The transmitter has more
than enough power to generate
a signal that can cover a large
room. You might be able to use
it (along with the receiver
described later) as a pager
between adjacent rooms. Try
taping the loop antenna to the
wall separating rooms and see if
you can pick up the signal in the
FIGURE 4. Audio Receiver
Telephone pickup coil
(e.g., RadioShack part #44-533 in 2002 catalog)
SPKR1 8 ohm speaker (also see parts list for
Audio Amplifier Module)
system, start by checking all connections. Make sure that
the transmitter and receiver antennas are connected
Place the receiver inside of the transmitter antenna
loop and turn it on. Set the volume to half maximum.
Next power up the transmitter and set the volume to minimum level. Gradually increase the volume until you can
hear the transmitter signal at the receiver. If you notice
power line interference try changing the orientation of the
One note of caution. The impedance matching resistor can get very warm (even hot). Handle it carefully while
the transmitter is operating. keep it away from material
that is flammable or can easily melt.
Building Your Own System
The circuit in Figure 3 is an
audio amplifier circuit using an
LM386. This basic circuit, combined with additional components, is used in the applications that follow. The LM386 s a very popular low power
audio amplifier chip and is widely available at low cost. It
requires a minimum of external parts, and has a wide
supply voltage range ( 4 V to 12 V for the LM386N-1, the
version used in this circuit). The voltage gain can be set
anywhere between 20 and 200.
The gain for our amplifier is fixed and set to maximum, by connecting capacitor C3 between pins 1 and 8.
Potentiometer R1 controls the output level. Input coupling
capacitor C1 should be selected so that at the lowest frequency of interest its impedance is small compared to R1
(about one tenth of R1).
Bypass capacitor C6 may or may not be required.
While working with this circuit I was getting significant
interference from a local radio station (especially when
the input was unconnected). Placing C6 between the
input and ground eliminated the problem.
Note that the National Semiconductor data sheet for
the LM386 indicates that high gain applications might
require a bypass capacitor between pin 7 and ground. I
found that it wasn't required.
It's easy to build your own simple wireless audio system from scratch. It takes few parts. They are readily
available and relatively inexpensive.
Audio Frequency Signal
NUTS & VOLTS
To illustrate just how easy it is we'll start with the
transmitter circuit in Figure 2. It's simply a 555 timer configured as an astable multivibrator. It generates a 1 kHz
square wave signal which switches transistor Q1 on and
off. Q1 drives the antenna L1 which is a loop of speaker
wire in series with a 10 watt (minimum), 20 ohm power
resistor. Remember the antenna needs to be long enough
to encircle the area of coverage.
Wireless Audio Receiver
To create a wireless receiver you simply connect a
pickup coil to the input of the amplifier circuit and an 8
ohm speaker to its output, as shown in Figure 4.
Wireless Audio Transmitter
Figure 5 shows a simple wireless audio transmitter
suitable for voice quality communication. The LM386
only provides about 1/4 W of output power. In order to
beef up the power, add the emitter follower circuit formed
by Q1, R3 and R4. Another option for the transmitter is to
use a more powerful audio amplifier chip. One candidate
is the LM384 5 W audio amplifier. Like the LM386, it