This project combines 15 years of experience in designing and making FM transmitters. As you can imagine, it has all the best features we can cram into a single design and consequently has a fairly high degree of complexity.

The circuit is quite advanced and should only be attempted by those who have already built some of our devices or at least an equivalent project - say a 3 or 4 transistor design. If you are just beginning, we have lots of simpler projects to start you off.
The first thing you will notice is the surface mount componentry. This is the way things are going. It is faster and easier to hand-assemble a surface mount board than a through-hole  board. 10 or 20 of them can be laid-out on the bench and the components added in a "production-line " arrangement. The boards don't have to be picked up, turned over or the component leads trimmed. This all saves handling time and the result is nearly half the assembly time. 
This method of assembly can be carried out on one board or a line of 20 boards. The only difference is accessing each component from a "carrier strip" as supplied in the kit or a "taped carrier" containing 5,000 of the same component (on a reel). With the carrier on a reel, 20 of the same component can be exposed by stripping off the top cover and it's just a matter of picking them up one-at-a-time with tweezers and soldering them to the boards. With the carrier supplied in the kit, you must only remove one component at a time to prevent them getting mixed up.
This design contains all the features we have developed over the past years and this includes an RFC (Radio Frequency Choke) to keep the RF signals away from the power rails, an air trimmer to adjust the frequency, replaceable batteries and our proven-reliable circuit.
This design has one new feature. The board has been designed for two different types of batteries. The kit comes with 2 lithium cells and a holder but you can opt for a 12v lighter battery . Let me point out the lithium cells will last more than 5 times longer than the lighter battery due to the energy-capacity of the cells. The 12v lighter battery is slightly smaller but the advantage of the lithium cells is they can be replaced.  The 12v lighter battery has to be desoldered to be replaced.

HOW THE CIRCUIT WORKS
The circuit consists of three stages - an audio amplifier, an RF oscillator and an RF output stage. 
The electret microphone contains a Field Effect Transistor (FET) and can be counted as a stage, if you wish.
The microphone detects audio in the form of air vibrations that enter the hole at the front of the microphone and move a very fine metallised plastic diaphragm. This diaphragm is made from mylar and is charged during manufacture. As the diaphragm moves, the charges on it are influenced by the charges on the case (like charges repel etc) and some of them move down a lead touching the diaphragm and into a FET transistor. This device amplifies the charges and produces a reading on the output lead. A FET has been used as it has a very high input impedance and does not have any loading effect on the static charges. 
The microphone must be placed in a circuit so that a small current flows through it. This current is modified when it detects audio and the result is a voltage swing that is fed into the first audio amplifier via a 22n capacitor. The surface-mount BC 848 transistor has a gain of about 70 -100 and amplifies the signal. It is connected in common-emitter mode with a 10k load resistor and 1M base-bias resistor. The signal is now large enough to be passed into an FM oscillator stage that is designed to produce the "carrier." In simple terms this is the "silence" produced by the transmitter on which the voice is "superimposed."
The oscillator is designed to operate at approx 88 - 108MHz and the frequency is set by the value of inductance of the 4 turn coil and the capacitance across it. The capacitance is made up of 47p and a 3-27p air trimmer and since these are in parallel, the capacitance varies from 50p to 74p.
The frequency is also determined to a lesser extent by the transistor, the 10p feedback capacitor and also the biasing components such as the 390R emitter resistor and 47k/100k base bias resistors. The supply voltage also has an effect as the oscillator can be classified as a voltage controlled oscillator. 
There are a lot of things that set the frequency and even though the parts have a 5%, 10% or 20% tolerance, the frequency can be set very accurately by moving the turns of the coil closer to lower the frequency or stretching them apart and then adjusting the air trimmer to the precise frequency you require. 
The circuit will stay at the desired frequency providing the supply voltage remains constant and the temperature of the components does not rise. 
Into the base of this stable circuit we inject a varying voltage via a 100n capacitor. Normally the base is held rigid by the 1n capacitor and the voltage on the base is governed by the 47k/100k base-bias resistors. These determine the amount of current delivered by the transistor, to the tuned circuit, during each "top up" cycle and when the audio is added or subtracted from this voltage, the frequency of the stage is altered. 
The frequency is altered by the fact that a higher voltage on the base will allow the transistor to turn on sooner and thus charge the capacitor with slightly more energy. This increases the discharge-time and lowers the frequency of operation of the tuned circuit. 
The 10p feedback capacitor turns on the transistor when the voltage across the tuned circuit has a polarity that decreases the voltage on the emitter. The voltage between base and emitter is increased and the transistor turns on slightly harder. See our website for an animation of the TANK CIRCUIT to see this happen.
Between the oscillator and antenna is a buffer stage. If the antenna is connected directly to the oscillator stage, it absorbs some of the energy and the amplitude of the waveform is reduced. The buffer transistor allows the oscillator to operate at maximum amplitude and a small amount of the waveform is tapped off the emitter and fed into the buffer stage. This stage can also be called an output stage. The transistor is connected in common-emitter mode as the capacitor on the emitter prevents the emitter voltage rising or falling at the frequency of operation of the stage. 
The RFC performs two functions. Firstly it prevents the oscillations on the collector entering the positive rail and this allows the transistor to generate a very good waveform. And secondly, when the current is collapsing in the RFC, it produces a voltage of opposite polarity. The RFC adds this to the waveform produced by the transistor so that the output of the buffer stage can be higher than the supply voltage. 
The end result is a very good output from a 3-transistor design, operating on 6v and drawing about 6 to 10 mA. 
We have classified the bug as a 400 metre device but this rating has been worked out with a 1/4 wave antenna. All our previous bugs have been rated with a 1/2 wave antenna.
Even though a 1/4 wave antenna is only slightly less efficient than 1/2 wave, a 1/4 wave has to be set up more accurately to achieve the same results.

