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Driver and command-line tool for Sensirion SHT1x and SHT7x sensors connected to GPIO pins.

Project description

Python driver and command-line tool for Sensirion SHT1x and SHT7x sensors connected to GPIO pins.

Originally written by Mike Kazantsev (mk-fg). Python 2.7-only version at mk-fg/sht-sensor for historial purposes. This repository includes support for Python 2.7/3.5.

Description

This is a pure-python module that only requires /sys/class/gpio interface, provided by the Linux kernel and should work on any device that has it (including RPi, Beaglebone boards, Cubieboard, etc - any linux).

Its main purpose is reading temperature (in degrees Celsius) and humidity (%RH) values from these devices, checking CRC8 checksums for received data to make sure it was not corrupted in transfer.

SHT1x (SHT10, SHT11, SHT15) and SHT7x (SHT71, SHT75) are fairly popular and accurate capacitive/band-gap relative humidity and temperature sensor IC’s, with digital output via custom 2-wire serial interface.

SHT1x differs from SHT7x in packaging, with SHT1x being surface-mountable one and latter having pluggable FR4 package.

Sensors include additional functionality available via the status register (like VDD level check, enabling internal heating element, resolution, OTP reload, etc) which may or may not also be implemented here, see “Stuff that is not implemented” section at the end.

Usage

Module can be imported from the python code or used via included command-line tool, which should be installed along with the module (or can be used via ./sht symlink in the repo root without installation).

See “Installation” section below on how to install the module.

GPIO numbers (to which SCK and DATA sensor pins are connected) must be specified either on command-line (for cli tool) or on class init (when using as a python module).

Example, for SCK connected to gpio 21 and DATA to gpio 17:

% sht -v -trd 21 17
temperature: 25.07
rh: 26.502119362
dew_point: 4.4847911176

GPIO “pin” numbers here (and in python module) use whichever numbering scheme kernel has in /sys/class/gpio, which is likely be totally different from the actual (physical) pin numbers on the board headers, and can potentially change between board revisions (e.g. RPi rev 1.0 -> 2.0) or even kernel updates, so be sure to check up-to-date docs on these.

For both the tool and module, also be sure to check/specify correct voltage (default is ‘3.5V’, value is from the datasheet table, not free-form!) that the sensor’s VDD pin is connected to:

% sht --voltage=5V --temperature 21 17
25.08

This voltage value is used to pick coefficient (as presented in datasheet table) for temperature calculation, and incorrect setting here should result in less precise output values (these values add/subtract 0.1th of degree, while sensor’s typical precision is +/- 0.4 degree, so mostly irrelevant).

If you’re using non-SHT1x/SHT7x, but a similar sensor (e.g. some later model), it might be a good idea to look at the Sht class in the code and make sure all coefficients (taken from SHT1x/SHT7x datasheet - google it, sensirion.com URL for it changed like 4 times in 2y) there match your model’s datasheet exactly.

See sht –help output for the full list of options for command-line tool.

Example usage from python code:

from sht_sensor import Sht
sht = Sht(21, 17)
print 'Temperature', sht.read_t()
print 'Relative Humidity', sht.read_rh()

Voltage value (see note on it above) on sensor’s VDD pin can be specified for calculations exactly as it is presented in datasheet table (either as a string or ShtVDDLevel enum value), if it’s not module-default ‘3.5V’, for example: sht = Sht(21, 17, voltage=ShtVDDLevel.vdd_5v).

It might be preferrable to use ShtVDDLevel.vdd_5v value over simple ‘5V’ string as it should catch typos and similar bugs in some cases, but makes no difference otherwise.

Some calculations (e.g. for RH) use other sensor-provided values, so it’s possible to pass these to the corresponding read_* methods, to avoid heating-up sensor with unnecessary extra measurements:

t = sht.read_t()
rh = sht.read_rh(t)
dew_point = sht.read_dew_point(t, rh)

If included sht_sensor.gpio module (accessing /sys/class/gpio directly) should not be used (e.g. on non-linux or with different gpio interface), its interface (“get_pin_value” and “set_pin_value” attrs/functions) can be re-implemented and passed as a “gpio” keyword argument on Sht class init.

