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By L. Abraham Smith N3BAH
PHREAK OUT WITH THE
For many years, hackers built special
electronic devices known as “blue boxes” for the
purpose of transmitting special audio tones over
telephone lines. The practice — known as blue
boxing — provided challenges to the subcategory
of hackers known as phone “phreaks.” They had to be able to both construct
the tone oscillator circuits to produce the telephone company’s proprietary
audio tones, and devise how to employ them to explore the telephone
switching networks. While the practice of blue boxing died out in the 1990’s
with the introduction of electronic switching systems which were not
susceptible to in-band tone manipulation, interest in the technical aspects
of telephone communications continued.
Now, a new generation of telephone experimenters are phreaking out over Asterisk™ ( www.asterisk.org) — the
open source Linux program which can put your own private
telephone system inside your computer! The proliferation of
computers running Asterisk interconnected over the Internet
and interfaced to the public switched telephone network
(PSTN) has given rise to a new form of legal phreaking
where users configure their Asterisk servers with special
features which can only be accessed by users transmitting
the old-school blue box tones in appropriate combinations.
This project which has gained worldwide popularity is
known as Project MF ( www.projectmf.com). Briefly stated,
Project MF recreates the techno environment which existed
“back in the day,” making it possible to experience the
challenges of blue boxing from the legal safety of the
Asterisk VOIP system.
Recently, I began experimenting with Asterisk and
Project MF. I needed a low cost programmable audio tone
generator capable of producing standard Dual-Tone Multi-Frequency (DTMF) telephone touchtones, as well as unique
Single- and Multi-Frequency (MF) tones. I wanted to design
a circuit using readily available components which was easy
to assemble and which could be modified by firmware
changes as my needs evolved. The solution I implemented
uses a microcontroller and provides the novice builder with
52 October 2010
an inexpensive and fun way to learn about the interaction of
micro hardware and software while constructing a unique
and useful tone generation device.
Why a Microcontroller?
While there is no shortage of PC software which could
generate the necessary tones, what fun is that? In order to
capture the full effect of the retro-tech environment
presented by Project MF, you need to build a hardware blue
box. I found a number of schematics on the Internet, most
of which either required obscure tone generator ICs or set
the tones with tedious potentiometer-tuned circuits. I
wanted a circuit which would use readily available parts and
allow tones to be easily programmed and changed via
embedded software. The desire for a combination of
hardware and modifiable software in one package led me to
consider a microcontroller.
Most microcontrollers share basic features with each
manufacturer’s products having their own unique
instructions or programming objects which make them
better suited for certain tasks. My experience has
demonstrated that the BASIC Stamp 2 from Parallax, Inc., is
uniquely suited for the purpose of audio frequency
generation. There are less expensive micro platforms than
