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Adafruit Si7021 Temperature + Humidity Sensor Created by lady ada https://learn.adafruit.com/adafruit-si7021-temperature-plus-humidity-sensor Last updated on 2022-03-02 05:16:13 PM EST ©Adafruit Industries Page 1 of 21 Table of Contents Overview 3 Pinouts 5 • Power Pins: • I2C Logic pins: 6 7 Assembly 7 • Prepare the header strip: • Add the breakout board: • And Solder! 7 8 8 Arduino Code 9 • Install Adafruit_Si7021 library • Load Demo • Library Reference 11 12 13 Python & CircuitPython 14 • • • • • • 14 15 17 17 18 18 CircuitPython Microcontroller Wiring Python Computer Wiring CircuitPython Installation of Si7021 Library Python Installation of Si7021 Library CircuitPython & Python Usage Full Example Code Python Library Docs 19 Downloads 19 • Files & Datasheets • Schematic and Fabrication Print STEMMA QT Version • Schematic and Fabrication Print Original Version 19 20 21 ©Adafruit Industries Page 2 of 21 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. ©Adafruit Industries Page 3 of 21 To get you going fast, we spun up a custom made PCB in the STEMMA QT form factor (https://adafru.it/LBQ), making them easy to interface with. The STEMMA QT connectors (https://adafru.it/JqB) on either side are compatible with the SparkFun Qwiic (https://adafru.it/Fpw) I2C connectors. This allows you to make solderless connections between your development board and the LIS331s or to chain them with a wide range of other sensors and accessories using a compatible cable (https:// adafru.it/JnB). We’ve of course broken out all the pins to standard headers and added a voltage regulator and level shifting so allow you to use it with either 3.3V or 5V systems such as the Raspberry Pi + Feather series or Arduino Uno respectively. ©Adafruit Industries Page 4 of 21 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 for use in a breadboard but it's easy to do. There are two versions of this board - the STEMMA QT version shown above, and the original header-only version shown below. Code works the same on both! 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! ©Adafruit Industries Page 5 of 21 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 ©Adafruit Industries Page 6 of 21 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. • STEMMA QT (https://adafru.it/Ft4) - These connectors allow you to connectors to dev boards with STEMMA QT connectors or to other things with various associated accessories (https://adafru.it/Ft6). 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 ©Adafruit Industries Page 7 of 21 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 (https://adafru.it/aTk)). ©Adafruit Industries Page 8 of 21 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! ©Adafruit Industries Page 9 of 21 • Connect Vin to the power supply, 3-5V is fine. (red wire on STEMMA QT version) 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 (black wire on STEMMA QT version) • Connect the SCL pin to the I2C clock SCL pin on your Arduino. (yellow wire on STEMMA QT version) 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. (blue wire on STEMMA QT version) 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 diagram.fzz Fritzing file https://adafru.it/rAA The Si7021 has a default I2C address of 0x40 and cannot be changed! ©Adafruit Industries Page 10 of 21 Install Adafruit_Si7021 library To begin reading sensor data, you will need to install the Adafruit_Si7021 library (code on our github repository) (https://adafru.it/rAw). 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 (https:// adafru.it/aYM) ©Adafruit Industries Page 11 of 21 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! ©Adafruit Industries Page 12 of 21 Library Reference The library we have is simple and easy to use You can create the Adafruit_Si7021 object with: Adafruit_Si7021 sensor = Adafruit_Si7021(); There are no pins to set since you must use the I2C bus! Then initialize the sensor with: sensor.begin(); this function returns True if the sensor was found and responded correctly and False i f it was not found Once initialized, you can query the temperature in °C with 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 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.readS erialNumber() to read out the 8 bytes of unique ID. Then you can access them from sensor.sernum_a and sensor.sernum_b ©Adafruit Industries Page 13 of 21 Python & CircuitPython It's easy to use the Si7021 sensor with Python or CircuitPython and the Adafruit CircuitPython SI7021 (https://adafru.it/BfV) 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-forPython compatibility library (https://adafru.it/BSN). 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: ©Adafruit Industries Page 14 of 21 • 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: I2C Clock Stretching on Raspberry Pi https://adafru.it/C7x ©Adafruit Industries Page 15 of 21 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 (https://adafru.it/BSN).  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) ©Adafruit Industries Page 16 of 21 CircuitPython Installation of Si7021 Library You'll need to install the Adafruit CircuitPython SI7021 (https://adafru.it/BfV) library on your CircuitPython board. First make sure you are running the latest version of Adafruit CircuitPython (https:// adafru.it/tBa) 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  (https://adafru.it/zdx).  For example the Circuit Playground Express guide has a great page on how to install the library bundle (https://adafru.it/Bf2) 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 adafrui t_si7021/, and adafruit_bus_device/ folders copied over. Next connect to the board's serial REPL  (https://adafru.it/pMf)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 (https ://adafru.it/BSN)! Once that's done, from your command line run the following command: • sudo pip3 install adafruit-circuitpython-si7021 ©Adafruit Industries Page 17 of 21 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 sensor = adafruit_si7021.SI7021(board.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 ©Adafruit Industries Page 18 of 21 while True: print("\nTemperature: %0.1f C" % sensor.temperature) print("Humidity: %0.1f %%" % sensor.relative_humidity) time.sleep(2) Python Library Docs Python Library Docs (https://adafru.it/C3f) Downloads Files & Datasheets • Fritzing object in Adafruit Fritzing library (https://adafru.it/aP3) • Arduino library on GitHub (https://adafru.it/rAw) • EagleCAD PCB files on GitHub (https://adafru.it/rAB) • Si7021-A20 datasheet (https://adafru.it/rAC) • K&R Smith calibration notes (https://adafru.it/BfU) If you try writing your own I2C driver, please be aware it is possible to brick a Si7021 by having it erase its calibration memory! https://www.silabs.com/ community/sensors/forum.topic.html/si7021_loses_serial-Lkb7 ©Adafruit Industries Page 19 of 21 Schematic and Fabrication Print STEMMA QT Version ©Adafruit Industries Page 20 of 21 Schematic and Fabrication Print Original Version ©Adafruit Industries Page 21 of 21