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An FM transmitter
for musicians
These three FM transmitters all use the same
circuit but you can vary the construction to
suit your application. One unit is configured
as a guitar FM transmitter, one as a handheld wireless microphone and the other for
use with a lapel microphone.
Design by BRANCO JUSTIC
This circuit is not new, having been
featured previously in the October
1993 issue of SILICON CHIP. However,
there have been a couple of minor
circuit modifications to tweak the
performance, while the PC board
has been redesigned to make the unit
easier to build.
54 Silicon Chip
In particular, the PC board is now
single-sided, whereas the earlier Mk.1
version used a double-sided PC board
that required soldering on both sides.
As with the previous design, the
new “FM Microphone Mk.2” was
designed by Oatley Electronics and
they will be making available a com-
plete kit of parts. So which version
should you order? Well, you don’t
have to worry about that because the
kit contains all the necessary parts for
each version. It even includes three
labels; you simply use the one that’s
right for your application.
The differences between the three
units are really quite minor. In fact,
the two wireless microphone versions
are identical except that the handheld
unit has the microphone attached to
the case while the second unit has the
microphone attached to a lapel clip.
Fairly obviously, the microphone is
left out of circuit for the guitar FM
transmitter unit and the guitar provides the input signals instead.
Physically, all three versions are
housed in the same plas
tic case,
which measures 125 x 40 x 24mm
Fig.1: the circuit is based
on three transistors (Q1,
Q2 & Q3). Q1 functions
as an audio preamplifier,
while Q2 & Q3 form a
modulated oscillator with
good isolation between
the antenna and the tank
circuit (L1 and its parallel
1pF capacitor).
(length x width x depth). As shown
in one of the photos, a standard 9V
alkaline battery sits at one end of the
case, while the PC board occupies the
other end. A miniature slide-switch
on the side of the case switches the
power on and off.
Performance
As before, this FM transmitter
design features excellent frequency
stability. Some FM wireless microphones can be temperamental devices
to use, particularly as far as frequency
drift is concerned and there are several causes for this. The first of these
is due to a drop in the supply voltage
as the battery ages. The second is due
to capacitance effects between the
user’s body and the dangling antenna.
Third, and not usually recognised, is
drift due to change in temperature.
When you set up an FM wireless
microphone to operate at a particular
frequency, say 95MHz, you don’t expect it to drift much. If it only drifts by
a small amount, the AFC (automatic
frequency control) circuits of your FM
tuner should cope with the change in
frequency so that the signal is always
received clearly. But there is a limit to
the AFC range of any FM tuner (per
haps ±100kHz) and beyond that, the
signal will start to distort badly and
ultimately, will not be received at all.
That is why drift caused by body
capacitance can be so annoying as it
varies all over the place.
This design does not have these
problems. We tested it in a number
of ways, including heating up the
PC board with a hot air gun and even
then, drift was not a problem. Nor do
power supply variations worry it. In
fact, drift due to supply voltage varia
tions of 1V for a 9V supply is quoted
as less than 0.03%.
The operating range is quoted as
better than 100 metres with a good
quality tuner. Other relevant specifications are: signal-to-noise ratio
>60dB; pre-emphasis 50ms; frequency response 40Hz to 15kHz.
Circuit details
Fig.1 shows the circuit which uses
three NPN transistors. Transistor Q1
(BC549) is an NPN audio preamplifier
stage which steps up the input signal
from the electret microphone or from
the guitar. The output from Q1 (at the
collector) is then coupled via a 0.1µF
capacitor and an 8.2kΩ resistor to the
Parts List
1 PC board (available from Oatley
Electronics)
1 electret microphone insert
1 9V alkaline battery
1 9V battery snap
1 subminiature former with core,
can and base (L1)
1 SPST miniature slide switch (S1)
1 50kΩ trimpot (VR1)
Semiconductors
1 BC549 NPN transistor (Q1)
2 BF199 NPN transistors (Q2,Q3)
Capacitors
4 0.1µF monolithic
1 0.047µF monolithic
2 100pF ceramic
1 33pF ceramic
1 22pF ceramic
2 15pF ceramic
1 1pF ceramic (see text)
Resistors (0.25W, 1%)
1 220kΩ
1 8.2kΩ
1 100kΩ
1 6.8kΩ
2 22kΩ
1 1kΩ
1 12kΩ
1 680Ω
1 10kΩ
1 270Ω
Kit Availability
This FM wireless microphone/
transmitter has been produced by
Oatley Electronics who own the
design copyright. They can supply
a complete kit of parts, as follows:
PC board, all on-board parts, a uni
directional microphone with clip, a
surplus plastic case, slide switch,
battery clip and stickers. The price
is $17.00 plus $5.00 for postage &
packing. The company’s ad
dress
is PO Box 89, Oatley, NSW 2223.
Phone (02) 9584 3563 or fax (02)
9584 3561.
November 1998 55
This view shows how the parts are installed inside the plastic case. This is the
handheld microphone version, which has the microphone attached to one end
of the case.
base of Q3 which is the lower half
of a cascode oscillator circuit. The
cascode configuration, involving Q2
& Q3, is the secret of the circuit’s excellent rejection of body capacitance
effects on the operating frequency.
The operating frequency is set by
a parallel LC network comprising
the 1pF capacitor and adjustable
coil L1 at the base of Q3. By virtue
of the cascode configuration, the
components which set the operating
frequency are well and truly isolated
from the antenna which is connected
to the collector of Q2. L1 allows the
operating frequency to be set to a vacant spot on the FM broadcast band
(88-108MHz).
