This kit is designed and manufactured by TALKING ELECTRONICS
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Download FM-Bug.pdf - note: the inside diameter of the coil is 3.5mm

The Complete FM Bug


FM BUG Circuit

Corporate espionage is reaching new heights in sophistication. The latest information to be released shows the depths firms will go to pry into a rival firm's operations.
By using the latest in electronic bugging, they have stolen information, secrets and even formulas known only to the inventors themselves.
Take the example of one firm:
Leaks from Top Management level remained a mystery until, one day, a bug was discovered inside the Managing Director's office.
Sitting prominently on his desk was a gift box of imported cigars!
Cleverly concealed in the lower part of the box was a miniature FM transmitter . . all a gift from a phony sales rep.
This is just one of the many bugging devices available on the eaves-dropping market. The range includes pen and pencil holders, trophies, framed pictures and office furniture with false bottom drawers.
These products are readily sold to fledgling companies, eager to nestle into big brother's market.
And for a while these bugging devices worked. Few firms knew of their existence, and even less on how to sniff them out.
But that has all changed now. If a corporation suspects a leak at any level, the first thing they order is an investigation into security. Not only personnel, but information and electronic security.
Debugging has grown into big business. Most large security organisations have a section concentrating on electronic surveillance including bugging and debugging.
They use scanners to detect hidden devices and can locate absolutely anything, anywhere, and on any frequency.
It was only after the firm above had commissioned a scan of the entire floor, that the cigar box was discovered. Its innocence had deceived everyone. And cost them a small fortune!
Bugging of this kind is completely illegal and we don't subscribe to this type of application at all.
But the uses for our SUPER-SNOOP FM WIRELESS MICROPHONE can be harmless, helpful and a lot of fun.
Our unit is both compact and very sensitive and can be used to pick up even the faintest of conversations or noises and transmit them 20 or so metres to any FM receiver.
When you build the FM BUG you will see why we consider the design to be very clever. We have used only low priced components and they are all easy to obtain.
No air trimmer capacitor is required as the coil is squeezed slightly to obtain the desired frequency. This has allowed us to fit the bug into a tooth-brush case so that it can be carried around or placed on a shelf.
If it is set between two books it will be hidden from view or as a supervision accessory it can be placed on a small child, etc. The transmitted signal will over-ride the background noise and the output will be clean. If the child wanders beyond the range of the transmitter, the background noise will come up and signal that the tot is out of range.
As an added bonus, you can listen to the chatterings and squabbles as the children amuse themselves in the back yard.
It is also great for picking up the first signs of a child awakening from his afternoon sleep or it can be used as an indicator from a bed-ridden patient.
The great advantage of the bug is the absence of wires. And since it draws only about 5-10 milliamps, the pair of AAA cells will last for many months.
The success of this FM BUG is the use of TWO transistors in the circuit. To create a good design, like this, each transistor should be required to perform only one task. In any type of transmitter, there is a minimum of two tasks.
One is to amplify the signal from the microphone and the other is to provide a high frequency oscillator.
The amplified microphone signal is injected into the oscillator to modify its frequency and thus produce a FREQUENCY MODULATED oscillator. If an aerial is connected to the output of the oscillator, some of the energy will be radiated into the atmosphere.
To increase the output of our design, an RF amplifier would be needed but this gets into legal technicalities with maximum transmitting power.
It may be of Interest to know that a record distance of 310 miles was achieved with a 350 micro-watt transmitter in the USA, some 15 years ago. This equates to an astounding ONE MILLION miles per watt!
In simple terms, an RF amplifier becomes a LINEAR amplifier.
We have opted for sensitivity and the first transistor is employed as a pre-amplifier. This will enable you to pick up very low-level sounds and transmit them about 20 to 50 metres.

The only critical component in the FM BUG is the oscillator coil. When I say critical, I am referring to its effect on the frequency. Its critical nature only means it must not be touched when the transmitter is in operation as this will detune the circuit completely.
It is the only component which needs to be adjusted or aligned and we will cover its winding and formation in detail.
The oscillator coil is made out of tinned copper wire and does not need any insulation. This is not normal practice but since the coil is small and rigid, the turns are unable to touch each other and short-out.
The coil is made by winding the tinned copper wire over a medium-size Philips screw-driver. The gauge of wire, the diameter of the coil and the spacing between turns is not extremely important and it will be adjusted in the alignment stage. However when the project is fully aligned, it must not be touched at all.
Don't be over-worried at this stage. Just follow the size and shape as shown in the diagram and everything will come out right in the end.

