SO SMALL IT FITS
IN A MATCHBOX
By GREG SWAIN
This sneaky FM bug is small enough to hide
in a matchbox. It's super-sensitive and can
be picked up on an ordinary FM receiver up
to 100 metres away.
We're not suggesting for one
minute that you use this little FM
wireless transmitter to listen in on
private conversations or to engage
in industrial espionage. That would
not be nice. Instead, this unit is
mainly intended as a fun project for
the budding electronics engineer.
Provided that you can handle a
soldering iron and follow a simple
wiring diagram, this project can be
assembled in about half an hour.
All the parts, including an electret
microphone and the batteries, are
mounted on a small PC board which
then slips inside a matchbox. The
only indication that something
devious is going on is an external
wire lead for the antenna.
A miniature slider switch on the
board lets you turn the unit off
when it is not in use to save batteries. When the unit is turned on,
signals picked up by the micro14
SILICON CHIP
phone are used to frequency
modulate a transmitter stage and
this then broadcasts the signal to
an FM receiver.
In practice, the transmitter is
tuned for an output frequency of
100MHz which is right in the middle of the commercial FM band
(88-108MHz). You do this simply by
adjusting a slug in the coil that
forms part of the transmitter tuned
circuit.
In addition to its obvious application as an FM bug, this unit also has
more serious applications. For example, it could also be used as a
baby monitor or to monitor a
telephone when you would otherwise be out of earshot.
By the way, this circuit is super
sensitive - so much so that it has
no difficulty picking up sounds in a
room even when the microphone is
fully enclosed in the matchbox. In
This larger than life
size photo shows just
how simple the FM
bug really is. Power
is supplied by two
1.5V cells.
fact, if you get too close to the
microphone, the circuit will be
overloaded.
The circuit also boasts excellent
frequency stability and has a power
output of about 3mW from a 3V
supply and 20mW from a 9V supply
(roughly twice that of previous
designs).
How it works
Fig.1 shows the circuit of the FM
Wireless Microphone. It uses three
NPN transistors, all of which are
low-cost BC548 types. The transistors are not critical however,
and BC547 and BC549 types will
work just as well.
A 3V battery supplies power to
the circuit via on/off switch St.
When the switch is closed, power is
supplied to the electret microphone
via resistor Rt. Any sound signals
in the room are now picked up by
the microphone and AC-coupled to
the base of transistor Qt via a
.01µF capacitor (Cl).
Qt functions as a common emitter amplifier with bias set by the
ON
r.t
C7
10pF
R6
47k'
C6
33pF
T
3-9V:
...I...
ELECTRET
MICROPHONE
L1 : ZA-2599
L2 : PCB PATTERN
B
EOc
VIEWED FROM
BELOW
MATCHBOX FM TRANSMITTER
Fig.1: signals picked up by the microphone are amplified by Qt and used to frequency modulate VHF
oscillator stage Q2. Q2's output is then fed to Q3 which drives the antenna.
lMO feedback resistor (R2) connected between its collector and
base terminals. The amplified output appears at the collector and is
coupled to the base of Q2 via C2
(O. lµF).
Q2 functions as a VHF (very high
frequency) oscillator stage. 11 and
C4 (33pF) function as a tuned collector load for Q2 while C5 provides
positive feedback to ensure that the
stage oscillates. A low inductance
is used for 11 and a high
capacitance for C4 to improve
stability and reduce the effect of
stray capacitance.
This VHF oscillator stage is fre-
quency modulated by the audio
signal on Q2's base. Thus, when
ever a signal is picked up by the
microphone, this stage produces a
corresponding FM signal. An adjustable ferrite slug in 11 allows the
oscillator to be tuned across the FM
broadcast band as indicated
earlier.
The RF (radio frequency) output
of the oscillator is taken from the
emitter of Q2 and coupled to the
base of Q3. This stage also functions as a common emitter amplifier. It drives the antenna via an
output resonant circuit consisting
of coil 12 and capacitor C7 (lOpF).
12 actually forms part of the copper pattern on the PC board, while
C7 tunes the resonant circuit to
100MHz.
If you intend operating the circuit
at 90MHz, then C7 should be increased to 12-15pF for maximum
efficiency. Note that in this circuit,
the antenna is isolated from the
oscillator tuned circuit. This ensures greater stability since the
oscillator frequency is no longer affected by movements of the
antenna.
Power for the circuit is derived
from two 1.5V N-size cells. These
give a supply voltage of 3V but you
CAPACITORS
D
D
D
D
D
D
No.
2
1
1
2
1
1
Value
0 .1µF
.01µF
.001µF
33pF
10pF
5.6pF
IEC
100n
10n
1n
33p
10p
5p6
EIA
104K
103K
102K
33K
10K
5 .6
Fig.2: keep all leads as short as possible when installing
the parts on the PC board. See text r~ orientation of Lt.
RESISTORS
D
D
D
D
No.
