Adafruit LPS35HW User guide

Adafruit LPS33/LPS35 Water Resistant

Pressure Sensor

Created by Bryan Siepert

https://learn.adafruit.com/lps35hw-water-resistant-pressure-sensor

Last updated on 2022-12-01 03:36:49 PM EST

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Table of Contents

Overview

• Capable sensors

• A Family Affair

Pinouts

• Power Pins

• I2C Logic pins:

• SPI Logic pins:

• Other pins

Assembly

• Prepare the header strip:

• Add the breakout board:

• And Solder!

Arduino

• I2C Wiring

• SPI Wiring

• Library Installation

• Load Example

• Example Code

Arduino Docs

3

8

10

13

17

Python and CircuitPython

• CircuitPython Microcontroller Wiring

• Python Computer Wiring

• CircuitPython Installation of LPS35HW Library

• Python Installation of LPS35HW Library

• CircuitPython & Python Usage

• Full Example Code

Python Docs

Downloads

• LPS33HW Files

• LPS33HW Schematic

• LPS33HW Fab Print

• LPS35HW Schematic

• LPS35HW Fab Print

18

22

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Overview

Sometimes you need to sense pressure when it's wet. And sometimes you need to

know the relative changes in pressure as well as the absolute pressure. For the times

you need to do both (or either), the LPS35HW is the pressure sensor for you.

Combining protection from water intrusion with support for high precision relative and

absolute measurements, this sensor will do what you need. With drivers for

CircuitPython, Arduino, and Raspberry Pi, and support for I2C or SPI (Arduino only SPI

support, for now) you'll be measuring pressure in moist situations in no time.

The sensor itself is advertised as Water Resistant but the breakout board for

testing out this sensor is not! If you want to use it in wet environments you'll need

to pot the rest of the board in a wateproof epoxy!

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The LPS35HW has a fraternal twin, the LPS33HW. In nearly all respects the LPS33HW

is the same as the LPS35HW however the LPS33HW has a bonus feature: a nozzle

that can be used to seal it against an enclosure or perhaps a properly fitted tube of

some sort. You'll want to make sure to use an appropriately sized o-ring to ensure a

good seal. Like the LPS35HW, the LPS33HW chip inside is encased in a protective

gel, however in the case of the LPS33HW, the gel is mentioned as being

...designed for and proven to protect electronic components from long-

term exposure to harsh environments such as water mixed with chlorine,

bromine, commercial washing detergent and fuels, solvents and chemicals.

Wow! That's a resilient sensor!

The LPS33HW like its sibling is advertised as being protected from all kinds of

horrible things however THE BREAKOUT BOARD IS NOT. For use in icky

environments, you'll want to protect the rest of the board with appropriate

measures. Since the LPS33HW has a sealable port, you may be able to use the

information in the application note on the downloads page to make a suitable

enclosure

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Capable sensors

The ST LPS35HW is a water resistant barometric pressure and temperature sensor

that is also safe to use in wet environments. The sensing element is nestled safely in

a ceramic package and is encased in a waterproof gel that prevents water that gets

into the sensor from interfering with readings. It does not carry any ratings for

resistance to moisture so you probably don't want to take it to the bottom of the

Mariana Trench, but it should work well for normal wet situations like weather stations

or high humidity.

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Similarly the LPS33HW is ensconced in a protective gel that protects it from harsh

environments. Additionally it is called out by the manufacturer (ST) in the datasheet as

being suitable for applications such as weather station equipment or a moist, mouth-

adjacent activity that we don't encourage. Let's just say it seems well suited for things

like sip-and-puff switches.

Along with not being afraid of getting wet, the LPS35HW and LPS33HW have 24bit

pressure data and 16 bit temperature data, allowing it to deliver pressure readings

with +/- 0.1% hPa accuracy. It can measure from 260 to 1260 hPa and is able to

withstand pressure up to 20 times their measurement range.

To help you take measurements to your requirements, the sensors also offer an

adjustable data rate, as well as a low pass filter to remove noise from the signal.

Finally, the onboard temperature compensation makes sure that your readings are

always good and won't vary as the temperature changes.

We placed these sensors on a breakout board with a 3.3V regulator and level shifting

circuitry so it can be used by 3V or 5V power/logic devices. A small piece of header is

also included, so you can solder it in for use with a breadboard.

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A Family Affair

The LPS33HW also joins our growing family of boards with SparkFun QUIIC()compati

bleSTEMMA QT() connectors, allowing you to combine it with all sorts of other

sensors(), and even displays(), all without needing to solder! Just plug and go/blow!

