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

Project description

sht-sensor

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

Pure-python module 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).

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 and SHT7x 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 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 pin 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 pin 21 and DATA pin 17:

% sht -v 21 17
TODO: copy-pasted output

For both the tool and module, be sure to check/specify correct voltage that the sensor is connected to:

% sht --voltage=5V --temperature 21 17
TODO: copy-pasted output

This voltage value is used to pick coefficient (as presented in datasheet table) for temperature calculation, and incorrect setting here should result in incorrect values (all of them, as RH also uses T in calculation).

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 SHT7x datasheet) there match your model’s datasheet exactly.

See sht --help output for more command-line arguments for the 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()

Again, be sure to specify correct voltage value (on sensor’s VDD pin, exactly as it is presented in datasheet table) for calculations, if it’s not module-default ‘3.5V’, for example: sht = Sht(21, 17, voltage='5V').

Some calculations (e.g. for RH) use other sensor-provided values, so it’s possible to pass these to corresponding read_* functions, 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, its interface (“get_pin_value” and “set_pin_value” attrs/functions) can be re-implemented and passed as 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.

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 (not 3.X).

Using pip is the best way:

% pip install sht-sensor

If you don’t have it, use:

% easy_install pip
% pip install sht-sensor

Alternatively (see also pip2014.com and install guide):

% curl https://raw.github.com/pypa/pip/master/contrib/get-pip.py | python
% pip install sht-sensor

Or, if you absolutely must:

% easy_install sht-sensor

But, you really shouldn’t do that.

Current-git version can be installed like this:

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

Note that to install stuff in system-wide PATH and site-packages, elevated privileges are often required. Use “install –user”, ~/.pydistutils.cfg or virtualenv to do unprivileged installs into custom paths.

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

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.skipcrc() method, but no idea why it might be preferrable to skip this check.

  • Changing SCK clock rate.

    Might be desirable for slower boards or more electric-noisy environments.

Project details


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