Category Archives: Tutorial

Beaglebone Black LESSON 6: Control PWM Signals on Output Pins from Python

In Lesson 4 and Lesson 5 we showed how to do digital writes to the GPIO pins using Python. (If you have not picked up your Beaglebone Black Rev. C yet, you can get one HERE) With digital writes, we could generate an output of 3.3 volts or 0 volts. For many applications, we would like analog output, or the in between voltages. The Beaglebone Black, as with most microcontrollers, can not produce true analog output. However, for many applications, an analog output can be simulated by creating a fast on/off sequence where the analog value is simmulated by controlling the ratio of on time and off time. This technique is called Pulse Width Modulation, or more simply, PWM. Consider a 3.3 volt signal, which is turning on and off with a frequency of 50 Hz.  A 50 Hz signal has a Period of: Period=1/frequency=1/50=.02 seconds, or 20 milliseconds. If during that 20 millisecond period, the signal was “High” for 10 milliseconds, and “Low” for 10 milliseconds, the signal would act like a 1.65 volt analog signal. The output voltage therefor could be considered the rail voltage (3.3 volts) multiplied by the duty cycle (percentage of time the signal is high.

For the Beaglebone Black, only certain pins can be used for PWM signals.

Beaglebone Black Pinout
Default Pin Configuration for the Beaglebone Black Rev. C.

In the chart above, the purple pins are suitable for PWM output. You can see there are 7 pins which can produce PWM signals. In this lesson we show you how to control those pins.

In order to control PWM signals, we are going to use Python and the Adafruid_BBIO Library. Recent versions of Beaglebone Black Rev. C are shipped with the library already part of the operating system. If you are getting errors indicating that you do not have the library, update your operating system to the latest Debian image for the Beaglebone Black.

In order to use PWM in Python, you must load the Adafruit Library. If you have the recent versions of Debian Wheezy for the Beaglebone black, the library will already be on your system. If you do not do an update and upgrade on your operating system.

To begin with, you will need to load the library.

 Next up, you will need to start the PWM on the pin you are using. We will use pin “P8_13”. Remember you must use one of the purple colored pins on the chart above. We start the PWM with the following command:

This command puts a 1000 Hz signal (Period of 1 mSec) on pin P8_13, with a duty cycle of 25%. This should yield a simulated analog voltage of .84 volts.

We can change the duty cycle after this initial setup with the command:

This command would change the duty cycle to 90%, which would simulate a voltage of 3.3 * .9 =  2.97 volts.

You can also change the frequency of the signal using the command:

This would change the frequency to 100 Hz (Period of 10 mSec). Changing the frequency does not really affect the net result of PWM in most applications, although it does matter for many servo applications.

After you are done, you can stop the PWM with the command:

And always remember to clean up after yourself with:

Play around with the Python Program below. Connect a DVM to your Beaglebone Black, and measure the DC voltage at the output pin. The DVM should show your anticipated voltages.

Considering that the simulated analog voltage V=3.365 X Duty Cycle, how would modify the program above to ask the user for the Voltage he desires, and then calculate the duty cycle that would give that voltage. Your assignment is to modify the program above where the user inputs desired voltage, and DC is calculated. Use a DVM to check your results

Comparing the Arduino, Raspberry Pi Model 2, and Beaglebone Black

In this video we do a head to head comparison of the Arduino, Raspberry Pi Model 2, and the Beaglebone black. We compare the pros and cons of each platform and discuss how to decide which platform to learn on and which is best for different types of projects.

You can pick up the gear discussed in this video below:

Arduino: This is a great place to start, and the device is very affordable.

Sparkfun Inventor Kit: Everything you need to learn microcontroller programming and circuits. This is the kit we use in our Arduino Lessons, and even includes the Arduino.

Raspberry Pi Kit: This kit has everything you need to follow along on our Raspberry Pi Lessons.

Raspberry Pi: If you already have the cords and cables, you can buy just the Raspberry Pi.

