Shallow Thoughts : tags : plug

Akkana's Musings on Open Source Computing, Science, and Nature.

Mon, 11 Apr 2011

Plug Computer + Arduino = Proximity Camera

In the last article, I wrote about how to get a very simple webcam demo running on a plug computer, also known as a Sheevaplug.

Okay, a webcam is sorta cool, but it's still a pretty easy thing to do from any laptop. I wanted to demonstrate some lower-level hardware control.

As I mentioned in the previous article, trying to run hardware directly from a plug computer is an exercise in frustration. So what do you do when you want to drive low-level hardware? Use an Arduino, of course!

Add the Arduino

[light sensor and Arduino] Happily, the sheeva.with-linux kernels include the FTDI driver you need to talk to an Arduino. So you can plug the Arduino to the plug computer, then let the Arduino read the sensor and write its value to the serial port, which you can read on the plug.

First I tried a simple light sensor from Adafruit, using the circuit from the LadyAda photocell tutorial.

I wrote a very simple Arduino sketch to read the analog output: lightsensor.pde. I'm allergic to Java IDEs, so I compiled the sketch from the commandline using this lightsensor Arduino Makefile. Edit the Makefile to point to wherever you've installed the Arduino software.

Now, on the plug, I needed a Python script to read the numbers coming in on the serial line. I ran apt-get install python-serial, then wrote this script: readsensor.py.

The script loops, reading the sensor and writing the output to an HTML file called arduino.html. Visit that in a browser from your desktop or laptop, and watch it reload and change the number as you wave your hand or a flashlight over the photocell.

Ultrasonic rangefinder for proximity detection

Pretty cool ... if you're extremely geeky and have no life. Otherwise, it's maybe a bit limited. But can we use this Arduino technique to do something useful in combination with the webcam exercise? How about an ultrasonic sonar rangefinder?

The rangefinder comes with a little PC board, and you have to solder wires to it. I wanted to be able to plug and unplug -- the rangefinger also has digital outputs and I may want to experiment with those some day. So I soldered an 8-pin header to the board. (The rangefinder board only has 7 holes, so I had to cut off the 8th pin on the header.)

I ran power and ground wires to 5v and Gnd on the Arduino, and a wire from the rangefinder's analog out to the Arduino's Analog In 2. A little heatshrink keeps the three wires together.

Then I rubber-banded the rangefinder to the front of the webcam, and I was ready to test.

[rangefinder and Arduino] [camera with rangefinder]

Use a sketch almost identical to the one for the light sensor: rangefinder.pde, and its rangefinder Arduino Makefile. I used pin 2 so I could leave the light sensor plugged in on Pin 1.

Now I ran that same readsensor.py script, paying attention to the numbers being printed out. I found that they generally read around 35-40 when I was sitting right in front of it (camera mounted on my laptop screen), and more like 150-250 when I got out of the way and pointed it across the room.

So I wrote a script, proximity.py, that basically does this:

  if data < 45 :
    if verbose :
      print "Snapping photo!"
    os.system("fswebcam --device /dev/video0 -S 1 output.jpg")
It also rewrites the HTML file to display the value it read from the rangefinder, though that part isn't so important.

Put it all together, and the proximity-sensitive camera snaps a photo any time something is right in front of it; otherwise, it keeps displaying the last photo and doesn't snap a new one. Sample uses: find out who's using your computer when you're away at lunch, or run a security camera at home, or set up a camera to snap shots of the exotic wildlife that's visiting your feeder or research station.

You could substitute an infra-red motion sensor and use it as a motion-operated security camera or bird feeder camera. I ordered one, but got bogged down trying to reverse-engineer the sensor (I should have just ordered one from Adafruit or Sparkfun).

I'm happy to say this all worked pretty well as a demo. But mostly, it's fun to know that I can plug in virtually any sensor and collect any sort of data I want. Adding the Arduino makes the plug computer much more fun and useful.

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[ 20:23 Apr 11, 2011    More tech | permalink to this entry | comments ]

Sun, 10 Apr 2011

A simple plug computer webcam

I was asked to give a talk on plug computers ("sheevaplugs") at a local LUG. I thought at first I didn't have much to say about them, but after writing down an outline I realized that wouldn't be a problem.

But plugs aren't that interesting unless you have something fun to demonstrate. Sure, plugs can run a web server or a file server, but that doesn't make for a very fun demo -- "woo, look, I'm loading a web page!" What's more fun? Hardware.

