Saving sensor data to a file

Overview

• In this lesson we are going to learn how to save our sensor data to a file.
• Saving data to a file will keep it safe.
• Once data is in a file, we can read the data again and make a graph.
• Reading, saving a graphing data is very important for all environmental and Citizen Science activities.
• Sensors often deliver data a numerical data. Some simple examples are temperature, time, speed, counts, etc.
• In this example we are going to save our data as a String (or text).
• Most files store data as text. In this example we also need to separate the data using commas (,).
• If data is separated by commas it makes each data element human readable and other programs, such as Spreadsheets, can also read the file more easily.

Learning Objectives

• Learn how convert numerical data into a String (or text)
• Learn how to save data to a new empty file
• Learn how to read a saved file and check for data integrity

Converting numerical data to a String

• In this example we will build on the python program from the previous lesson.
• Our python program was used to download data from a sensor (temperature, atmospheric pressure, battery voltage).
• We will now convert this data into a String and put all the data into a single variable named data.
• Each piece of data will be separated by a comma.

Open up the Python file named atm_sensor_get.py

• From the Raspberry Pi main menu drop down select Programming > Python3 (IDLE).

• This will open the Python Shell.

• From the File drop down menu select Recent Files.

• Choose the file named atm_sensor_get.py
• Edit the file so that it appears like the example below.

• Saving and Running the file should produce an output in the Python Shell like the output below.

Arranging numerical data in a String

• We can create (or define) a new empty variable named data to hold all the temperature, pressure and battery data using the statement data = “”

• We need to add or concatenate all the data together into one long String.
• To do this we need to:
  • convert the numerical data to String using the str() function
  • then add each data String together using the plus (+) symbol
• The final code is presented below.
• We also added a print(data) statement so that we can see data variable.

• Save and Run the program. The result should be like the following.

3. Adding spaces between data values

• The issue with the format of this data is that there is no gap between the data.
• To put a gap between the data we need to add a space using this additional code + “ “ +
• See the example below as a reference.

• The output of this program in the Shell is reproduced here.

Adding a human readable header to our data

• To make the data easier to read we can add an additional print() statement to act as a header for our data.
  • print(“Temp, Pres, Bat”)

• With the additional print() header, it makes the data easier to read.
• We also remove the data = “” statement, because we don’t really need it in Python.

Comma separated data

• Rather than having a space separating the data it is better to use a comma (,).
• Replace all the “ “ code with “,” as in the example below.

• After saving and running the program you should get an output like the one below.
• This is an important first step in preparing data to be saved to a file.

Saving data to a file

• Data can be saved in a text file so that we don’t lose the data, and so that other programs on our computer can access the data.
• With bigger data sets you can also save data in a specialised database or data warehouse.
• Saving data to a file is much easier and it also allows us to more easily check the data for integrity.
• In this example we will be creating an empty file named data.txt and then saving new data to this file.

Navigating and listing files using the Terminal

• Open the Terminal.
• When you open the Terminal you will be in your pi home directory. You should see the tilde (~) symbol.
• The path to the pi home directory is /home/pi yet for simplicity this is abbreviated to ~.
• Enter the command ls to list all the contents of the pi home directory.
• In my pi home directory, my project folder is named botanica-park-lake