Beaglebone Black: We are not working on a series of lessons showing you how to use the Beaglebone Black. Now would be a good time to go ahead and order your Beagle.

I hope you enjoyed this video lesson, and hope you will jump in and take our lessons on using the Arduino, Raspberry Pi, and the Beaglebone Black

Raspberry Pi LESSON 29: Configuring GPIO Pins as Inputs

We are now ready to learn how to “read” values from the Raspberry Pi GPIO pins. In order to demonstrate this, we will show a simple example using buttons. If you ordered the Raspberry Pi kit we recommend, you already have everything you need, or you can pick your kit up HERE. To start with, you need to put together a simple circuit that connects two push buttons to your Raspberry Pi. Connect according to this schematic.

Raspberry Pi Buttons
Simple Circuit Connecting Two Push Buttons to the Raspberry Pi

Note that one leg of each button is connected to the ground rail on the breadboard, that is connected to the Pi ground at physical pin 6. Then we connect the left leg of the left button to physical pin 16, and the left leg of the right button to physical pin 12.

In order to read the state of these buttons, that is, whether they are being pressed or not, we need to write a python program. To begin with we must import GPIO library and specify that we want to

 Now we are ready to set the pin modes on the pins we are using. We are using pins 12 and 16. We will set up variables so that we can reference the pins by descriptive variables.

Note in our GPIO.setup commands, we are not just defining the pins as inputs, we are also activating pullup resistors with

With this command, the raspberry pi places a pullup resistor between the designated pin and the 3.3 V rail. This means that if we simply read the pin, we will read a “1”, “True”, or “High”, since the pin will see the rail through the pullup resistor. If we connect the pin to ground by pressing a button or switch, the pin will then read a “0”, “False” or “Low” because it will be a straight connection to ground, and as current flows through the pullup resistor, the 3.3 Volts will drop across the pullup resistor. Hence, the pin sees 0 volts.

The result is that with the pullup resistor activated, the pin will always report a “1” until something connects the pin to ground, and then it will read a “0”. This configuration should work for most things, but if you are getting unpredictable results which can result from electrical noise, then try using external pullup resistors.

 Now we are ready to read the values from the pins.

Notice that we read from the pin using the GPIO.input command. Also note that for reliable results you need to usually put a small delay in your code. This will help debounce the button, and will also give more stable results.

OK, so our final code is as follows:

This code will sit and monitor the buttons, and when one is pressed it will report that that button has been pressed.

Python with Arduino LESSON 17: Sending and Receiving Data Over Ethernet

Arduino Ethernet
This circuit contains an Arduino Nano and Pressure Sensor Communicating over Ethernet

In LESSON 16 we showed a simple Client Server model that allows us to send strings between Python running on a PC and the arduino over Ethernet. That lesson simply passed strings back and forth to show a very basic Server on Arduino, and Python acting as the Client. In this lesson we show a more practical example, with the Arduino connected to an Adafruit BMP180 Pressure Sensor. In order to complete this lesson, you will need an Arduino, an Ethernet Shield, and the Pressure Sensor. If you do not have this particular pressure sensor, you can probably follow along in the lesson using whatever sensor you have that is of interest. The video will take you through the tutorial step-by-step, and then the code we developed is shown below.

The key issue in getting this project to work is to get your mac address and IP address from your router or network. If you are at school, simply speak to your network administrator, and he will help you get an IP address for your arduino. If you are at home, you will need to connect to your router from a browser, and configure it to assign an IP address and agree on a mac address for your arduino. Some arduino Ethernet shields have a sticker with a mac address. If your Ethernet shield has a sticker with mac address, use that one. If it does not, you will need to come up with a unique mac address. There are thousands of possible routers and networks out there, so I can not help you with that part. But if you look in the router documentation, you should be able to get the IP address and mac address worked out. The arduino itself does not have a hard wired mac address, but you set the mac address in the arduino software, and the IP address as well. The key thing is that the mac address is unique on your network, and the router and arduino agree on the IP address and mac address. If you have a clearer way to explain this, please leave a comment below.

