Monday, 24 June 2019

An excellent course for Jetson Nano owners

Jetson Nano
Regular readers will know than I'm a keen Jetson Nano owner.

Recently I posted a series about how to started with the computer but NVIDIA have now published an excellent course,  'Getting Started with the Jetson Nano', which is  free for members of the NVIDIA developers' program.

The course comes with a pre-built image which can run the Nano in headless mode. That's very useful - I had to buy a new monitor to get going, as none of my old monitors had native HDMI support.

The image provide with the course just needs a Nano and a Laptop or Desktop computer with a USB port.

The course is a great introduction to deep learning with a GPU. Once you've completed it you may want to delve deeper; there are lots of excellent Deep Learning courses available on-line, and many of them use Google's Colab for practical sessions.

Google Colab gives you free access to top-of-the range NVIDIA hardware, and if you want to run your trained models locally it's easy to move them onto the Nano. Of course the Nano is small enough to use for mobile robotics!

I'm designing an autonomous robot with the Nano, and I have been delighted with Adafruit's Nano port of their Blinka library. It makes it really easy to write Physical Computing code that runs without change on the Nano, the Raspberry Pi and  Adafruit's CircuitPython-enabled boards.

Come and see the Nano

Tomorrow I'll be at the London Raspberry Pint meet-up showing a Nano driving Adafruit, Grove and SparkFun peripherals using a simple interface board (the babelboard) and some straightforward Python code.

If you're in or near London, do come along. The meet-up starts at 7 PM. It's at CodeNode, and you'll save a lot of time if you pre-register at the CodeNode site.

If you can't join us, a video of the talk will be available in a few days and I will be blogging more about the babelboard later this week.

Thursday, 20 June 2019

Use websockets to build a browser-based Digital Voltmeter

This is the third and final part of a series about building a browser-based six-channel Digital Voltmeter (DVM) using an Arduino and a Raspberry Pi.

Part one covered the software on the Arduino and the USB connection to the Raspberry Pi.

Part two showed you how to use the pyserial package to read that data into a Python program and print the results as they arrived.

In this final part you'll see how to use Joe Walnes' brilliantly simple websocketd. You'll see how you can turn the Python program into a server without writing any additional code! Finally, you'll view a web-page that displays the voltages in more or less real-time.

Web sockets with websocketd

HTTP makes it easy for a web browser to ask for data from a web server. That approach is known as client pull because the client (the web browser) pulls (requests) the data from the server.

For the DVM application you want to use server push; in other words, you want to send changes in the voltages from a server on the Raspberry Pi to your browser. Websockets give you a simple, well-supported way of implementing web push.

You'll use websocketd to send the data from the Pi to the browser. websocketd is a really useful program which allows you to wrap any program or script that prints output and send that data to a browser via a websocket. (You can send data back from the browser as well, but the Web DVM doesn't need to do that.)

Four steps to use websocketd

I followed four simple steps to install and use websocketd with the Python program form part 2.

  1. I installed the Python program which you saw in Part two.
  2. Next  I copied the websocketd binary program onto your Raspberry Pi.
  3. I installed a web page with some simple javscript as well as some html.
  4. To wrap the program, I started websocketd and told it where to find the program it should run. For the web DVM I wanted webstocketd to act as a conventional web server. To do that, I provided an extra parameter telling websocketd where to find the web pages to serve.
To make things as simple as possible I have set up a public repository on GitHub which contains all the required software as well as the web page you'll need.

As you'll see below, you just need to download and unzip a file and then run the command that starts the server.

If you're a confident GitHub user you can fork or clone the repository instead of downloading the zip file.

The code on GitHub assumes that you are using a Raspberry Pi with the hostname of raspberrypi. If you want to use your own choice of hostname, you'll need to make one simple change to the web page. I'll cover that in the final step of this post.

Installing the Web DVM project on the Pi

Open a terminal window on the Pi and move into a directory of your choice.


cd cd WebDVM-master/
./websocketd --port=8080 --staticdir=. ./

You should see websocketd displaying messages as it starts up.


 Testing the application

If you open a browser on rasberrypi:8080  you should see a web page like this >>

For demo purposes, I've connected the analog inputs to various fixed and variable voltages. For example, A0 is ground,  A2 is connected to a light-dependent resistor and A3 is connected to the Arduino's 5V rail.

How does the web page work?

The web page is a basic html page with a little JavaScript.