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Pinouts

Power Pins

Vin - this is the power pin. Since the sensor chip uses 3.3 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, for a feather use 3.3V

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3Vo - 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:

SCK - this is the I2C clock pin, connect to your microcontrollers I2C clock line.

•

SDI - this is the I2C data pin, connect to your microcontrollers I2C data line.

•

Leave theSDOandCSpins disconnected

SPI Logic pins:

All pins going into the breakout have level shifting circuitry to make them 3-5V logic

level safe. Use whatever logic level is on Vin!

SCK - This is alsothe SPI Clock pin, its an input to the chip

•

SDO - this is the Serial Data Out / Microcontroller In Sensor Outpin, for data

•

sent from the LPS35HW to your processor

SDI - this is alsothe Serial Data In / Microcontroller Out Sensor Inpin, for data

•

sent from your processor to the LPS35HW

CS - this is the Chip Select pin, drop it low to start an SPI transaction. Its an input

•

to the chip

If you want to connect multiple LPS35HW's to one microcontroller, have them share

the SDI, SDO and SCK pins. Then assign each one a unique CS pin.

Other pins

INTis the interrupt output pin. You can configure the interrupt to trigger for

•

various 'reasons' such as going over or under a configured pressure threshold.

Voltage level is the same asVcc.

The low pressure threshold interrupt only works when the LPS33W/LPS35HW are

operating in relative mode.

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Assembly

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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

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And Solder!

Be sure to solder all 8 pins for reliable

electrical contact.

(For tips on soldering, be sure to check out

ourGuide to Excellent Soldering()).

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You're done! Check your solder joints

visually and continue onto the next steps.

Arduino

I2C Wiring

Use this wiring if you want to connect via I2C interface

By default, the i2c address is 0x5d. If you add a jumper from SDO to GND, the

address will change to 0x5c.

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Connectboard VCC (red wire) toArduino

5Vif you are running a 5V board Arduino

(Uno, etc.).If your board is 3V, connect to

that instead.

Connect board GND (black

wire)toArduino GND

Connect board SCL (yellow

wire)toArduino SCL

Connect board SDA (blue wire)toArduino

SDA

The final results should resemble the illustration above, showing an Adafruit Metro

development board.

SPI Wiring

Since this is a SPI-capable sensor, we can use hardware or 'software' SPI. To make

wiring identical on all microcontrollers, we'll begin with 'software' SPI. The following

pins should be used:

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Connect Vin to the power supply, 3V or 5V

is fine. Use the same voltage that the

microcontroller logic is based off of

Connect GND to common power/data

ground

Connect the SCK pin to Digital #13 but any

pin can be used later

Connect the SDO pin to Digital #12 but any

pin can be used later

Connect the SDI pin to Digital #11 but any

pin can be used later

Connect the CS pin Digital #10 but any pin

can be used later

Later on, once we get it working, we can adjust the library to use hardware SPI if you

desire, or change the pins to others.

Library Installation

You can install the Adafruit LPS35HW Library for Arduino using the Library Manager in

the Arduino IDE. This will work for both the LPS35HW and LPS33HW:

Click theManage Libraries ... menu item, search for Adafruit LPS35HW,and select

theAdafruit LPS35HWlibrary:

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Then follow the same process for theAdafruit BusIOlibrary.

Load Example

Open upFile -> Examples -> AdafruitLPS35HW -> lps35hw_test and upload to your

Arduino wired up to the sensor.

Depending on whether you are using I2C or SPI, change the pin names and comment

or uncomment the following lines.

if (!lps35hw.begin_I2C()) {
//if (!lps35hw.begin_SPI(LPS_CS)) {
//if (!lps35hw.begin_SPI(LPS_CS, LPS_SCK, LPS_MISO, LPS_MOSI)) {

Once you upload the code, you will see the temperature and pressure being printed

when you open the Serial Monitor (Tools->Serial Monitor) at 115200 baud, similar to

this:

Temperature is calculated in degrees C, you can convert this to F by using the classic

F = C * 9/5 + 32 equation.

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Pressure is returned in the SI units of Pascals. 100 Pascals = 1 hPa = 1 millibar. Often

times barometric pressure is reported in millibar or inches-mercury. For future

reference 1 pascal =0.000295333727 inches of mercury, or 1 inch Hg = 3386.39

Pascal. So if you take the pascal value of say 100734 and divide by 3386.39 you'll get

29.72 inches-Hg.