The first step to running any hardware off a plug computer is to get an upgraded kernel. The kernels that come with these things can't drive any useful external gizmos.

I've often lamented how the folks who build plug computers seem oblivious to the fact that a large part of their potential customer base wants to drive hardware -- temperature and light sensors, weather stations, garage door openers, servos, whatever. By not including drivers for GPIO, 1-wire, video and so forth, they're shutting out anyone who doesn't feel up to building a kernel.

And make no mistake: building a kernel for a sheevaplug is quite a bit harder than building one for your laptop or workstation. Some of the hardware isn't supported by fully open source drivers, and most Linux distros don't offer a cross-compiler that can do the job. I covered some of the issues in my LinuxPlanet article on Cross-compiling Custom Kernels for Plug Computers.

Fortunately, the sheeva.with-linux kernels include a webcam driver. That seemed like a good start for a demo.

A simple webcam demo

My demo plug is running Debian Squeeze, which has a wealth of webcam software available. Although there are lots of packages to stream live video to a web server, they all have a lot of requirements, so I settled for a simple snapshot program, fswebcam.

The command I needed to snap a photo is:

fswebcam --device /dev/video0 -S 1 output.jpeg
The -S 1 skips a frame to account for the fact that my cheap and crappy webcam (a Gearhead Sound FX) tends to return wildly striped green and purple images otherwise.

So I run that in a loop, something like:

while /bin/true; do
  fswebcam --device /dev/video0 -S 1 output.jpeg
  sleep 2
done

Now that I have a continuously updating image, I need to run some sort of web server on the plug. Plugs are perfectly capable of running apache or lighttpd or whatever server you favor. But for this simple demo, I used a tiny Python server script: simpleserver.py.

Then all I have to do is a simple web page that includes <img src="output.jpg"> and point my computer at http://192.168.1.102:8080 to see the image. Either refresh the page to see the image update, or add something like

<meta http-equiv="Refresh" content='2'>
to make it refresh

The next parts of the demo added an Arduino to the mix. But this is already getting long and I'm out of time ... so the second part of this demo will follow in a day or two.

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[ 21:18 Apr 10, 2011    More tech | permalink to this entry | comments ]

Fri, 25 Feb 2011

Article: Building kernels for plug computers

This week's Linux Planet article continues the Plug Computer series, with Cross-compiling Custom Kernels for Little Linux Plug Computers.

It covers how to find and install a cross-compiler (sadly, your Linux distro probably doesn't include one), configuring the Linux kernel build, and a few gotchas and things that I've found not to work reliably.

It took me a lot of trial and error to figure out some of this -- there are lots of howtos on the web but a lot of them skip the basic steps, like the syntax for the kernel's CROSS_COMPILE argument -- but you'll need it if you want to enable any unusual drivers like GPIO. So good luck, and have fun!

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[ 11:30 Feb 25, 2011    More tech | permalink to this entry | comments ]

Thu, 10 Feb 2011

Unbricking Plug Computers

This week's Linux Planet article continues my Plug Computing part 1 from two weeks ago. This week, I cover the all-important issue of "unbricking": what to do when you mess something up and the plug doesn't boot any more. Which happens quite a lot when you're getting started with plugs.

Here it is: Un-Bricking Linux Plug Computers: uBoot, iBoot, We All Boot for uBoot.

If you want more exhaustive detail, particular on those uBoot scripts and how they work, I go through some of the details in a brain-dump I wrote after two weeks of struggling to unbrick my first GuruPlug: Building and installing a new kernel for a SheevaPlug. But don't worry if that page isn't too clear; I'll cover the kernel-building part more clearly in my next LinuxPlanet article on Feb. 24.

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[ 09:15 Feb 10, 2011    More tech | permalink to this entry | comments ]

Thu, 27 Jan 2011

Getting Started with Plug Computers

My article this week on Linux Planet is an introduction to Plug Computers: tiny Linux-based "wall wart" computers that fit in a box not much bigger than a typical AC power adaptor.

Although they run standard Linux (usually Debian or Ubuntu), there are some gotchas to choosing and installing plug computers. So this week's article starts with the basics of choosing a model and connecting to it; part II, in two weeks, will address more difficult issues like how to talk to uBoot, flash a new kernel or recover if things go wrong.

Read part I here: Tiny Linux Plug Computers: Wall Wart Linux Servers.

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[ 18:36 Jan 27, 2011    More tech | permalink to this entry | comments ]