IEEE Spectrum - North American - March 2016 - 23

raspberry pi radio: The RS-UV3 was
screwed to wooden supports [bottom left] and
mounted. A USB adapter allows audio to pass
between the RS-UV3 and a Pi, while commands
are passed via a DB-9 connector [top left].
To reduce interference, I housed the RS-UV3 in
a foil-lined enclosure [bottom right and middle].
Signals are decoded with Fldigi [above].

But most people who have a radio want to
tune it to more than one frequency. You accomplish this, and other functions such as
adjusting the volume, by setting up a serial
connection with the RS-UV3's processor
and sending it commands. (The setup is
reminiscent of the old dial-up modem Hayes
command codes.)
You can establish the serial connection
in a number of ways, such as mounting an
Arduino directly to the RS-UV3 using the
provided through-holes and then wiring
two of the Arduino's input/output pins to a
header on the shield. Another option is to use
a header intended for FTDI serial cables and
connect it to a PC's USB port.
Or you can use the shield's versatile DB-9
connector. Along with serial transmit and
receive lines, the connector also provides
audio input and output to the transceiver and a
PTT control line. These serial lines use 3.3 volts,

rather than the typical 5 V. This is handy because 3.3 V is the operating voltage of the
Raspberry Pi. Consequently, you can connect
the RS-UV3 directly to the general-purpose
input/output header (GPIO) of the Pi without
much in the way of interface electronics.
And thanks to the upgrade in processing power that came with the release of
the Raspberry Pi 2, the Pi now has enough
horsepower to run Fldigi in addition to controlling the RS-UV3. So I did just that.
First, I built a simple hardware interface between the Pi's GPIO port and the
RS-UV3's DB-9 connector using an old
prototyping "hat" from Adafruit I had lying
around. I also created a wooden enclosure
(as is my wont) for the shield-lined with tinfoil to reduce radio frequency interference-
with a power switch and a PTT button on top
and a slot to hold the Pi.
I used the Pi's configuration tool so that
the Raspbian operating system wouldn't reserve the serial pins on the GPIO for its own
use. I tweaked a Python script I found online,
written by Fabio Varesano, to create a basic
command-line terminal to send commands
to the RS-UV3.
Then I turned my attention to using Fldigi
to listen to and generate signals for the

transceiver. I wired up two mono audio
jacks to the RS-UV3's audio input and output feeds. Unfortunately, while the Pi has a
built-in socket for audio output, it doesn't
have one for audio input. So, following the
advice of Lior Elazary's website, I purchased
a Syba USB audio adapter for the Pi and
plugged the jacks into that.
Installing Fldigi from its source code was
straightforward (I followed Jeffrey Kopcak's
online instructions, although without the remote access steps).
Firing everything up (with the Pi connected
to a keyboard, mouse, and monitor), I was able
to set the RS-UV3 to a suitable frequency and
communicate via a number of digital modes
to a nearby test rig (which consisted simply
of me holding up a handheld ham radio to a
laptop running Fldigi through its built-in loudspeakers and microphone in the quiet of IEEE
Spectrum's offices late at night). Success!
I now have a dedicated digital radio rig, for
less than $150 all told.
Future plans will require obtaining an amplifier, but I hope to build on this basic setup
to create a complete packet radio bulletinboard system that will fit in a relatively small
box. Just in case that whole mobile phone/
Internet thing has a hiccup. -stephen Cass

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MaR 2016

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Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2016