SURFACE MOUNT
If you havn't worked with surface-mount before it is suggested you try one of our other kits, such as the VOYAGER. It contains only 5 surface mount resistors and is a good place to start.  Surface-mount resistors are the largest of the surface mount components and if you have trouble working with these, you will have enormous difficulty working with some of the other components, such as capacitors and transistors. The transistors are half the size of the resistors and have three leads! Plus, they must not get too hot when soldering or they may be damaged.
If you don't have a fine tipped soldering iron, forget it! The component will stick to the end of the iron and come off the board as easy as picking up a pin with a magnet!
I don't want to put you off, but honesty is the best policy and if you know the facts before you start, you won't be disappointed. Obviously you can graduate to this project as we have a range of over 30 different FM transmitters and if you work your way up, everything will work as you go, and you will be another satisfied builder.
Now that we have laid the ground-work, let's get on with the project and show exactly what you can expect.

ASSEMBLY
As we mentioned in the introduction, this is a surface-mount project and unless you have the correct tools for the job, you are going to have a difficult time. The three things you MUST have are:
1. A fine tipped soldering iron (constant temperature type preferred)
2. A pair of fine tweezers to pick up the components
3. Fine solder  and a piece of blu tack to hold the board onto the bench.

Here is the order for assembly:
  xx surface mount resistors andxx surface mount capacitors
2 surface mount transistors
1- jumper link, coil, RFC, air trimmer through-hole transistor, switch microphone, antenna, battery holder .
Its best to fit the surface-mount resistors first so that most of the solder lands are used up and this will make it easier to fit the capacitors.  Refer to the large placement guide before fitting any of the components. The surface mount

THE COILS
There are two coils in this project. One is an air wound coil and the other has a ferrite core. The main reason why a coil is wound on a ferrite core is to increase the inductance and reduce the size. The ferrite has a higher permeability than air (air = 1) and everything can be made much smaller. But at 100MHz, ferrite has losses (due to eddy currents) that equal the gain normally associated with ferrite, with the result that ferrite performs almost the same as an air core.
The advantage of the air coil is the turns can be stretched apart to alter the frequency and providing the coil is made from thick wire, they will not move when the project is carried.
The kit comes with a pre-wound air coil because the diameter is very critical and if we say the coil has to be wound on a 2.5mm former, most constructors will think "near enough is good enough." The air coil has 4-turns.
On the other hand, the RFC has been supplied as a slug and wire. 
The turns can easily be wound on the slug when you know how.
Start at the middle of the slug and the middle of the wire. Wind from the centre of the slug to one end. Then from the centre to the other end. This allows you to hold the slug while you are winding. If you have wound the turns very close to each other you will have 10 turns. 
You now have two coils for inserting into the board. Cut the leads so they are only 1cm long. 
Here comes the most important part:
The enamel must be scraped off the ends so the shiny copper wire is visible. This is done with a razor-blade or knife so the copper is visible all around the lead. 
The leads are now tinned and any surplus is flicked off so they can be fitted to the board. 

