Simple Wireless Audio System
tiny coil of wire, called a Telecoil. The Telecoil allows the
user to hear audio signals being transmitted over an induction loop system (note that due to the trend towards smaller hearing aids and cochlear implants, Telecoils had
begun to disappear from hearing aids in the US — but
there has been a renewal of interest lately).
Short Range Communication
Induction loop systems have been used by cave
explorers as alternatives to RF systems. Radio signals in
the LF and VLF bands can propagate through rock, however they can require fairly large antennas. An induction
loop system requires a relatively small loop in order to
cover the short distance between the cave and the surface.
Short Range Data Transmission
Magnetic induction is being proposed as an alternative to RF and IR in a number of applications including
wireless data transfer between computer peripherals, and
wireless headsets for cell phones.
At least one company, Aura Communications
( www.auracomm.com/Technology), has produced a
chip set enabling designers to construct devices capable
of short range communication using this technology. They
have recently released a reference design for mobile telephone headsets named "Docker" that is receiving positive
reviews in the press.
FIGURE 2. A Simple Signal Generator
.01 µF capacitor
.001 µF capacitor
20Ω, 10 watt resistor
TIP31 NPN transistor (and heatsink)
LM555 timer IC
SPST momentary contact switch
Constructing An Induction Loop
In the remainder of this article we'll describe how to
build a one-way voice quality induction loop communication system. It's intended to be a starting point for further
investigation into applying this approach to creating low
bandwidth short range communication systems.
The easiest way to get started is to use an existing
HiFi receiver (radio, CD player, etc.) with an external
speaker connection, for the transmitter. The output power
should be at least 4 W. Run a length of speaker wire
around the room. Connect one end of the wire to the
speaker output terminal of the amplifier.
The total impedance and power rating of the antenna
loop should match that of the amplifier. For example, if
the amp output is rated at 10 W into 8 ohms, then use an
8 ohm resistor with a minimum rating of 10 W, as an
impedance matching resistor.
Connect the other end of the wire, in series with the
impedance matching resistor, to the other speaker output
terminal. The receiver can be a portable battery powered
The receiver antenna is an inductive pickup coil which
can be purchased from an electronics parts store. For
example, RadioShack carries a telephone pick-up coil (part# 44-533 in the 2002 catalog), and
a battery powered amplifier (part# 277-1008)
which are suitable for this project. To test the
FIGURE 3. Audio Amplifier Module
C1 . 22 µF capacitor
C2 .1 µF capacitor
C3 10 µF electrolytic capacitor
C4 .047 µF capacitor
C5 220 µF electrolytic capacitor