Arduino LESSON 21: Log Sensor Data to an SD Card

In most of our work so far, we have just watched our data go by on the Serial Monitor. In most cases, you will want to have some means to store your data. The easiest way to do this is to use a simple SD card reader. For this example, we use the Virtuabotix SD Card Reader.

SD Card Reader
Arduino connected to a BMP180 pressure sensor and an SD Card Reader

In this tutorial, we will need to have some sensor hooked up so we will have some data to store. We will be using the BMP 180 Pressuer and Temperature sensor from adafruit.  We have a complete tutorial on this sensor  HERE.  You will need to go to that lesson and get the sensor hooked up, the library installed, and the software done. All this is explained step-by-step in the LESSON.

The BMP180 is connected to the arduino as follows:

Connecting Up the BMP180 Pressure and Temperature Sensor
BMP180 Pin Arduino Pin
Vin 5V
GND GND
SCL A5
SDA A4

Once you have the BMP180 connected, test and make sure your code is working, and you are getting good pressure and temperature readings. Once that is working, you are ready to connect your SD card Reader/Writer.

The SD card reader should be connected as follows:

Connecting the SD Card Reader
Sd Card Reader Pin Arduino Pin Details
GND GND Common Ground
3.3 V – (NOT USED)
+5 5V Power
CS 4 Chip Select
MOSI 11 SPI Data
SCK 13 Clock
MISO 12 SPI Data
GND GND Common Ground

In the video we will show step-by-step how to develop the software. You should follow along in the video, and not copy and paste the code below. You will never learn to program if you do not write your own code. The code below is to help you in case you get stuck.

If you have the BMP180 and the SD card connected correctly,  this should create a file called PTData.txt on the card, and write comma delimited data to the file. Note that if the file does not exist on the card, the command:

will create the file. If the file already exists, this command will append data to the existing file. If you want to start with a clean new data set, erase the old PTData file.

When you run the program, you end up with a PTData.txt file on the SD card. When you have finished logging your data, you can pop the card out, put it into your PC, and then import the data into excel. You should now be able to plot, graph or analyze the data using all the powerful features of Excel.

Python with Arduino LESSON 13: Calculating Height from Pressure measurements from BMP180 Pressure Sensor.

It is time to bring together a lot of things we have learned in our earlier lessons to create a Height-O-Meter, which will plot how high our BMP180 pressure sensor is above the floor. For this lesson we make simplifying assumption of constant temperature. When we use the sensor for our space probe or other high altitude experiments we will need to derive the equation again to take into account changing temperature. We went through the math of calculating height from changing pressure in LESSON 12.

In this lesson, we start with the software we developed in LESSON 11 for measuring, streaming, and plotting pressure and temperature data from the BMP180 sensor.

Remember, we connect the sensor to the Arduino as follows:

Connecting Up the BMP180 Pressure and Temperature Sensor
BMP180 Pin Arduino Pin
Vin 5V
GND GND
SCL A5
SDA A4

 

The software we are using on the arduino side is shown below, from LESSON 11.

We modify the Python code from LESSON 11 as explained in the video above to get this code for the Python side.

 Please go through video for complete description of this software. Remember this is only valid for small changes in height over which temperature is constant.

Python with Arduino LESSON 12: Approximating Changes in Height from Changes in Pressure

In LESSON 9 we learned how to hook up a BMP180 Pressure Sensor and make pressure and temperature readings. Then in LESSON 11 we learned how to stream that data to Matplotlib and create live graphs and charts of our data that update in real time. We could see that as we moved the pressure sensor up and down, we could see the pressure change, as the pressure decreases with increasing elevation.

This leads to the interesting question of whether we can use our circuit developed in LESSON 9 to create a Height-O-Meter . . . a simple device that will measure the height above the floor.

The math to calculate altitude vs. pressure turns out to be very complex. Particularly, if we wanted something for our high altitude balloon flights, or for model rocketry. It turns out that for the case of measuring height inside and for relatively small changes in height we can make simplifying assumptions that make things much easier. The assumption we will make is that temperature does not change much over the range of our experiment. With this assumption, we can create our own Height-O-Meter. To do this though, we do need to to through some math. I show my math below, and go through it step-by-step in the video. Remember, this simplified approach is only valid for playing around with small changes in height. We will have to do the more complicated math when we make our high altitude balloon probe. For now though, this math will work pretty well.

Height and Pressure
Calculate Changes in Height from Changes in Pressure

We can rearrange the equation to solve for height as a function of pressure.

Height Pressure
Calculating Height from Pressure Changes

Python with Arduino LESSON 11: Plotting and Graphing Live Data from Arduino with Matplotlib

We now have all the pieces put together to allow us to plot live data from the Arduino. If you have kept up with the earlier lessons, you will now have everything you need. If you have not done the earlier lessons, make sure you have python 2.7, vPython and pySerial installed from Python with Arduino LESSON 2.  Make sure you have installed matplotlib (Python with Arduino LESSON 7), and install drawnow (Python with Arduino LESSON 10). Also, you need to build the BMP180 circuit and get the arduino programmed up as explained in Python with Arduino LESSON 9. With this business taken care of, you are now ready to start plotting live data.

Pressure Data
This chart shows live pressure and temperature data being plotted in real time

We are using the Adafruit BMP180 pressure sensor.  We showed how to hook it up and program it in LESSON 9. As a reminder, we are using this code for the arduino. LESSON 9 explained in detail how the code works.

The video in this lesson above explains step-by-step how to develop the code on the Python side, and how matplotlib and drawnow work together to make live graphs and plots of data streaming from the arduino in real time. The code below is what we developed in the video. Do not simply cut and paste this code, but make sure that you understand it so you are able to create your own live graphing programs from scratch. If you are in my class, you will be required to be able to develop live graphing code like this from scratch, so don’t take a shortcut and copy and paste.

You should be seeing data like the graph on the top of this lesson. You will probably need to adjust your y-axis scale parameters in Python to ensure the scale is suitable for the data you are taking. If your chart is blank, likely your y-scales are not right for your data measurements.

Python with Arduino LESSON 10: Installing Drawnow to Allow Live Plotting with Matplotlib

Our objective with this series of lessons is to plot live data coming from arduino using Python and Matplotlib. We have taken a few lessons to get familiar with Matplotlib, and we have built a circuit to stream live data from arduino to python. We need to install one more library to enable Matplotlib to plot live sensor data in real time. The magic library is called drawnow. The bad news is that this library is hard to install on windows. The good news it that PIP installs it very easy. So, if you have not done so already, you need to go to Python with Arduino LESSON 6 and install PIP. PIP makes it very easy to install drawnow.

Making The World a Better Place One High Tech Project at a Time. Enjoy!