SETTING THE FREQUENCY

The Spymite is designed to operate at about 88-90MHz and you should search for a free spot on the dial at about this frequency.
The coarse frequency setting is done by moving the turns of the coil together to lower the frequency, or moving them apart to raise the frequency. This should be done with a plastic screwdriver so that you can monitor the frequency of the bug as you move the turns.
If you set your radio to a quiet spot on the dial and listen for the bug, you will hear a squeal due to feedback, when it is at exactly the same frequency.
It's best to have the air trimmer adjusted to "half-mesh" while you are doing this so the frequency can be shifted up or down the dial after the course setting is made.
You will be able to see the blades of the trimmer engaging as you turn the rotor and a "half setting" will allow final adjustment after the project is heat shrunk.

IF IT DOESN'T WORK
This is going to be a real test of your skills. Not only will you have contend with working on a miniature layout but some of the components are unmarked.
Unfortunately the capacitors are not marked and their size is no indication of their capacitance as they are made up of layers of metallised ceramic so, depending on how they are constructed, a 22n can be smaller than 1n! This is not always the case, it depends on the manufacturer and the 22n in your kit may not be smaller!
That's why it's absolutely essential to follow the construction steps exactly as we have laid them out. If any of the capacitors are swapped over, the project will not work at all as the values are spread widely across the range.
The first problem will be trying to determine the value of the capacitors.  The only solution is to send $10 to Talking Electronics for a replacement strip of capacitors and the two surface-mount transistors.  (price includes postage)

With the new capacitors in a "carrier strip" you can compare the size of the components on the board with those in the strip.  Take them out of the carrier strip one at a time and sit them on either blu tack, sticky tape or place them in a small zip top bag  and mark the bag with the value. Don't let any chance of a mix-up occurring.
If you have a capacitor tester - such as on a multimeter - or as a piece of test equipment, it can be used to check the capacitors "in-circuit." Some of the readings may not be absolutely accurate due to the effects of the surrounding components but if you get a reading that is close to the nominated value, you can assume the value is correct.
Talking electronics also has a capacitor tester project xxx The cost of the kit is less than $20.00  xx and it's very handy for times like this when you need to know an approximate reading.
The only other way of solving the problem is to remove the capacitors very carefully and replace them with leaded types. This will only be a temporary measure so the capacitors can be connected to the underside of the board and the leads left fairly long. This won't upset the operation of the bug and when you get it working, they can be replaced, one-at-a-time, until everything is put back together.
The capacitors will withstand soldering and desoldering but the transistors are another matter. It all depends on the temperature of your iron. At the correct soldering temperature, all surface-mount components will withstand 10 seconds immersion in molten solder.
How do you know the correct temperature without a soldering iron temperature meter? If you have an adjustable temperature soldering station, it should be set to the lowest temperature 320°C (600°F). Solder melts at 260°C so any temperature above this will melt the solder. But a solder joint becomes cleaner, faster and better-quality as the temperature of the iron increases. The reason is the flux in the solder has to act on the impurities on the surface of the component and board. These impurities include grease, dirt and chemicals. These have to be "lifted" so the solder can attach directly to the metal below. It is the flux in the solder that carries out this operation. That's why you must add flux to the iron when it is actually on the joint. It is no use adding solder to the iron then taking the iron to the joint. By this time the flux has burnt off the iron and there is nothing to clean the solder-joint.  As the temperature of the iron increases, the cleaning action of the flux improves. So you have to come to a compromise between temperature and good cleaning action. 
In addition, the temperature of the iron must be higher than the melting point of solder due to a temperature gradient between the iron and the PC board and component. The board is virtually at zero degrees (20°C) and has to be raised to at least the melting point of solder. This is the only way solder is going to attach to the surface of the board (and the component). If this temperature is not achieved, the joint is said to be "dry." If you pull on the lead or wiggle it, the joint will come apart.  Sometimes the stickiness of the flux and dirt is the only thing keeping the solder in place. In this case the connection will look perfect but no current will flow at all! 
To prevent this from happening, you have only one choice. The temperature must be kept as low as possible and the soldering time as short as possible to prevent damage to the components. Without a demonstration video there is no way we can relate these skills. Every soldering iron is different and every component is different. A leaded component requires additional time due to the lead "drawing away" the heat energy from the iron. A surface-mount component, on the other hand, requires very little thermal energy to heat it up and the land on the board is very thin, so it's just a matter of a second or less and the connection is done. 
Surface mount components are very rugged "temperature-wise" but it's important to realise you cannot solder one end of a resistor then try to move it on the board to "square-it-up." You will produce a crack between the metallised end and the resistive layer. 
One last point about soldering these tiny devices. You really need three hands when soldering the first end of a surface-mount component. One to hold the solder, one to hold the component and one to hold the iron. The answer is to tack it in place then solder the other end. Don't forget to come back and solder it properly. It's more important to get the component sitting flush on the board and square with the edges, than trying to create a perfect first connection. You can always come back and clean up a joint by re-soldering it.   

