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HTU21D Humidity Sensor Hookup Guide
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NAT E
HTU21D Overview
The HTU21D is a low-cost, easy to use, highly accurate, digital humidity
sensor. All you need is two lines for I
2
C communication and you’ll have
relative humidity readings such as “45.2%” or “23.1%” and very accurate
temperature readings as a bonus!
Things you should know about this sensor:
•
•
•
•
Uses the I
2
C interface
Typical humidity accuracy of ±2%
Typical temperature accuracy of ±0.3C
Operates from 0 to 100% humidity but this sensor isn’t recommended
for harsh environments where it could come in contact with water
(such as rain).
• 3.3V sensor - use inline logic level converters or 10k resistors to limit
5V signals
• Here’s the datasheet
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• Only one HTU21D sensor can reside on the I
2
C bus at a time
This sensor is ideal for environmental sensing and data logging. Perfect for
a weather station or humidor control system. It is a very good replacement
for digital humidity sensors such as the SHT15, SHT21, SHT25, HIH-4030,
HIH6130 and capacitive humidity sensors such as the HH10D.
Suggested Reading
Things you might need to know:
•
•
•
•
•
•
I
2
C Protocol
Using the PCA9306 voltage translator
Logic Levels
Installing an Arduino library
What are pull-up resistors?
How to use a breadbaord
Hooking It Up
Wiring up the HTU21D humidity sensor is very easy! We recommend
soldering four male headers to the breakout board and using the HTU21D
in a breadboard.
Connections: Breakout board to Arduino
•
•
•
•
VCC
→
3.3V
GND
→
GND
SDA
→
A4
SCL
→
A5
There are only four pins that need to be hooked up in order to start using
this sensor in a project. One for VCC, one for GND, and two data lines for
I
2
C communication. On an Arduino board connect the SDA pin on the
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breakout board to A4 and SCL to A5. If you have a newer Arduino, you can
connect the SDA and SCL lines directly to the SDA and SCL lines broken
out on the Arduino headers.
This board runs at 3.3V. Be sure to power the board from the 3.3V pin!
Because I
2
C is an open drain signal, there’s no need to worry about level
shifting the signal; the 3.3V signal will be adequate to communicate with the
Arduino and the signal will never reach a dangerous level for the pins on
the HTU21D.
Note:
This breakout board has built in 4.7k pull up resistors for I
2
C
communications. If you’re hooking up multiple I
2
C devices on the same bus,
you may want to disable these resistors.
HTU21D Library and Functions
We’ve written an Arduino library and some example code to make using the
HTU21D easy to get up and running. Grab the HTU21D library for Arduino
here. Copy and paste the folder “HTU21D_Humidity” into your Arduino
libraries folder. Not sure how to install an Arduino library? Read here for
more information.
The examples menu expanded to show Humidity example
Once the library is installed, open Arduino, and expand the examples
menu. You should see the HTU21D_Humidity submenu.
Load this example onto the Arduino. Open the serial terminal at 9600bps.
You will see the current humidity and temperature in the room!
Explanation of Functions:
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The library and example code demonstrate some of the smaller functions
supported by the HTU21D. These are generally not used by regular users
but are documented here in case you need them:
•
myHumidity.readHumidity()
will return a float containing the
humidity. Ex: 54.7
•
myHumidity.readTemperature()
will return a float containing the
temperature in Celsius. Ex: 24.1
•
myHumidity.setResolution(byte: 0b.76543210)
sets the resolution
of the readings.
•
myHumidity.check_crc(message, check_value)
verifies the 8-bit
CRC generated by the sensor.
•
myHumidity.read_user_register()
returns the user register. Used
to set resolution.
When you call the ReadHumidity or ReadTemperature functions you will
get a float with the sensor reading or an error code:
•
57.8
is an example of a valid reading. 0.0 to 100.0 for humidity and
-40.0 to 125.0 for temperature.
•
998
indicates that I2C timed out (100ms max). Check your
connections.
•
999
CRC was wrong. The HTU21D calculates an internal CRC and
transmits it along with the temperature and humidity readings. It’s
highly unlikely that you will
ever
see a bad CRC, but the library
supports CRC checking.
SetResolution()
allows the user to change the humidity and temperature
resolution. The vast majority of users do not need to change the resolution.
By default the sensor will be in its highest resolution settings. This function
is useful if you need to decrease the amount of time between readings or to
save power. See page 12 of the datasheet. As an example, to change the
resolution to 11 bit RH and 11 bit temperature, you would call
myHumidity.SetResolution(0b10000001);
to set bit 7 and bit 0.
check_crc()
inputs the message and check_value from the HTU21D and
then does a 8-bit polynomial CRC to verify that the message the sensor
sent was valid. It’s extremely rare that you will see a bad CRC but if you’re
looking into how to decipher a 8-bit CRC, this is a good example. The
HTU21D uses the same CRC as 1-wire products.
Disabling I2C Resistors
There are two 4.7k resistors on the breakout board that pull-up the SDA
and SCL lines. If you’ve got other devices on the I
2
C bus you may need to
disable the on-board resistors. To do this you need to clear the solder
jumper on the board. This will disconnect the resistors from VCC and from
the I
2
C bus.
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Resources and Going Further
You should now have a good idea of how to add humidity and temperature
sensing into your next project. Need some inspiration? Check out these
other tutorials:
• Make an automated terrarium that manages heat and humidity
levels.
• Build a humidor control box that maintains a constant humidity in a
controlled space.
Resources:
• Example Arduino Sketch demonstrating how to talk to the HTU21D
Humidity Sensor
• HTU21D Datasheet
• HTU21D Breakout Board Schematic
• HTU21D Breakout Board Eagle Files
• Github repo containing all the latest files and code
https://learn.sparkfun.com/tutorials/htu21d-humidity-sensor-hookup-guide?_ga=1.46692769.725293211.142... 2/12/2015
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