ShtComms class is an implementation of 2-wire protocol that sensor uses and probably should not be used directly. All the coefficients, calculations and such high-level logic is defined in Sht class, extending ShtComms.

Installed python module can also be used from cli via the usual python -m sht_sensor ... convention.

Installation

It’s a regular package for Python 2.7 and 3.5.

Using pip is the best way:

% pip install sht-sensor

(add –user option to install into $HOME for current user only)

Or, if you don’t have “pip” command:

% python -m ensurepip
% python -m pip install --upgrade pip
% python -m pip install sht-sensor

On a very old systems, one of these might work:

% easy_install pip
% pip install sht-sensor

% curl https://bootstrap.pypa.io/get-pip.py | python
% pip install sht-sensor

% easy_install sht-sensor

% git clone --depth=1 https://github.com/kizniche/sht-sensor
% cd sht-sensor
% python setup.py install

Current-git version can be installed like this:

% pip install 'git+https://github.com/kizniche/sht-sensor.git#egg=sht-sensor'

Note that to install stuff to system-wide PATH and site-packages (without –user), elevated privileges (i.e. root and su/sudo) are often required.

Use “…install –user”, ~/.pydistutils.cfg or virtualenv to do unprivileged installs into custom paths.

More info on python packaging can be found at packaging.python.org.

Alternatively, ./sht tool can be run right from the checkout tree without any installation, if that’s the only thing you need there.

Misc features / quirks

Description of minor things that might be useful in some less common cases.

ShtCommFailure: Command ACK failed on step-1

Very common error indicating that there’s no response from the sensor at all.

Basically, command gets sent on a wire and at the very first step where there should be response (acknowledgement) from the sensor, there is none.

This would happen if specified pins are not connected to anything for example, which is the most likely issue here - probably worth double-checking GPIO-line/pin numbering scheme (usually GPIO numbers are NOT the same as physical pin numbers, and their wiring may vary between board revisions) and whether controlling specified pins via /sys/class/gpio can be measured - e.g. lights up the LED connected to the pin/gnd or shows up on the multimeter display.

For example, to control voltage on GPIO line number 17 (again, note that it can be connected to any physical pin number, check device docs):

# cd /sys/class/gpio
# echo 17 > export
# echo high > gpio17/direction
# echo low > gpio17/direction

Another simple thing to check is whether used sensor package needs a pull-up resistor, and whether that is connected properly.

Might also be some issue with the sensor of course, but that should be extremely unlikely compared to aforementioned trivial issues.

Max bit-banging frequency control

Max frequency value Can be passed either on command-line with –max-freq or when creating an Sht instance, with separate values for SCK and DATA pins, if necessary.

Sensor can work just fine with very low frequencies like 20Hz - e.g. sht --max-freq 20 -trv 30 60 - though that’d obviously slow things down a bit.

Separate SCK:DATA frequencies (in that order): sht --max-freq 100:200 -trv 30 60

Same from python module: sht = Sht(21, 17, freq_sck=100, freq_data=200)

Stuff that is not implemented

  • Everything related to the Status Register.

    In particular, commands like VDD level check, enabling internal heating element, resolution, OTP reload, etc.

  • Temerature measurements in degrees Fahrenheit.

    These just use different calculation coefficients, which can be overidden in the Sht class. Or degrees-Celsius value can easily be converted to F after the fact.

    Metric system is used here, so I just had no need for these.

  • Lower-resolution measurements.

    Sensor supports returning these after changing the value in the Status Register, so interface to that one should probably be implemented/tested first.

  • Skipping CRC8 checksum validation.

    Code is there, as ShtComms._skip_crc() method, but no idea why it might be preferrable to skip this check.

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