The coil has 5 turns and is wound on a 3.5mm shaft. To be more specific, it has 5 loops of wire at the top and each end terminates at the PC board. The coil must be wound in a clock-wise direction to fit onto the board and if you make a mistake, rewind the coil in the opposite direction.

Construction is quite straight-forward as everything is mounted on the printed circuit board. The only point to watch is the height of some of the components. The electrolytic must be folded over so that the board will fit into the case.
Positioning of the parts is not as critical as you think as the final frequency is adjusted by squeezing the coil together or stretching it apart.
However it is important to keep the component leads as short as possible and the soldering neat due to the high frequencies involved. The components must be soldered firmly to the board so that they do not move when the transmitter is being carried.
Even the poorest of soldering will work but who wants to see poor soldering on a project?
The soldering may not affect the resulting frequency but poor layout of the components certainly will.
All the resistors must be pressed firmly against the PC board before soldering and the two transistors must be pushed so that they are as closes as possible to the board.
Some BC 547 transistors will not work in the circuit. Maybe the frequency is too high. SGS BC 547 transistors did not work at all. The other two types: f BC 547 and Philips BC 547 worked perfectly.
All the small-value capacitors are ceramic as they are not critical in value and do not need to be high stability. But you must be careful when identifying them. It would be a very simple mistake to buy a 56p instead of 5p6 because there is no difference in the size. 22n may be identified with 223 or 22n or .022. A capacitor marked 22k will be a 22p cap and will not be suitable. The 1n capacitor may be marked 1n or .001 or 102. These are all the same value. The value 101 or 103 is NOT 1n so be careful, the caps may be about the same size. The rule is: don't use a capacitor unless its markings are clear and you are sure of the value.


The complete FM BUG

The switch is mounted on the PC board with its three terminals fitted into the large holes.
The final items to add to the board are the two AAA cells. These come with the kit and we have chosen them for slenderness so that they can be fitted side-by-side.
It is very difficult to solder to the zinc case but if you roughen the surface with a file and use a large, HOT, soldering iron, the job can be done very quickly. Use a piece of tinned copper wire to join the positive of one to the negative of the other. At the other end, solder longer lengths of wire so that they can be connected directly to the PC board. Make sure the positive terminal connects to the plus on the PC board.

Top and bottom of the FM BUG PC board

AAA cells are also obtainable at photographic shops. The only alternative is an 'N' cell which is nearly as thin as an AAA cell but only half the length.
The terminal marked A on the board is the antenna output. For a frequency of 90MHz, the antenna should be 165cm long. This is classified as a half-wave antenna and provides one of the most effective radiators. If you find the antenna gets in the way you can opt for a quarter-wave antenna and this will be 83cm long. If you only require to transmit 10 to 20 metres the antenna can be as short as 42cm or even as low as 5 or 10 cm.
The most suitable length will depend on the sensitivity of the FM radio used to pick up the signal and the obstructions between the transmitter and receiver. It will be a good experiment for you to 'cut' your own antenna and determine which is the most suitable for your application.

The circuit consists of two separate stages. The first is an audio pre-amplifier and the second is a 90MHz oscillator.
The first stage is very simple to explain. It is a self-biasing common-emitter amplifier capable of amplifying minute signals picked up by the electret microphone. It delivers these to the oscillator stage. The amplification of the first stage is about 70 and it only operates at audio frequencies. The 22n capacitor isolates the microphone from the base voltage of the transistor and allows only AC signals to pass through. The transistor is automatically biased via the 1M resistor which is fed from the voltage appearing at the collector. This is a simple yet very effective circuit. The output from the transistor passes through a 2.2u electrolytic. This value is not critical as its sole purpose is to couple the two stages.
The 47k, 1n, 470R and 22n components are not critical either. So, what are the critical components in this circuit?
The critical components are the coil and 47p capacitor. These determine the frequency at which the bug will transmit. In addition, the effective capacitance of the transistor plays a deciding factor in the resulting frequency.
This stage is basically a free-running 90MHz oscillator in which the feedback path is the 5p6 capacitor.
When the circuit is turned on, a pulse of electricity passes through the collector-emitter circuit and this also includes the parallel tuned circuit made up of the oscillator coil and the 47p capacitor. This pulse of electricity is due to the transistor being turned on via the 47k resistor in the base circuit.
When ever energy is injected into a tuned circuit, the energy is firstly absorbed by the capacitor. The electricity will then flow out to the coil where it is converted to magnetic flux. The magnetic flux will cut the turns of wire in the coil and produce current and voltage which will be passed to the capacitor.
In theory, this current will flow back and forth indefinitely, however in practice, there are a number of losses which will cause the oscillations to die down fairly quickly.
If a feedback circuit is provided for the stage, the natural RESONANT frequency of the coil/capacitor combination will be maintained. The 5p6 provides this feedback path and keeps the transistor oscillating.
The 5p6 feeds a small sample of the voltage appearing at the collector, to the emitter and modifies the emitter voltage. The transistor sees its base-to-emitter voltage altering in harmony with the resonant frequency of the tuned circuit and turns the collector on and off at the same frequency.
Thus there is a degree of stability in the oscillator frequency.
The actual frequency of the stage is dependent upon the total capacitance of the circuit and this includes all the other components to a minor extent.
Once the basic frequency of 90MHz is set, the variations in frequency are produced by the changes in effective capacitance of the transistor. This occurs when its base voltage is increased and reduced. The electret microphone picks up the sound waves which are amplified by the first transistor and the resulting frequency is passed to the base of Q2 via the 2.2u electrolytic.
This alters the gain of the transistor and changes its internal capacitance. This junction capacitance modifies the oscillator with a frequency equal to the sound entering the microphone thus FREQUENCY MODULATING the circuit. A short length of antenna wire is connected to the collector of the oscillator via a coupling capacitor and some of the energy of the circuit will be radiated to the surroundings.
Any FM receiver will pick up this energy and decode the audio portion of the signal.

