Simple Wireless Audio System
takes very few additional components to build a working
As we described earlier, the transmitter antenna is
simply a loop of speaker wire long enough to go around
the perimeter of the area of coverage. Be sure to use an
appropriate impedance matching resistor with the proper
power rating. Remember to observe the handling precautions described earlier.
For the receiver antenna, you'll save a lot of time and
effort if you use a telephone pick-up coil such as the one
described above. Although we won't go into detail here,
you should know that it is possible to make your own
coils. I experimented with a few homemade coils ranging
from an air core version made by wrapping speaker wire
around a plastic tube, to winding smaller diameter wire
around a ferrite rod. I had varying results depending upon
the type of wire, number of turns, and type of core used.
This is a good opportunity for some experimentation!
FIGURE 5. Audio Transmitter
R4 20Ω, 10 watt resistor
Q1 TIP31 NPN transistor (and heatsink)
ANT1 A length of speaker wire
(also see parts list for Audio Amplifier Module)
The transmitter and receiver circuits are simple
enough to be constructed using point to point soldering.
Use general purpose prefabricated PC boards (purchased
at most electronics suppliers).
This will help to reduce component wiring. In order to
reduce power line hum, RF noise, etc., make sure that
component leads and connecting wires are as short as
The LM386 and LM555 ICs should be mounted in
sockets as this eliminates the possibility of damaging
them while soldering. Additionally, it's easier to replace an
IC if it fails. Remember to observe the proper polarity for
all electrolytic capacitors.
1. Looping the World: Rediscovering a simple and
effective hearing assistance system, by Paula
Hendricks and Norman Lederman.
You can get it at:
2. Low Frequency Induction Cave Radio Electronics
We've just scratched the surface in this article. There
are a number of new ideas to investigate.
For example, there is communication over a modulated carrier, rather than the baseband system described in
this article. Another idea would be a bidirectional voice
communication system. A third would be short range data
transmission for microprocessor based data acquisition