1
2
2
1
Value
1MO
47k0
4.7k0
4700
4-Band Code (5%)
brown black green gold
yellow violet orange gold
yellow violet red gold
yellow violet brown gold
5-Band Code (1%)
brown black black
yellow violet black
yellow violet black
yellow violet black
yellow brown
red brown
brown gold
black brown
NOVEMBER 1989
15
PARTS LIST
1 PC board, code ZA-1 431 ,
50 x 31mm
1 DPDT miniature slider switch
1 pre-wound coil with ferrite
slug, DSE Cat. ZA2599
1 electret microphone insert
2 1 .5V N-size batteries
1 7 50mm-length of insulated
hook-up wire (for antenna)
3 BC54 7, BC548 or BC549
NPN transistors (01-03)
Capacitors
2 0. 1 µ.F monolithics or
ceramics (C2,C8)
1 .01 µ.F ceramic (C1)
1 .001 µ.F ceramic (C3)
2 33pF ceramics (C4,C6)
1 1OpF ceramic (C7)
1 5.6pF ceramic (C5)
Resistors (0.25W, 5%)
1 1 MO (R2)
2 47k0 (R4,R6)
2 4 . 7k0 (R1 ,R3)
1 4700 (R5)
can use a 6V or 9V supply if you
want longer range and space is not
a consideration. The current consumption is about 11.5mA.
Construction
Construction is straightforward.
A complete kit for this project is
available from Dick Smith Electronics for $16.95 (see panel), so obtaining the necessary parts is ncit
difficult. All you have to supply is
the matchbox.
The batteries are secured by soldering their terminals to loops of tinned
copper wire which in turn are soldered to the copper pads on the PCB (see
Fig.2). Tin the ends of the batteries first by scraping away some of the plating
and then applying a hot soldering iron.
Fig.2 shows how to install the
parts on the PC board. Install the
two wire links first, then the
resistors and the capacitors. These
parts should all be pushed flat
against the PCB before soldering
their leads.
The three transistors can now be
installed. Push them down onto the
board as far as they will comfortably go and be sure to match the
transistor cases to the outlines on
the wiring diagram. Solder their
leads, then install the slider switch.
The switch mounting lug adjacent
to the edge of the board should be
snapped off using a pair of pliers
(see photo) otherwise it will foul the
side of the case.
Above: the square section at the top of coil L1 should be
trimmed off, preferably before mounting it on the PCB.
Orient the coil as shown. Left: the circuit fits neatly into
a matchbox.
16
SILICON CHIP
Similarly, the square section at
the top of coil 11 must be cut off to
provide clearance for the matchbox
lid. Use a sharp razor blade for this
job but be sure to remove the ferrite
slug first. Install 11 so that the
highest shoulder on the plastic
former is adjacent to the switch.
The electret microphone is
mounted on the board using two
short lengths of tinned copper wire
(see photo). To do this, first solder
the leads to the pads on the back of
the microphone, then bend them so
that they match the holes in the
PCB. The microphone can then be
installed on the board with its
positive terminal adjacent to R5
(4700).
You can set the transmitter output to virtually any spot on the FM band but we recommend a frequency of about
100MHz. Don't forget to turn the unit off when it is not in use (the switch is shown here in the 'on' position).
It's quite easy to identify the
microphone terminals. The negative terminal is the one connected
to the metal case.
Now for the batteries. These are
mounted on one end of the PCB and
are secured by soldering their terminals to loops of tinned copper
wire. These wire loops in turn are
soldered to the copper pads on the
PCB.
Be sure to install the batteries
the right way around and note that
they face in opposite directions to
each other.
Finally, connect a 750mm-long
antenna (use insulated hookup
wire) to 12 on the copper side of the
board. Of course, you can use a
shorter antenna if you wish but
note that this will give a reduced
range.
Testing
A commercial FM receiver is required for the test and adjustment
procedure. Set the volume control
to the usual level, then turn the
receiver on and tune it to 100MHz.
Now turn the FM transmitter on
(just slide the toggle switch towards
the centre of the board) and adjust
11 using the insulated alignment
tool. When the transmitter is tuned
to the correct frequency, you will
hear a corresponding whistle in the
receiver. If you now speak, you
should be able to hear your voice
via the receiver but note that it will
probably sound quite distorted due
to overloading and feedback
effects.
To fully test the unit, slip it into
the matchbox with the antenna lead
emerging through a hole in one end
of the drawer. It can then be taken
into a room where other people are
speaking (or the TV is going). Any
conversations should now come
through loud and clear on the
receiver.
That's it! Don't forget to move the
slide switch to the off position when
the unit is not in use otherwise
you'll quickly flatten the batteries.
De-bugging
Now, what if you suspect that
your room has been bugged by one
of these FM wireless microphones?
How do you detect it? It's quite
easy. All you do is turn on your FM
radio and wind the volume control
well up. You then tune slowly right
across the FM band. If at any time
you come across a whistle, you
have been bugged. The whistle will
be caused by acoustic feedback between the microphone in the bug
and your radio's loudspeaker.
It's then a matter of ferreting the
little blighter out. You do this by
moving your FM radio around the
room. As you get closer to the bug
the whistle (or howl) will become
louder.
[§;I
Where to buy the kit
A complete kit of parts for this project is available from Dick Smith Electronics stores or by mail order from PO Box 321 , North Ryde, NSW
2113 . Phone (02) 888 2105.
The kit comes with all parts (including two 1.5V batteries) and sells for
$16.95 plus $2 .00 p&p (standard mail only) . Quote Cat. K-5006 when
ordering .
Note: copyright of the PC artwork for this project is owned by Dick Smith
Electronics .
NOVEMBER 1989
17
