A great project for "Fox Hunt"
All the parts, PC
board and 3 button cells $8.75
You will also need an FM radio to pick up the
This bug can be attached to
anything from a glider to a pot plant and you can track it with a
Why track a pot plant?
The Beeper is the result of many requests for a mini tracking device
and introduces a new world of tracking.
The circuit is very compact and consumes very little power. It is small
enough to be hidden in anything you suspect will be lost or
By using a mercury switch or "grasshopper" the bug can be
"primed" for the time when it is moved and you can track it with an FM radio.
A grasshopper is a switch that is ready to go off at any time. A piece
of plastic is placed between the two switch contacts to keep them apart
and connected to a length of cotton thread that is fixed to the floor.
When the object is moved, the cotton thread pulls the plastic out of
the switch and the bug is turned ON. A bug like this would make
an ideal detector to track down anything going astray. By attaching it
to the product under surveillance, you can follow its path an maybe
turn up a few surprises!
Why a pot plant? Because one of our customers wanted to track the
person taking her pot-plants from her front perch. The result? It was
one of her neighbours!
HOW THE CIRCUIT WORKS
The circuit consists of three blocks. The first is the Phase-Shift
Oscillator. It produces a sinewave and is turned on for a very
short period of time by a two-transistor pulse generator made up of the
two lower transistors.
At the same time, the 88MHz carrier oscillator is turned on and the
tone is superimposed on the carrier to get a brief BEEP.
The two stages are coupled via a 22n capacitor. The frequency of the RF
oscillator is determined by a number of things but mainly the value of
the capacitor and inductor in the parallel tuned circuit.
The final building block is the pulse circuit made up of a feedback
amplifier using a BC 557 and BC 547.
The circuit starts its cycle by charging the 10u via the 330k resistor.
If you can't see how this occurs, remember a capacitor and resistor in
series form a DELAY CIRCUIT and it does not matter if the resistor is
the top component or the lower component.
As the capacitor charges, it brings the base of the BC 557 towards the
negative (0v) rail and this turns the transistor ON.
This effect is passed directly to the BC 547 via the 100n (and 1k
resistor) and the BC 547 turns ON.
The voltage on the collector of the BC 547 falls very quickly and
brings the 10u down with it.
The small voltage across the 10u forces the base of the BC 557 lower
and turns the transistor ON even HARDER.
Both transistors become turned on fully and the energy in the 10u feeds
the BC 557 until it is used up.
The 330k can no longer keep the BC 557 turned on and it turns
off slightly. This effect is passed to the BC 547 where the
collector voltage rises slightly and the rise is transferred directly
to the BC 557 via the 10u and the circuit starts to move in the other
Both transistors become fully turned off and the 10u begins to charge
again via the 330k.
The circuit takes almost no current between beeps and the duty-cycle of
the tone is about 10%. This makes the circuit very economical on
batteries and you should get many hours of operation from a set of
The block diagram above
shows the 3 building blocks for the Beeper Bug. The widely differing
frequencies produced by each of the blocks makes it difficult to show a
combined output waveform. The diagram below shows how the Pulse
generator block turns on the Phase Shift Oscillator and 108MHz
Oscillator sections to produce short beeps.
Building blocks (a) and (b)
combine to create the signal (the beep) shown in the diagram as
amplitude lines. Signal (a) modulates signal (b) to produce a tone of
approx 1kHz on the 108MHz carrier. Building block (c) turns the beep on
and off. All this is shown on the diagram above but due to the
enormous difference in the frequencies of the three waveforms, the
figure above is not to scale.
The diagram above
shows three things:
1. When the pulse generator is off, all the blocks are off and
the circuit consumes almost NO current.
When the pulse generator turns on (it turns on for a very short
period of time) the voltage across it is very small and it effectively
puts the top two building blocks across the power rails. This is how
they are turned on.
2. The phase shift oscillator produces about 10 cycles during this
period of time.
3. For each cycle of the phase-shift oscillator, the 108MHz oscillator
produces about 100,000 cycles!
During the production of the 100,000 cycles, the waveform from the
phase-shift oscillator causes the 108MHz oscillator to increase
frequency very slightly then decrease frequency slightly so that the
end result is a tone from the receiving radio.
When the circuit turns off, the radio picks up background
"hiss" and so the result is a beep with background hiss
between the beeps.
The animation below shows another way of
"seeing" the pulse generator in operation. Study the
animation carefully and you will see the bar-graphs indicating the
voltage on the collector of the output transistor.
When the circuit is NOT TURNED ON, the voltage on the collector
is HIGH. When the circuit is TURNED ON,
the voltage on the collector of the output transistor is LOW.
This is normal with all circuits like this. The voltage on the output
of the circuit must be low so that most of the voltage will be across
the LOAD. In this case the LOAD is the two oscillator blocks and
when the pulse generator is TURNED ON, the two oscillators will see RAIL
The second diagram in the
animations above shows two things:
1. The voltage across it is VERY SMALL when it is turned ON.
2. The circuit changes from OFF to ON (and back again) VERY
QUICKLY. In the next page of the course you will learn
about circuits that turn on and off very quickly. They are very
important in electronics - they are the main building block for
All the parts fit on to a small PC board and by using miniature
components, the size is kept to a minimum. Keep this in mind when collecting the parts. If you use old-style
components, they will not fit. The safest way is to buy a kit.