Example Code

The following example code is part of the standard library, and illustrates how you can

retrieve sensor data from the LPS35HW or LPS33HW for pressure and temperature:

#include <Adafruit_LPS35HW.h>

Adafruit_LPS35HW lps35hw = Adafruit_LPS35HW();

// For SPI mode, we need a CS pin
#define LPS_CS 10
// For software-SPI mode we need SCK/MOSI/MISO pins
#define LPS_SCK 13
#define LPS_MISO 12
#define LPS_MOSI 11

void setup() {
Serial.begin(115200);
// Wait until serial port is opened
while (!Serial) { delay(1); }

Serial.println("Adafruit LPS35HW Test");

if (!lps35hw.begin_I2C()) {
//if (!lps35hw.begin_SPI(LPS_CS)) {
//if (!lps35hw.begin_SPI(LPS_CS, LPS_SCK, LPS_MISO, LPS_MOSI)) {
Serial.println("Couldn't find LPS35HW chip");
while (1);
}
Serial.println("Found LPS35HW chip");
}

void loop() {
Serial.print("Temperature: ");
Serial.print(lps35hw.readTemperature());
Serial.println(" C");

Serial.print("Pressure: ");
Serial.print(lps35hw.readPressure());
Serial.println(" hPa");

Serial.println();
delay(1000);
}

Arduino Docs

Arduino Docs()

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Python and CircuitPython

It's easy to use the LPS33HW or LPS35HW sensors with Python and CircuitPython,

and the Adafruit CircuitPython LPS35HW() module. This module allows you to easily

write Python code that reads the pressure and temperature and will work with either

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 LPS35HW or LPS33HW to your board for an I2C connection, exactly as

shown below. Here's an example of wiring a Feather M4 to the sensor with I2C:

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Board 3Vtosensor VIN (red wire)

Board GNDtosensor GND (black wire)

Board SCLtosensor SCL (yellow wire)

Board SDAtosensor SDA (blue wire)

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:

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Pi 3V3 to sensor VIN

Pi GND to sensor GND

Pi SCL to sensor SCK

Pi SDA to sensor SDI

CircuitPython Installation of LPS35HW Library

Next you'll need to install theAdafruit CircuitPython LPS35HW()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

(). Our introduction 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, you'll need to manually install the

necessary libraries from the bundle:

adafruit_lps35hw.mpy

•

adafruit_bus_device

•

adafruit_register

•

You can also download theadafruit_lps35hw.mpyfromits releases page on Github().

Before continuing make sure your board's lib folder or root filesystem has theadafruit

_lps35hw.mpy,adafruit_bus_device, and adafruit_registerfiles and folderscopied

over.

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

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Python Installation of LPS35HW 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-lps35hw

•

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 pressure levels from the board's Python REPL.

Run the following code to import the necessary modules and initialize the I2C

connection with the sensor:

import time
import board
import busio
import adafruit_lps35hw

i2c = busio.I2C(board.SCL, board.SDA)
lps35hw = adafruit_lps35hw.LPS35HW(i2c)

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

pressure- The barometric pressure in hPa.

•

temperature- The temperature in degrees C.

•

For example to print the pressure and temperature values:

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print("Pressure: %.2f hPa" % lps35hw.pressure)
print("Temperature: %.2f C"% lps35hw.temperature)

For more details, check out the library documentation().

That's all there is to using the LPS33HW/LPS35HW sensor with CircuitPython!

Full Example Code

# SPDX-FileCopyrightText: 2019 Bryan Siepert, written for Adafruit Industries
#
# SPDX-License-Identifier: Unlicense
import time
import board
import adafruit_lps35hw

i2c = board.I2C() # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C() # For using the built-in STEMMA QT connector on a
microcontroller
lps = adafruit_lps35hw.LPS35HW(i2c)

while True:
print("Pressure: %.2f hPa" % lps.pressure)
print("Temperature: %.2f C" % lps.temperature)
print("")
time.sleep(1)

Python Docs

Python Docs()

Downloads

LPS33HW Files

LPS35HW Datasheet()

•

LPS33HW Datasheet()

•

LPS33HW System Integration App Note()

•

EagleCAD files on GitHub()

•

LPS33HW Fritzing object from Adafruit Fritzing Library()

•

LPS35HW Fritzing object from Adafruit Fritzing Library()

•

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LPS33HW Schematic

LPS33HW Fab Print

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LPS35HW Schematic

LPS35HW Fab Print

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