Here's index.html:

<html lang="en">
    <meta charset="UTF-8">
    <title>Web DVM</title>
<h1>Web DVM - 6 Analog Channels</h1>
<div>A0: <span id="A0"></span></div>
<div>A1: <span id="A1"></span></div>
<div>A2: <span id="A2"></span></div>
<div>A3: <span id="A3"></span></div>
<div>A4: <span id="A4"></span></div>
<div>A5: <span id="A5"></span></div>
  // setup websocket with callbacks
  var ws = new WebSocket('ws://raspberrypi:8080/');
  ws.onopen = function() {
  ws.onclose = function() {
  ws.onmessage = function(event) {
      data ='=');
      if (data[0].length === 2) {
          document.getElementById(data[0]).textContent = data[1];

The work is done by a small bit of JavaScript. The code
var ws = new WebSocket('ws://raspberrypi:8080/');
opens a websocket.

That's the bit of code you'll need to change if you want to use a different hstname for the Pi.

Whenever the websocket receives a message, the
code processes it and updates the contents of the web page.


In this three-part series you've met several useful techniques, and built the WebDVM. That's a useful piece of Test Equipment!

You can use the approach to  make lots of different measurements, and there are several ways in which you can add functionality.

In a future blog post I'll show you how to add scrolling graphics to your web-based DVM.

The babelboard is coming

Before that I'll be exploring the babelboard, another simple tool which you can use to quickly prototype physical computing applications.

The babelboard can be used with a Raspberry Pi or a Jetson Nano. Later posts will show how to build versions for the Adafruit feather and the BBC micro:bit.

The babelboard is a simple piece of open source hardware which you can build in an hour using readily available components.

To make sure you don't miss the next series, follow @rareblog on Twitter!

Friday, 14 June 2019

Using pyserial on a Raspberry Pi to read Arduino output - WebDVM part 2

This series describes how to build a simple web-based DVM (digital volt meter) that can display up to six voltages in a web browser.

It's a really useful hack if you need to measure several voltages at once.

In part 1 you saw how to send analog voltage data over a serial link to a Raspberry Pi. For testing purposes you displayed the data in a terminal window on the Pi.

In this part you will use read that data using Python on the Pi and print the data to standard output.

In the next part you'll turn your Python script into a dynamic application that displays the 6 voltages in a browser, updating the web page as the values change.

The code (available on github) uses the pyserial package. It's a simple approach, but there are a few issues to find your way around. You'll see the problems and solutions below.


pyserial is a Python package that allows Python programs to read and write data using serial ports. You can install it via pip.

pip3 install pyserial

When you connect the Arduino to the Pi via a USB cable, the Arduino connection appears as a serial port.

That's where the first problem bit me.

For part one of this series I used a Raspberry Pi zero with an Arduino Uno.
When I connected them the Arduino connection showed up as device /dev/ttyACM0.

For part two, I decided to use an Arduino Nano instead of the Uno. The Nano is smaller and cheaper than the Uno. Like the Uno it has six analog ports, so it looked ideal.

When I ran the software it fell over.

I did a quick check listing the teletype devices in /dev/tty*

There was no /dev/ttyACM0 but there was another new device: /dev/ttyUSB0.

I'd forgotten that the Nano uses a different approach for its USB-to-serial converter, and the Pi sees it as a different device.

So the software now starts with a check to see which device is available.

devices = [port.device for port in list_ports.comports()]
ports = [port for port in devices if port in ['/dev/ttyACM0','/dev/ttyUSB0']]
if len(ports) != 1:
    raise Exception('cannot identify port to use')
port = ports[0]

Once the script has found which prot to use,  pyserial reads from that device in much the same way as you would read from or write to a file.

Unlike a file, though, data will only be available to read once the Arduino has sent it! For that reason, the script wraps the serial connection in a buffer.

It also needs to  convert the serial input from a series of bytes to a unicode string.

Unfortunately, there is no way to synchronise the Pi and Arduino when they first connect. This means it's possible that the Python program starts reading an invalid sequence of bytes, so the program needs to cope with an error when the bytes are converted to a string.

Here's the code that does all that:

with serial.Serial(port, 9600, timeout=TIMEOUT_SECONDS) as ser:
    # We use a Bi-directional BufferedRWPair so people who copy + adapt can write as well as read
    sio = io.TextIOWrapper(io.BufferedRWPair(ser, ser))
    while True:
            line = sio.readline()
        except UnicodeDecodeError:
            continue # decode error - keep calm and carry on

Next the program checks to see that a non-empty line was actually received, prints it to standard output, and flushes the newly printed information. That last step is necessary to make sure that new data is printed out as soon as it has been received. That will become vital in part three of this series.

       if len(line) > 0:
           sys.stdout.flush() # to avoid buffering, needed for websocketd

Part three will describe how to make the anaolg data available in a browser that updates in real time. While you're waiting you can take a look at the code on github.