Adafruit Industries HTU21D-F User manual

Adafruit HTU21D-F Temperature & Humidity Sensor
Created by lady ada
Last updated on 2016-08-24 05:33:19 PM UTC

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Guide Contents
Guide Contents
Overview
Pinouts
Power Pins:
I2C Logic pins:
Assembly
Prepare the header strip:
Add the breakout board:
And Solder!
Wiring & Test
Download Adafruit_HTU21DF
Load Demo
Library Reference
Downloads
Files & Datasheets
Schematics
PCB Print
© Adafruit Industries https://learn.adafruit.com/adafruit-htu21d-f-temperature-humidity-
sensor Page 2 of 18

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 52% when it feels more like 77%. Enter the HTU21D-F Temperature
+ Humidity Sensor - the best way to prove the weatherman wrong!
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sensor Page 3 of 18

This I2C digital humidity sensor is an accurate and intelligent alternative to the much
simpler Humidity and Temperature Sensor - SHT15 Breakout (http://adafru.it/1638) It has a
typical accuracy of ±2% with an operating range that's optimized from 5% to 95% RH.
Operation outside this range is still possible - just the accuracy might drop a bit. The
temperature output has an accuracy of ±1°C from -30~90°C. If you're looking to measure
temperature more accurately, we recommend the MCP9808 High Accuracy I2C
Temperature Sensor Breakout Board. (http://adafru.it/1782)
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sensor Page 4 of 18

Such a lovely chip - so we spun up a breakout board that includes the Filtered version (the
white bit of plastic which is a PTFE filter to keep the sensor clean), a 3.3V regulator and I2C
level shifting circuitry. This lets you use it safely with any kind of microcontroller with 3.3V-
5V power or logic. 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.
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sensor Page 5 of 18

Pinouts
The HTU21D-F 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:
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sensor Page 6 of 18

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.
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sensor Page 7 of 18

Assembly
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
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sensor Page 8 of 18

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
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sensor Page 9 of 18

reliable electrical contact.
(For tips on soldering, be sure to
check out our Guide to Excellent
Soldering (http://adafru.it/aTk)).
You're done! Check your solder
joints visually and continue onto
the next steps
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sensor Page 10 of 18

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sensor Page 11 of 18

Wiring & Test
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 HTU21D-F has a default I2C address of 0x40 and cannot be changed!
Download Adafruit_HTU21DF
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sensor Page 12 of 18

Download Adafruit_HTU21DF
To begin reading sensor data, you will need to download Adafruit_HTU21DF_Library from
our github repository (http://adafru.it/dw5). You can do that by visiting the github repo and
manually downloading or, easier, just click this button to download the zip
Download Adafruit HTU21D-F Library
http://adafru.it/dw6
Rename the uncompressed folder Adafruit_HTU21DF and check that the
Adafruit_HTU21DF folder contains Adafruit_HTU21DF.cpp and Adafruit_HTU21DF.h
Place the Adafruit_HTU21DF library folder your arduinosketchfolder/libraries/ folder.
You may need to create the libraries subfolder if its your first library. Restart the IDE.
We also have a great tutorial on Arduino library installation at:
http://learn.adafruit.com/adafruit-all-about-arduino-libraries-install-use (http://adafru.it/aYM)
Load Demo
Open up File->Examples->Adafruit_HTU21DF->HTU21DFtest and upload to your
Arduino wired up to the sensor
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sensor Page 13 of 18

Thats it! Now open up the serial terminal window at 9600 speed to begin the test.
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sensor Page 14 of 18

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_HTU21DF object with:
Adafruit_HTU21DF htu = Adafruit_HTU21DF()
There are no pins to set since you must use the I2C bus!
Then initialize the sensor with:
htu.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
htu.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!
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sensor Page 15 of 18

Reading the humidity is equally simple. Call
htu.readHumidity()
to read the humidity also as a floating point value between 0 and 100 (this reads %
humidity)
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sensor Page 16 of 18

Downloads
Files & Datasheets
Datasheet for the HTU21D-F (http://adafru.it/dKR) (the -F part is for the PTFE Filter,
which is the white insert on top of the sensor)
Fritzing object in Adafruit Fritzing library (http://adafru.it/aP3)
EagleCAD PCB files in GitHub (http://adafru.it/r5F)
Schematics
PCB Print
Dimensions in Inches!
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sensor Page 17 of 18

© Adafruit Industries Last Updated: 2016-08-24 05:33:18 PM UTC Page 18 of 18