Building it
Fig.2 shows the assembly details for
the PC board. The assembly is a pretty
straightforward process; it’s simply
a matter of inserting and soldering
each component in turn. The most
important point to remember is to
keep all the component leads to an absolute minimum length. Because the
circuit operates in the FM broadcast
band, even short lead lengths have
significant inductance and this can
prejudice the performance.
If you intend building either of the
FM microphone versions, leave trim
pot VR1 out. Conversely, for the guitar
transmitter version, include VR1 but
delete the 22kΩ resistor on its wiper
and delete the microphone.
Note that all the resistors are soldered “end-on” to save space on the
PC board. The length of the antenna
wire is up to you. You can have it
short and unobtrusive or long and
thereby obtain better range. We suggest a length of about 65-90cm for
good range; any longer and the range
will be reduced.
Once all the parts have been soldered to the board, you are ready to
test it and set the operating frequency. For this you need an FM radio.
Connect the 9V battery and turn on
your FM radio. Now tune across the
band until the speaker squeals. The
frequency on your dial is now the
operating frequency of the circuit.
If you want to adjust the frequency
of operation, you reverse the above
process. Tune your radio to a vacant
part of the band – let’s say this frequency is 99MHz. All you should be
getting is hiss from the loudspeaker
of the radio. Now adjust the slug of
coil L1 until you get a continuous
squeal from the radio. That’s it, the
job is complete.
In more detail, the tuning range of
Resistor Colour Codes
No.
1
1
2
1
1
1
1
1
1
1
56 Silicon Chip
Value
220kΩ
100kΩ
22kΩ
12kΩ
10kΩ
8.2kΩ
6.8kΩ
1kΩ
680Ω
270Ω
4-Band Code (1%)
red red yellow brown
brown black yellow brown
red red orange brown
brown red orange brown
brown black orange brown
grey red red brown
blue grey red brown
brown black red brown
blue grey brown brown
red violet brown brown
5-Band Code (1%)
red red black orange brown
brown black black orange brown
red red black red brown
brown red black red brown
brown black black red brown
grey red black brown brown
blue grey black brown brown
brown black black brown brown
blue grey black black brown
red violet black black brown
SMART® FASTCHARGERS
One charger for all your
Nicad & NiMH batteries
As featured in ‘Silicon Chip’ Jan. ’96
Fig.2: keep all leads as short as possible when installing the
parts on the PC board. Note particularly that Q1 is a BC549
type and is different to Q2 and Q3 which are both BF199s.
the wireless microphone can be adjusted upwards by removing the 1pF
capacitor. With this capacitor in circuit, the tuning range of L1 will be in
the lower region of the FM band: from
below 88MHz to about 102MHz. With
the 1pF capacitor in circuit, the tuning range will be from about 95MHz.
You have to decide which portion
of the band you want your circuit to
operate in and then pull the capacitor
out or leave it in. You then adjust the
slug of L1 as described above.
After you have adjusted coil L1 to
your satisfaction, move the microphone well away from the radio so
that the acoustic feedback squeal and
distortion is no longer apparent. You
should now be able to speak into the
microphone and your voice should
come from the radio with clean reproduction.
You can now complete the construction by wiring up the on-off
switch and then installing the board
and battery inside the plastic case
– see photos. The PC board is positioned at the top of the case and is
secured using silicone sealant. You
will have to drill a hole in the end
of the case to accept the lead for the
guitar or microphone (lapel version).
If you intend building the handheld
microphone version, you will need
to mount the microphone on the end
of the case before installing the PC
board. This will involve drilling a
small pilot hole (with the two halves
of the case attached together) and then
carefully reaming the hole to size.
Once this has been done, separate
the two halves of the case and secure
the plastic base of the microphone to
the bottom half only using silicone
Capacitor Codes
Value IEC Code EIA Code
0.1µF 100n
104
.047µF 47n
473
100pF 100p
101
1 33pF 33p 33
22pF 22p 22
15pF 15p 15
1pF 1p0 1
Designed for maximum battery capacity
and longest battery life
Charge: Power tools Torches Radio
equipment Mobile phones Video
cameras Radio controlled models
Field test instruments Lap-top computers
Toys Dust busters Others
The REFLEX® charger is powered from a
Power Supply (optional) or from 12 or 24V
batteries.
AVOIDS THE WELL KNOWN MEMORY EFFECT.
SAVES MONEY and TIME. Restore Nicads
with memory effect to remaining capacity and
rejuvenate many 0V worn-out Nicads.
CHARGES VERY FAST plus ELIMINATES THE
NEED TO DISCHARGE: charge standard batteries
in max. 1 hour and the ‘fastcharge’ batteries in max.
15 min. Partially emptied batteries are just topped
up. Batteries always remain cool, increasing both
the total battery life and the useful discharge time.
DESIGNED AND MADE IN AUSTRALIA
For a FREE detailed technical description please
Ph: (03) 6492 1368 or Fax: (03) 6492 1329
2567 Wilmot Rd, Devonport, TAS 7310
sealant. The PC board can then be
installed, as before.
You will also have to drill a small
hole at the other end of the case for
the antenna lead. Drill this hole in
the bottom half of the case only, then
make a small cutout for the slider of
the plastic switch. This cutout can
be made by drilling some small holes
and then filing the hole to shape.
The switch is also secured to the
side of the case using epoxy resin.
Be careful not to get any of the epoxy
inside the switch – it will be ruined
if you do.
Also supplied with the kit is a small
piece of 3mm-thick foam rubber. This
should be attached to the bottom of
the case at the end opposite the PC
board. It’s there to stop the battery
from rattling around.
Once everything is in place, the
two halves of the case can be secured
together using the supplied self-tapping screw. This is installed from the
back and screws into a central boss
in the top half of the case. Finally, fit
the appropriate front panel label and
SC
the job is complete.
November 1998 57
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