.sernum_a and sensor.sernum_b

Python & CircuitPython

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 to the sensor with I2C:

  • Board 3V to sensor VIN
  • Board GND to sensor GND
  • Board SCL to sensor SCL
  • Board SDA to sensor SDA

Python Computer Wiring

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
  • Pi GND to sensor GND
  • Pi SCL to sensor SCL
  • Pi SDA to sensor SDA

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.

Remember for non-express boards like the Trinket M0, Gemma M0, and Feather/Metro M0 basic you'll need to manually install the necessary libraries from the bundle:

  • adafruit_si7021.mpy
  • adafruit_bus_device

You can also download the adafruit_si7021.mpy from its releases page on Github.

Before continuing make sure your board's lib folder or root filesystem has the adafruit_si7021.mpy, and adafruit_bus_device files and 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:

Download: file
import board
import busio
import adafruit_si7021
i2c = busio.I2C(board.SCL, board.SDA)
sensor = adafruit_si7021.SI7021(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%.
Download: file
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

import time
import board
import busio
import adafruit_si7021

# Create library object using our Bus I2C port
i2c = busio.I2C(board.SCL, board.SDA)
sensor = adafruit_si7021.SI7021(i2c)


while True:
    print("\nTemperature: %0.1f C" % sensor.temperature)
    print("Humidity: %0.1f %%" % sensor.relative_humidity)
    time.sleep(2)

Downloads

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