All the parts must be kept close to the board when soldering so that
everything is kept compact. All
the resistors stand-up on the board and the easiest way to carry out
construction is to start at one end of the board and fit each part as
you come to it.
The overlay makes it easy to see where everything goes and the only
things you have to be careful with are: 1.The BC 557 transistor
(don't confuse it with the BC 547's) and 2. The polarity of the
3. Don't forget to scrape the ends of the coil with a sharp blade to
remove the enamel before tinning.
The on-off switch can be mounded on top of the board with short lengths
of wire or fitted directly. The supply comes from three button cells
and these are soldered together very quickly to prevent them getting too hot.
Finally, a length of antenna wire is soldered to the last remaining
hole on the board and the project is ready for testing.
1 - 330R
1 - 1k
1 - 10k
2 - 22k
1 - 47k
1 - 330k
1 - 1M
1 - 10p ceramic
1 - 27p ceramic
1 - 1n ceramic
3 - 4n7 ceramics
2 - 22n ceramics
1 - 100n monoblock
1 - 10u 16v PC mount electrolytic
1 - 22u 16v PC mount electrolytic
3 - BC 547 transistors
1 - BC 557 transistor
1 - 6 turn 3mm dia coil enamelled wire
1 - SPDT mini slide switch
3 - button cells
1 - 1.7 metre wire for antenna
BEEPER BUG PC BOARD
With the power switched off, connect a multimeter across the switch
terminals and you will see the needle jump very briefly to indicate the
circuit is beeping.
You will not be able to detect the average current as the duty cycle is
too short, but you can see it is very small by the slight movement of
Next test the current consumption and the frequency of operation.
Remove the 10u and retest the current. It should be about 5-8mA and
this shows everything is operating.
Switch the unit on and place the antenna as it will appear on the
glider or pot plant etc.
Tune an FM radio to 108MHz and expand the turns of the coil until the
tone is detected. Keep the radio away from the transmitter to prevent
picking up harmonics (side tones). Replace the 10u and the project is
ready for installation.
IF IT DOESN'T WORK
If the Beeper doesn't work, you will have to determine which blocks
are not functioning.
If the circuit gives out a constant tone, the pulse section will be
faulty. If a clicking sound is emitted, the tone oscillator will be
faulty. If no signal is picked up at all, the RF oscillator section may
Firstly check the current consumption. If the needle "jumps"
but no RF is detected, the RF may be off the FM band.
If RF is detected, you should check the value of components around the
oscillator section and the number of turns on the coil.
The spacing of the turns and the diameter of the coil is also critical.
The kit comes with a pre-wound coil (6 turns @ 3mm dia) and the holes
on the board give you an idea of the spacing of the turns.
If you have made the coil yourself, check the dimensions carefully,
they are most critical. Removing the 10u will cause the tone to be
emitted continually and if a carrier is detected (blank spot on the
dial) but no tone, you should check the voltage on the collector of the
tone stage. It should be about 3v. A CRO will be the easiest way to see
the output however an audio amplifier will be just as good.
Take the emitter of the transistor directly to the negative rail (to
remove the pulse section) and check the soldering on the phase-shift
components (4n7's and 22k's) to make sure a dry joint is not
If you are still having trouble, buy another kit and build just the
tone section (don't forget to take the emitter to negative).
Alternatively, build the circuit on bread-board and detect the tone
with an audio amplifier.
The amplifier can also be used to make sure the tone is passing through
the 22n stage-coupling capacitor, to the RF stage.
As we mentioned above, if RF is not detected, the fault will lie in the
The components making up this stage are the 47k, 1n, 27p, 6 turn coil,
BC 547, 10p, 330R and 22n across the battery.
The capacitance of the circuit will have an effect on the frequency as
will the battery condition, the earthing effect of your hand, the
length of the leads of the battery (and switch) and the length of the
Once you get the circuit working you can change things around but keep
everything compact at the start.
Finally we come to the pulse circuit. It is simply a high-gain DC
coupled amplifier. By removing the 10u, the circuit will turn on. The
voltage between the collector and emitter will be small (about 0.35v to
0.5v). If not, short between collector-emitter of the BC 557. If the
circuit turns on, the BC 557 is faulty. If not, the BC 547 is
Shorting across the 10u with a jumper will turn the circuit off and the
voltage between collector and emitter of the BC 547 will rise to about
If the circuit doesn't work when the 10u is fitted, the problem may be
a leaky electro. Try another and experiment with different values.
Everything has now been checked and the circuit should be working
The kit comes with 3 button cells but you can use 2 lithium
cells for a 6v supply. The output will be increased considerably and
this will give a longer range. The circuit has been designed to operate
at the high end of the band (108MHz) and if required to operate at
88MHz, the 27p tuning capacitor should be changed to 47p.
The Beeper Bug can be used for many applications and its range will
depend on the effectiveness of the antenna.
Obviously a long straight antenna will be the best but you can't always
get what you want when you are trying to hide things. A long fine wire
will be better than a short thick one and if you want to hide the
Beeper Bug in a pot plant, you can drape the wire over the branches
like a thread.
If it is to be used in a glider, the main struts can be the antenna. If
it is to be hidden in a box of tissues, the antenna can be taped to the
inside of the box.
To get the best range, you must experiment before placing the bug on
location and when it's working perfectly, I'm sure you will be pleased
with its performance.