THE ANTENNA
The antenna is woven "in and out" of two holes in the board to prevent the wires breaking. Only one set of holes is used for this. The others are used when the board is cut down for the 12v version. The idea of the two sets of holes is to keep the antenna in the centre of the board for either version. 
Because of the short antenna, it has to be kept straight and preferably allowed to hang vertically. This gives it a vertical polarisation and if the antenna on the receiver is also upright, the range will be the greatest.  The bug should be placed as high as possible in a room to achieve the maximum range. On top of a cupboard, or ledge is ideal and away from any metal objects such as refrigerator or exhaust canopy. The only other thing that will reduce the range is reinforcing mesh in pre-cast concrete walls and floors. The signal finds it very difficult to pass through this mesh and prefers to go through windows and wooden doors. Obviously metal clad sheds or metal roofs produce a total barrier to the signal however some of it seems to leak through the cracks so you can get a short range. 
Winding the antenna up or coiling it reduces the range as does reducing the length. If you require only the length of the house for instance, the antenna can be shorted to only 10cm or so. In some countries, the maximum length of the antenna must be 10cm and the maximum allowable range is 30 metres (100f). If these are you limits, you will have to abide by them. 

SELECTING THE BATTERY
The current consumption for the Spymite is approx 6mA for 6v supply and 9mA for 12v supply. You have to take this into account when deciding on the type of battery you want to use.
Because of the very small cells inside the 12v lighter battery (30mAHr) it  will only last about 1 to 2 days. On the other hand, the lithium cells (180mAHr) will last about 3 to 5 days. You cannot simply divide the capacity of the cell by the current consumption to arrive at the number of hours of operation because as the voltage decreases, the current reduces also and you get a "tailing-off" effect that extends the hours of the battery. The difference in overall size of the project will vary only a very small amount, between the 6v and 12v batteries and if you are considering life expectancy, the lithium cells are the best decision.

The advantage of these cells is the ease of replacement. They can easily be taken from the battery holder. On the other hand, the 12v lighter battery has to be desoldered.
   

RESISTOR VALUES
The value of resistance printed on the surface-mount resistors follows a 3-digit code according to the following:

All values of resistance on surface-mount resistors are in "ohms" The first two digits provide the two digits in the answer  and the third digit is the number of zero's to add to the digits. The resistors used in this project are:

HOUSING THE PROJECT
The Spymite is designed to be heatshrunk to keep it as small as possible. This is another operation that requires a fair degree of skill. The hardest part is heating up the plastic with a hair dryer then sealing the ends with long nose pliers. The plastic has to be just hot enough to create a permanent seal. If the joint is too cold it will simply come apart. Obviously you don't want to heat up the batteries too much so you have to carry out a quick, hot operation. You can also use a gas stove to heatshrink the project or a miniature gas blow-torch. Even a cigarette lighter can be used and after a bit of practice I have seen the staff heatshrink a bug in less than a minute!