When the FM BUG is complete, checked and ready for insertion into its case, there is one slight adjustment which must be made to align it to the correct frequency.
As we have said, the only critical component is the oscillator coil. It is the only item which is adjustable.
Since we are working with a very high frequency, the proximity of your hand or even a metal screw-driver will tend to de-tune the oscillator appreciably.
For this reason you must use a plastic aligning stick to make the adjustment. Any piece of plastic will do. A knitting needle, pen barrel or plastic stirring stick can be used.
Place the bug about a metre from the FM radio and switch both units on. Tune the radio to an unused portion of the band and use the alignment stick to push the turns of the coil together. Make sure none of the turns touch each other as this will short out the operation of the oscillator.
All of a sudden you will hear the background noise diminish and you may even get feed back. This amount of adjustment is sufficient. Place the BUG in its case and tape up the two halves.
The fine tuning between radio and transmitter is done on the radio. Peak the reception and move the BUG further away. Peak the fine tune again and move the BUG into another part of the house and see how far it will transmit.

If the bug fails to operate, you have a problem. Simple digital tests will not fix it nor will ordinary audio procedures. The frequency at which the BUG operates is too high.
You have to use a new method called comparison.
This involves the comparing of a unit which works, with the faulty unit.
This means it is ideal for a group of constructors to build a number of units and compare one against the other.
This will not be possible with individual constructors and they will have to adapt this fault-finding section.
The first fact you have to establish is the correct operation of the FM receiver.
If you have another BUG and it is capable of transmitting through the radio you know the radio is tuned to the correct frequency. Otherwise you will have to double-check the tuning of the dial and make sure the radio is switched to the correct setting.
The next stage is to determine if the BUG is functioning AT ALL. The only voltage measurements you can make are across the collector-emitter terminals of the first transistor (1 v to 1.5v) and across the collector-emitter terminals of the second transistor (1.3v to 1.5v) These values won't tell you much, except that the battery voltage is reaching the component.
Tune the radio to about 90MHz and lay the radio antenna very close to the antenna of the BUG. Switch the BUG on and off via the slide switch. You should hear a click in the radio if the BUG is on a frequency NEAR 90MHz. Move the turns of the aerial coil together or apart with a plastic stick as you switch the unit ON and OFF.
If a click is heard but no feed-back, the oscillator will be operating but not the pre-amp stage. This could be due to the electret microphone being around the wrong way, the transistor around the wrong way, a missing component or an open 2.2u electro.
If the fault cannot be located, compare your unit with a friend's. You may have made a solder bridge, connected the batteries around the wrong way, made the coil too big or used the wrong value capacitor for one of the values.
If all this fails, put the unit aside and start again.

1 - 470R
1 - 10k
1 - 22k
1 - 47k
1 - 1M
1 - 5.6p ceramic = 5p6
1 - 22p ceramic or 27p or 33p
1 - 47p ceramic
1 - 1n ceramic = 1,000p or 102
1 - 22n ceramic = .022 or 223
1 - 2.2u 16v or 25v
2 - BC 547 transistors
1 - mini slide switch spdt.
1 - electret microphone (insert)
2 - AAA cells
10cm tinned copper wire
2 - metres aerial wire

1 - FM BUG PC board