Our smallest FM Beeper Bug
Size: 0.8in x 0.6in
incl all parts and PCB
plus $4.50 postage to anywhere in the world
This project uses the Micro Tracker PC board.
The component values are marked on the board.
The output produces a thump-thump-thump- squeal-squeal-thump-thump-thump . .
. . . at approx 90MHz but the turns of the coil can be expanded to raise the
Bug Tracker Circuit
Read the Micro Bug Article for
This article will just refer to the soldering of the components onto the
HOW THE CIRCUIT WORKS
The circuit consists of two stages, a digital stage consisting of an ON-OFF
waveform and an RF oscillator. The digital stage is a flashing LED and it
has an in-built oscillator to turn red-blue and green LEDs on and off.
This action takes current from the supply when the LED is illuminated and
almost no current when the LED is not illuminated.
The flashing LED is supplied via a 10k resistor and the flashing still
occurs but the LEDs are much duller.
The flashing LED is no different to a transistor turning ON and OFF. The
voltage across the LOAD resistor (10k) is detected by a buffer
transistor and this transistor turns on and off.
The transistor supplies base current to the RF oscillator and when the
oscillator turns ON it produces a carrier that removes the background noise
in the receiving radio.
This is how you get the thump-thump-thump in the audio. It is just the
difference between the silence and background noise.
The LED also produces a tone during part of the cycle when the LEDs are
increasing or decreasing in brightness. This is heard as a squeal on the
The Flashing LED cannot be put in the base of the oscillator stage as this a
common-base design as the base voltage rises to nearly the supply voltage
and the LED drops at least 1.7v when active and the oscillator stage would
The RF oscillator is designed to operate at about 88MHz and the frequency is
set by the inductance of the 5 turn coil together with the 47p capacitor. These
two components make up a circuit called a parallel resonant circuit (tank
The frequency is also determined by the transistor, the 10p feedback capacitor
and also to a lesser extent by the biasing components (47k, 100k and 390R
When the buffer transistor is tuned ON, the 1n base capacitor will charge via the
resistor and turn the oscillator transistor ON.
The base voltage will continue to rise and the 10p will have the effect of
trying to prevent the emitter from moving. A point in time is reached when the
energy from the capacitor is exhausted and it can no longer resist the movement
of the emitter. The base-emitter voltage decreases and turns the transistor
off. The current flow in the coil then ceases and the magnetic flux collapses.
This collapsing magnetic field produces a voltage in the opposite direction and
whereas the collector voltage may have been 2.9v, it will now rise to over 3v
and charge the 47p in the opposite direction. This voltage will have the effect
of charging the 10p and the voltage drop across the 390R emitter resistor will
be such that the transistor will be turned more firmly OFF.
As the 10p charges, the emitter voltage will drop to a point where the
transistor will begin to turn ON and the current flow through the coil will
oppose the collapsing magnetic field.
The voltage across the coil will reverse and the collector voltage will drop.
This change will be passed to the emitter via the 10p and the result will be
that the transistor will turn ON very hard and short out the 10p. After this
the cycle begins again.
What we have is an oscillator that produces AC energy at 88MHz with the
amplified audio signal fed into this stage via the 100n, varying the frequency
of oscillation to produce the FM signals.
|1 - 390R (marked 391)
1 - 2k2 marked 222)
2 - 10k (marked 103 )
2 - 4p7 surface mount
1 - 47p surface mount
1 - 1n surface mount
1 - 22n surface mount
1 - BC 848 transistor marked ‘1k’
1 - BC 858 transistor
1 - 3mm flashing LED
1 - 5 turn 3mm dia enamelled coil
3 - 1.5v button cells
1 - battery box
15cm very fine solder
1 - 80cm hook-up wire for antenna
1 - MICRO TRACKER PC board
Micro Tracker components
Before starting, place a tiny piece of blu-tack on the top-side of the
Micro-Bug Tracker PC board and push it onto your work-bench so it does not move - all
the components are mounted on the "underside" of the board.
All the components for this project are surface-mount.
This includes the two transistors,
Let me warn you once again, the components are very tiny.
Before opening the kit of parts you must prepare a clean space on your
workbench so the components can be taken out of the holding cells without being
You will also need a soldering iron with a fine tip.
We use a soldering iron that is commonly called a SOLDERING PENCIL. It is a
very small iron with a tip that is sharp enough to prick you!
If you do not have this type of soldering equipment, assembly will be much more
difficult and the neatness will not be as good as our photos.
You will be amazed at the improvement you get by simply using the correct
equipment and FINE SOLDER.
To get more information, go to our
see the range of soldering irons from a large supplier -
Howard Electronic Instruments Inc.
Ask Howard Electronic Instruments to quote on a very small iron and a roll of 0.7mm solder.
See hundreds of
and pictures on soldering on Howard Electronic Instruments website.
Also see this
ARTICLE on soldering by WELLER.
