A South Plugin Example In Python: the DHT11 Sensor¶
Let’s try to put all the information together and write a plugin. We can continue to use the example of an inexpensive sensor, the DHT11, used to measure temperature and humidity, directly wired to a Raspberry PI. This plugin is available on github, Fledge DHT11 South Plugin.
First, here is a set of links where you can find more information regarding this sensor:
The Hardware¶
The DHT sensor is directly connected to a Raspberry PI 2 or 3. You may decide to buy a sensor and a resistor and solder them yourself, or you can buy a ready-made circuit that provides the correct output to wire to the Raspberry PI. This picture shows a DHT11 with resistor that you can buy online.
The sensor can be directly connected to the Raspberry PI GPIO (General Purpose Input/Output). An introduction to the GPIO and the pinset is available here. In our case, you must connect the sensor on these pins:
VCC is connected to PIN #2 (5v Power)
GND is connected to PIN #6 (Ground)
DATA is connected to PIN #7 (BCM 4 - GPCLK0)
This picture shows the sensor wired to the Raspberry PI and this is a zoom into the wires used.
The Software¶
For this plugin we use the ADAFruit Python Library (links to the GitHub repository are above). First, you must install the library (in future versions the library will be provided in a ready-made package):
$ git clone https://github.com/adafruit/Adafruit_Python_DHT.git
Cloning into 'Adafruit_Python_DHT'...
remote: Counting objects: 249, done.
remote: Total 249 (delta 0), reused 0 (delta 0), pack-reused 249
Receiving objects: 100% (249/249), 77.00 KiB | 0 bytes/s, done.
Resolving deltas: 100% (142/142), done.
$ cd Adafruit_Python_DHT
$ sudo apt-get install build-essential python-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
build-essential python-dev
...
$ sudo python3 setup.py install
running install
running bdist_egg
running egg_info
creating Adafruit_DHT.egg-info
...
$
The Plugin¶
This is the code for the plugin:
# -*- coding: utf-8 -*-
# FLEDGE_BEGIN
# See: http://fledge-iot.readthedocs.io/
# FLEDGE_END
""" Plugin for a DHT11 temperature and humidity sensor attached directly
to the GPIO pins of a Raspberry Pi
This plugin uses the Adafruit DHT library, to install this perform
the following steps:
git clone https://github.com/adafruit/Adafruit_Python_DHT.git
cd Adafruit_Python_DHT
sudo apt-get install build-essential python-dev
sudo python setup.py install
To access the GPIO pins fledge must be able to access /dev/gpiomem,
the default access for this is owner and group read/write. Either
Fledge must be added to the group or the permissions altered to
allow Fledge access to the device.
"""
from datetime import datetime, timezone
import uuid
from fledge.common import logger
from fledge.services.south import exceptions
__author__ = "Mark Riddoch"
__copyright__ = "Copyright (c) 2017 OSIsoft, LLC"
__license__ = "Apache 2.0"
__version__ = "${VERSION}"
_DEFAULT_CONFIG = {
'plugin': {
'description': 'Python module name of the plugin to load',
'type': 'string',
'default': 'dht11'
},
'pollInterval': {
'description': 'The interval between poll calls to the device poll routine expressed in milliseconds.',
'type': 'integer',
'default': '1000'
},
'gpiopin': {
'description': 'The GPIO pin into which the DHT11 data pin is connected',
'type': 'integer',
'default': '4'
}
}
_LOGGER = logger.setup(__name__)
""" Setup the access to the logging system of Fledge """
def plugin_info():
""" Returns information about the plugin.
Args:
Returns:
dict: plugin information
Raises:
"""
return {
'name': 'DHT11 GPIO',
'version': '1.0',
'mode': 'poll',
'type': 'south',
'interface': '1.0',
'config': _DEFAULT_CONFIG
}
def plugin_init(config):
""" Initialise the plugin.
Args:
config: JSON configuration document for the device configuration category
Returns:
handle: JSON object to be used in future calls to the plugin
Raises:
"""
handle = config['gpiopin']['value']
return handle
def plugin_poll(handle):
""" Extracts data from the sensor and returns it in a JSON document as a Python dict.
Available for poll mode only.