This is the server side software to run on the arduino. Again, you should use a suitable IP address and mac address for your network. Do not think you can just copy the ones I use in the code below.

Once you have this on your arduino, and the arduino connected to the internet via an Ethernet cable, you can test by opening a command line in Windows. Then ping the address you have assigned to the Arduino. If it pings correctly and you get a reply, you are ready to develop the Python code. The Python will be the client. It will send the requests to the Arduino, and the Arduino will respond with data. Since our circuit can measure pressure or temperature, you can request either of those. When the arduino receives a request for temperature, it will go out, make the temperature measurement and then return the data to Python. Similarly, if you request Pressure the arduino will read the request, will make the Pressure measurement, and then return pressure reading to the client (Python).

 This python code will request Temperature, will then read the response, and then will print the data. It then requests Pressure, reads the response, and then prints it. If you look at our earlier lessons you can see graphical techniques to visually present the data. The hard part is getting the data passed back and forth, which we show how to do in this lesson.

Python with Arduino LESSON 16: Simple Client Server Configuration over Ethernet

Arduino and Ethernet Shield
Ethernet Shields are available which allow the arduino to act as a server

In the previous lessons we have seen that powerful analytic and graphic programs can be written that allow data taken from the arduino to be displayed on a PC via Python. We have shown how the arduino can be connected to a PC by either a serial cable or Xbee radios. To fully unleash the power of the arduino, it can be set up as a server, and connected to a network via Ethernet. In this lesson, we will show how to set the arduino up as a server which is controlled and queried by clients on PC’s on the same network. In order to complete this tutorial, you will need an Arduino Uno and an Ethernet shield. The Ethernet Shield is a relatively expensive component, but I suggest getting an authentic arduino made shield, as I have had poor results from the cheap knock offs. Understand that this lesson is intended for High School students to show them a simple technique for connecting the arduino to a network. It is not an exhaustive treatise on Ethernet communication. The goal is to provide a simple protocol which should allow you to get your arduino talking over Ethernet. It requires that you already know, or can figure out how to assign a mac address and IP address in your router. Some arduino Ethernet shields have a sticker with a mac address. If your Ethernet shield has a sticker with mac address, use that one. If it does not, you will need to come up with a unique mac address. If you are at school, the network administrator can help you get the router configured. I can not provide support in getting this to work on your network, as there are many variables. The techniques provided in this tutorial should work for most networks. This tutorial does not present the most efficient or elegant solution, but the goal is a simple protocol for people just getting started. The video below gives a step-by-step demonstration of setting up the arduino as a server, and Python on a PC as the client. It uses UDP protocol to transfer data packets.

In order to get the arduino to work over Ethernet, you must first assign an IP address to the arduino in your router.  If you just plug the arduino with shield into the network, your router might assign an IP and report a mac address. If this is the case, you need to have the router assign those addresses permanently to the arduino. The bottom line is that the IP address and mac address you identify in the arduino code must match the IP address and mac address assigned in the router. I can not provide any additional help on that issue as there are so many different possible networks. You have to figure out how to do that.

Once you have the IP address and mac address sorted out, you will need to set up the arduino as a server. The video above explains how to do this, and results in this code. Note that your IP address and mac address must be set to a suitable configuration for your router and network. The numbers I use in this code would not work for your network.

This is the code developed in the video to set the arduino up:

Once you have this code in your arduino, ping the IP address of the arduino from your PC cmd line. Make sure your PC can talk to the arduino by successfully pinging it. If you get an error while you are trying to ping the arduino, you will have to stop and get figured out what is wrong. There is no reason to proceed with the lesson until you can successfully ping the arduino.

Once you can ping the arduino, you are ready to set up a client in Python. The code below is what we developed in the video. You will need to watch the video in order to understand the code. Also note, that the IP address in the code below is the IP address of the Arduino, NOT the PC. You should set this to whatever IP address is of YOUR arduino.

 Again, this is a bit of a tricky thing to set up, but if you get the PC successfully pinging the arduino, everything else should be straightforward.