Now we can get down to assembly:
Use the photo above to identify the placement of the parts aas the board is
designed to take two different circuits and the overlay is for the voice
The oscillator section is the same for both projects but the LED and
transistor are in different places for this project, plus 3 resistors.
Lay out a clean sheet of paper and place the strip of components so the
black band is to the left.
The first components to fit onto the board are the resistors and these are in
the strip in ascending order. This
means the values are: 390R, 2k2, 10k, 10k and are clearly marked with
a 3-digit or 4-digit code.
The next components to fit to the board are the 5 capacitors. Unfortunately they are not
marked and so you will have to be very careful when removing them so that they
are not mixed up.
The size of a capacitor chip is no indication of its value and so you have to
take our word that we have placed them in ascending order.
Even the colour of the substrate is no indication of a value so please don't
ring us up and say “I have a tiny pink capacitor and a tiny mauve capacitor and
a tiny purple capacitor, which is the 1n!”
The surface-mount capacitors are housed in a carrier strip, with a black
line at one end. The black line corresponds to the lowest value of
The colours and size will change
from one batch to another and from one manufacturer to another.
I know some people are going to plough into construction and lose a component
or two so the replacement strips will be available for $2.00 for the set of
capacitors $2.00 for the resistors and $2.00 for the transistors plus $3.00 for postage.
This is the best we can do so don't ask for individual components - you're
lucky we have a service for replacement parts.
When fitting the components to the board, open up the strip from the black-band
end and allow one chip at a time to fall out of its cell to a clean sheet of
paper on the bench.
If the component is a resistor, it will have markings on it. Turn it over to
reveal the numbers.
This is a three number or four number code to indicate the value in ohms. Make sure it is
turned around so the numbers make sense. Sometimes you can hold a resistor
around the wrong way and it will appear to be a different value!
The markings for each resistor have been supplied in the parts list so
double-check the value before fitting it.
Pick up the resistor with a pair of tweezers or a paper clip that has a tiny
piece of Blu-Tack placed on the end and carry the chip to the board.
Hold the chip in place with the paper clip but take the Blu-tack away so that it
does not melt and contaminate the joint.
Bring up the soldering iron to one end of the chip and with your third hand,
add a small amount of fine solder and make a connection.
The art of soldering surface-mount components will take a little time to
master, but you
will find they are easier and quicker to fit than conventional
We build hundreds of items for customers and the assembly workers have said
surface-mount devices are easier and quicker to fit.
If you are using tweezers, you can add a small amount of solder to one end
of the chip and when you place it on the board, it will be very quick to
solder. Push on the tweezers to make sure the chip sits firmly on the board.
Then solder the other end.
Everything takes time to get perfect and since electronics is going in the direction
of surface-mount, now is a good time to get your hand in.
Every year, more and more of our consumer electronics are being converted to
Already more than 40% mount is surface-mount and the percentage is increasing all the
time. The driving force is economics.
Even though surface mount components are slightly more expensive, the cost of
insertion is much lower as they are placed on the board with a
pick-and-place machine. These can fit components five-times faster than
But the real saving is in the size of the board. This can be reduced by as much
as 50% with components on one side or as much as 70% with components on both
The board can be made of much thinner material or even flexible, so the scope
for new produces is enormous.
Board for cameras, for instance, are made of paper-thin flexible material so
it can fit around the body of the camera with
flexible wiring to connect to the button, motor, battery and flash.
But the greatest advantage of surface-mount is the aesthetics. Surface-mount
looks professional and makes your product stand out from the rest.
Now back to the topic.
When you get to the capacitors, you must look into the
cell before letting it fall out so you can recognise it once it hits the
Refer to the diagram of the strip for the value and then the enlarged diagram
of the board for the position of the component.
It doesn't matter which way around the resistors or capacitors are placed and
they can even be turned over but there is one thing you must not do.
When you have soldered one end of a component you must not try to straighten it
up. This also applies to a component that is standing up slightly.
Any amount of force will fracture the substrate. Even slight pressure can
the metallised end caps from their substrate and cause the component to go open
That's why it is important to hold the component in place with the tip of a
paper clip while soldering the first end.
If the component shifts, you are in big trouble.
This is because YOU are going to take much longer to re-heat the end and
If this happens with a transistor, forget it. The transistor will be damaged.
It a component moves, leave it misaligned and try to solder the other end as
best as possible. The damage you will cause in trying to straighten it will be
a nightmare to fault-find.
Another problem you will find is some soldering irons are magnetic and the
resistor will stick to the tip of the iron if you don't hold them down with
something such as a pair of tweezers or paper clip.
Just before we go to the transistors, let me give one final hint.
To reduce the heat build-up in a component, it is best to solder one end then
wait a few seconds for the component to cool down before soldering the other
This is very important when soldering the transistors.
The Micro Bug Tracker from the
bottom. Notice how the parts are positioned and soldered
Micro Tracker PC Board
enlarged view of the underside of the Micro Tracker PCB.
It shows the
placement of the
A 4p7 is placed on top of a 4p7 to make 10p capacitor.