Args:
handle: handle returned by the plugin initialisation call
Returns:
returns a sensor reading in a JSON document, as a Python dict, if it is available
None - If no reading is available
Raises:
DataRetrievalError
"""
try:
humidity, temperature = Adafruit_DHT.read_retry(Adafruit_DHT.DHT11, handle)
if humidity is not None and temperature is not None:
time_stamp = str(datetime.now(tz=timezone.utc))
readings = {'temperature': temperature, 'humidity': humidity}
wrapper = {
'asset': 'dht11',
'timestamp': time_stamp,
'key': str(uuid.uuid4()),
'readings': readings
}
return wrapper
else:
return None
except Exception as ex:
raise exceptions.DataRetrievalError(ex)
return None
def plugin_reconfigure(handle, new_config):
""" Reconfigures the plugin, it should be called when the configuration of the plugin is changed during the
operation of the device service.
The new configuration category should be passed.
Args:
handle: handle returned by the plugin initialisation call
new_config: JSON object representing the new configuration category for the category
Returns:
new_handle: new handle to be used in the future calls
Raises:
"""
new_handle = new_config['gpiopin']['value']
return new_handle
def plugin_shutdown(handle):
""" Shutdowns the plugin doing required cleanup, to be called prior to the device service being shut down.
Args:
handle: handle returned by the plugin initialisation call
Returns:
Raises:
"""
pass
Building Fledge and Adding the Plugin¶
If you have not built Fledge yet, follow the steps described here. After the build, you can optionally install Fledge following these steps.
If you have started Fledge from the build directory, copy the structure of the fledge-south-dht11/python/ directory into the python directory:
$ cd ~/Fledge
$ cp -R ~/fledge-south-dht11/python/fledge/plugins/south/dht11 python/fledge/plugins/south/
$
If you have installed Fledge by executing
sudo make install, copy the structure of the fledge-south-dht11/python/ directory into the installed python directory:
$ sudo cp -R ~/fledge-south-dht11/python/fledge/plugins/south/dht11 /usr/local/fledge/python/fledge/plugins/south/
$
Note
If you have installed Fledge using an alternative DESTDIR, remember to add the path to the destination directory to the cp command.
Add service
$ curl -sX POST http://localhost:8081/fledge/service -d '{"name": "dht11", "type": "south", "plugin": "dht11", "enabled": true}'
Note
Each plugin repo has its own debian packaging script and documentation, And that is the recommended way to go! As above method(s) may need explicit action for linux and/or python dependencies installation.
Using the Plugin¶
Once south plugin is added as an enabled service, You are ready to use the DHT11 plugin.
$ curl -X GET http://localhost:8081/fledge/service | jq
Let’s see what we have collected so far:
$ curl -s http://localhost:8081/fledge/asset | jq
[
{
"count": 158,
"asset_code": "dht11"
}
]
$
Finally, let’s extract some values:
$ curl -s http://localhost:8081/fledge/asset/dht11?limit=5 | jq
[
{
"timestamp": "2017-12-30 14:41:39.672",
"reading": {
"temperature": 19,
"humidity": 62
}
},
{
"timestamp": "2017-12-30 14:41:35.615",
"reading": {
"temperature": 19,
"humidity": 63
}
},
{
"timestamp": "2017-12-30 14:41:34.087",
"reading": {
"temperature": 19,
"humidity": 62
}
},
{
"timestamp": "2017-12-30 14:41:32.557",
"reading": {
"temperature": 19,
"humidity": 63
}
},
{
"timestamp": "2017-12-30 14:41:31.028",
"reading": {
"temperature": 19,
"humidity": 63
}
}
]
$
Clearly we will not see many changes in temperature or humidity, unless we place our thumb on the sensor or we blow warm breathe on it :-)
$ curl -s http://localhost:8081/fledge/asset/dht11?limit=5 | jq
[
{
"timestamp": "2017-12-30 14:43:16.787",
"reading": {
"temperature": 25,
"humidity": 95
}
},
{
"timestamp": "2017-12-30 14:43:15.258",
"reading": {
"temperature": 25,
"humidity": 95
}
},
{
"timestamp": "2017-12-30 14:43:13.729",
"reading": {
"temperature": 24,
"humidity": 95
}
},
{
"timestamp": "2017-12-30 14:43:12.201",
"reading": {
"temperature": 24,
"humidity": 95
}
},
{
"timestamp": "2017-12-30 14:43:05.616",
"reading": {
"temperature": 22,
"humidity": 95
}
}
]
$
Needless to say, the North plugin will send the buffered data to the PI system using the OMF plugin or any other north system using the appropriate north plugin.
