Reading data from an atmospheric sensor

Overview

• In this lesson we are going to write code that will allow us to download data from an atmospheric sensor.
• The sensor will give us data on temperature, atmospheric pressure, battery voltage and signal strength.
• We will use the python language to get this data.
• Once we have data we can do other amazing things such as save the data and even graph the data.
• Environmental data is very important because it allows us to monitor the environment we live in more closely.

Learning Objectives

• Learn how to use the requests library in python to get data from a URL
• Learn how to interpret data presented in the JSON format
• Learn how to process data in python so that it can be saved in variables

Requesting new data from a sensor URL in Python

• Sensors are now commonly connected to the internet.
• This has given rise to the term – The Internet of Things.
• If a sensor is connected to the internet then it will often have a URL (Universal Resource Locator) to allow us to interact with the sensor.
• One common interaction is to be able to see or download data collected by the sensor.
• To get data from a sensor we need to import the requests library in python.
• In this lesson we will download some data from an atmospheric sensor.
• The sensor data includes temperature, air pressure, sensor battery voltage and signal strength.
• In this example we write a python3 program to download sensor data using the requests library.
• The data will be presented in the python Shell.

Create a new file in python named atm_sensor_get.py

• From the Raspberry Pi main menu drop down select Programming > Python3 (IDLE).
• This will open up the Python Shell.
• From the File drop down menu select New File.
• This will open up a new file Editor Window.
• We are going to name the file atm_sensor_get.py

Adding Comments within files

• We can add notes within our program called Comments.
• Comments always start with a # (hash symbol) at the start of the line.
• Computers ignore comments because comments are only read by humans.
• To signal that the text has been commented out, the text colour will change to red.
• In this example the comment is the file name.

• To save the new file, select File > Save As

• Navigate to your working directory and then save the file as atm_sensor_get.py
• In Linux we never use names that include spaces.
• Replace spaces with either the _ (underline) symbol or the – (hyphen) symbol.

Request data from sensor using python

• Enter the following lines of request to request data from the atmospheric sensor.
• The key items in the code are described:
  • import requests – this statement imports a python library so that the python program can get data from the internet.
  • r = requests.get(URL) – this statement gets the data from a specified URL and stores the result in the variable object named r. URL is short for Uniform Resource Location, also known as a web address.
  • print(r) – this statement prints a code that determines if the request was successful or not.
• Save and the Run the program.

• The response in the Python Shell should be [200].
• A value of [200] means that the request was successful.
• Other numbers or errors will indicate that there is a problem either with the Python code or with communication with the sensor.

3. View sensor data in JSON format

• We can now add some more code so that we can look at the actual data.
• Add the statement print(r.json()) and run the code.
• The method json() means that the data will be presented in JSON format.
• JSON stands for JavaScript Object Notation.
• It is an open standard to help format data for data exchange between computers and it uses human-readable text.
• The data is stored in attribute-value pairs' similar to the format of a dictionary.

• You can see the data output in the python Shell Window.

• Unfortunately, presenting the data this way is not very readable.
• In the next section of this lesson we will learn how to view and interpret data stored in the JSON format.

Interpreting data in json format

• As previously mentioned, JSON is an open standard that allows data to be exchanged more easily between computers.
• JSON data is stored as text, and this makes it human-readable however you need a special application to see this data more clearly.
• In this lesson we will use the web browser Firefox to visualise the data so we can see the arrangement of attribute-value pairs.

Copying, editing and saving JSON formatted data

• One way to see the data more easily is to view it in the web browser Firefox.
• To do this we need to Copy the JSON data output from the Raspberry Pi and Paste it in a file on your PC.
• We also need to replace all ‘ characters with “ otherwise Firefox wont show the JSON data correctly.
• Copy the data above from the Python Shell.
• To copy, right-mouse button click and select Copy.
• Paste it into a text editor on your PC.
• To paste, right-mouse button click and select Paste.
• Replace all ‘ characters with “ using the in-build Find/Replace function in your Text Editor.
• Then Save the data as a file with the file extension .json on your PC.
• In this example the file name is atm_data.json