• Raspberry Pi Mobile: Part 1 - Building the Pi


    Several months ago I purchased a 256MB Raspberry Pi computer off the internet as it seemed like a neat little toy. For $35 I got an ARM-processor based computer, ethernet, HDMI video, and two USB ports...all in a form-factor about the size of a credit card. It seemed like a great deal on a neat little machine; however, I had no planned application for it when it arrived, so it sat on my desk for a number of months. Recently, I pulled the Pi out, installed a copy of the Raspbian OS on an SD card, and got the Pi up and running. After some thought, I downloaded and installed several ham radio applications out of the Raspbian repository (Raspbian is a variant of the Debian Linux distribution recompiled for the ARM processor, so chances are if the software is available for Debian and its derivatives, it's available for Raspbian.) With the software installed, it dawned on me that the Pi would be the perfect guinea pig for a mobile computing experiment.

    This article is the first of a multi-part series on my project to get my Pi mobile. Click on the thumbnail pictures in the article for a full-size view.


    I had three goals for this project;

    1) Test the Pi as a terminal for my D710 internal TNC. For this test, I wanted to see if the Pi would function reliably as a packet radio terminal. The low overhead of the CLI for a packet terminal was an attractive fit for a low-power computer like the Pi. At a minimum, the Pi permanently mounted in my truck would eliminate the need for me to bring along a separate laptop and connect it to the D710 when I wanted to do mobile packet.

    2) Test the Pi running the Linux soundmodem software suite for packet operations. Soundmodem is a Linux software TNC package, and functions in a very similar manner to AGWPE for Windows users. Soundmodem eliminates the need for a hardware TNC for a radio to do packet operations. Using a soundcard interface, the goal was to be able to connect any available radio to the Pi and use it for on-the-fly packet operations. This would allow the use of HT's and other add-on radios in the truck in the event the D710 was tied up in cross-band operations, or other packet needs.

    3) Test the Pi running fldigi software as an HF digital mode host platform. For this test, I wanted to see if the Pi would run fldigi at an acceptable speed, recognize my USB soundcard-radio interface, and reliably function as a host computer for HF digital mode operations.


    Building the Pi was trivial, as the Pi comes fully operational out of the box. All you need to add is an SD card formatted with the Raspbian image (link given above). You will need at least a 4GB SD card, and I would recommend a Class 10 SD card to minimize read/write latency, as the card will be the "hard disk" for the Pi. I purchased a 32GB Class 10 SD card online for about $25. My intent was to utilize space on the card not used by the OS to store MP3 music files and AVI movies to provide entertainment on the road. This is not necessary; however, if you have an external USB harddrive or thumbdrive, as the Pi will mount and access those devices with no problems.

    A wide variety of Pi cases are available on the internet, depending on your desire or form factor. I wanted something simple & cheap for this project and found this available online for $13 shipped:
    The case is simply two clamshell halves which snap together to house the Pi. The case is in no way weather-resistant, and is designed primarily to keep the Pi from suffering crush damage. I dropped the Pi board into the bottom half of the clamshell...
    ...and then snapped the top on to achieve this simple, but effective housing for my Pi:
    The Pi comes with two USB connectors; however, I knew that I would need more USB ports to support all the peripherals which I would add to the basic computer. I searched the internet and found this D-Link 4-port hub online for about $24. The USB hub is powered by an external AC adapter; however, for my mobile installation, I plan on building a 5V power bus in the truck and using that to power the USB hub with DC direct.
    The D-Link hub was a very small form-factor, which was my primary motivation for selecting it...any powered USB hub will do:
    Next I needed a small keyboard and mouse to use with the Pi. I didn't want to have to carry a traditional keyboard in the truck, as I thought it would be too big, and in the way. After a brief search of the internet I found this small thumb-board with an attached touchpad:
    The thumb-board has an internal lithium ion battery and charges via a USB mini connector. It has a small USB dongle which plugs into the Pi and operates in the 2.4GHz band. (Other versions are available which connect to the Pi using Bluetooth.) The board has a switchable backlight for low-light operations, and a small power switch on the side to power it off when not needed. The touchpad is small, but intuitive. I purchased the keyboard for $25 shipped.

    In the photo above you will also see two dongle devices that I attached to the USB hub. One is an 802.11b/g/n adapter to allow me to connect the Pi to wireless networks (this is a separate adapter from the one that comes with the keyboard.) The other dongle is a Bluetooth adapter, to allow wireless serial connection to Bluetooth enabled devices.

    Finally, I needed a monitor for my Pi computer. The Pi is configured to allow tremendous flexibility when it comes to graphical output. The Pi has two outputs...one is a standard VGA output via a composite connector. The other is a full HDMI A/V output. The Pi has an on-board HD graphics card and when connected via HDMI is a very capable machine for GUI environments. Using one of these two outputs, there is a good chance you can quickly connect the Pi to an existing DVD player in your mobile, if you have one. I did not have such capability in my truck and went another route. My wife has an existing DVD player in the dash of her mobile and I will utilize the AUX video input on that to a 7" screen when I put one of these Pi computers in her car.

    For my application, I wanted something cheap and small. I don't have a lot of dash space for a screen, and really just wanted something about the size of my Garmin Nuvi 350 which was previously running with my D710. After searching the internet, I located a very cheap 3.5" display powered by 12V DC for $23 shipped:
    The display is VGA-quality, so I was concerned that the video resolution of a VGA output coupled with the small display would make it marginally useful for the Pi; however, at only $23 I figured it was worth a shot, and I could always upgrade the monitor at a later date if desired.

    Once I put all the components together, I got a full computer in a VERY compact little package:
    Yes, that is a $1 bill, and my computer footprint is not much bigger than that! The Pi can be seen on the bottom of the stack. On top the Pi is the USB hub, and on top the hub is the 3.5" display. This was exactly what I had in mind for the truck, and would be easy to install taking up a very minimum amount of space.

    Although my objective for the build was a compact package, realize that the Pi computer can be used with full-sized keyboard & mouse components, as well as any size VGA or HDMI-capable display. You can even use one setup, and have a more robust component set that you keep in a go-box in case you need it for protracted or high-intensity level exercises. I did not have such a need, so my build was designed for casual & weekend operating from remote parts of AZ.

    Stay tuned for Part 2, where we test the Pi as a TNC terminal with my D710 radio.

    Until next time...

    This article was originally published in forum thread: Raspberry Pi Mobile: Part 1 - Building the Pi started by KE7KUS View original post