See Phone Ring. It is a surface-mount project to test your skills as soldering SM components.

Something we have been asked on a number of occasions is: “Why do we keep producing so many FM transmitters projects?
Apart from the fact that they are always extremely popular, each of the designs either incorporates an improvement over a previous design or a new feature.
A reader wrote to us suggesting we design one enormous project incorporating the features of our VOX bugs, our Wall Bug and our long range bugs, with a switch to select a particular feature.
This would be wonderful idea if it were practical, but it would defeat the purpose of the designs as each demonstrates a different feature.
There is no circuit simpler than an FM transmitter. Nothing produces crystal clear audio from two or three stages and transmits to a tuner to give the equivalent of a high wattage amplifier. To make the circuit more complex by adding extra features would defeat the purpose of starting with something simple.
Each of the transmitter circuits we have designed incorporates a number of BUILDING BLOCKS and studying these will help you; not only to understand how the circuit works, but also with designing your own circuits.
Building FM transmitters is definitely a great place to start, as the cost is low and the experience you get is invaluable.

DESIGN IMPROVEMENTS
Many of the designs in our transmitter series are on their second or third improvement.
It would be wonderful to design a project that is perfect from the start, but unfortunately this is not reality. There are always improvements and additions that come along after we produce a project. Sometimes the improvements are only small and sometimes they are considerable.
No-one is perfect and we would be the last to admit we are perfect. However we can state our designs are by far the best we have seen, simply due to the fact that we update and improve them to get them to the stage of near perfection.
It's the “I help you, you help me” principle. I sell you something cheap and you buy it in enormous quantity. That's how we work. Everyone can afford our low-cost kits and that's how we stay in business. We help you with designs and you give us feedback and improvements.
This has been the case with all of our projects. When they go into the field we get feedback from constructors. We also have the advantage over other designers in that we sell made-up devices as well as kits and we get first-hand ideas on improvements. That's how we have got to the stage we are at.
And now for a really challenging project.

SMALLER AND SMALLER DEVICES
We are constantly asked for smaller and smaller bugs. Some constructors think it is easier to hide a smaller device.
But the problem with size is the antenna and the batteries.
It's no good having a micro-miniature bug if the antenna has to be 50-100cm long!
The other problem is the size of the battery. As the battery gets smaller and smaller, not only does the life of the bug reduce but the earth plane produced by the mass of the battery is also reduced and the output of the transmitter is reduced.
That's why really small bugs are placed in a metal case. It acts as a ground-plane.
It all boils down to the optimum size for a transmitter and this seems to be about the size of a matchbox.
This size gives us adequate room to fit two AAA cells, a switch and electret microphone as well as a board containing conventional components. But sometimes we have to pander to demand.
At the present, the demand is for a miniature bug.
That's why we have designed the MICROBUG. It is the smallest we can go using readily available surface-mount componentry - even then the components are not readily available, although some suppliers are listing packs of 10 of each component. We  had to import the very tiny slide switch and microphone.
The circuit is our tried and proven two transistor design and has already been presented as the Earwig project.
Even though the circuit is very simple, no-one has been able to improve its performance and all we have done is use surface-mount components to reduce the size.
I am going over the operation of the circuit once again because every time I describe it, I include a few more details of how each component works.

HOW THE CIRCUIT WORKS
The circuit consists of three stages, an electret microphone, an audio amplifier and an RF oscillator.
The electret microphone consists of only two parts - a FET transistor and a plastic diaphragm. There is nothing else inside the case. The 3-leaded FET has an input, output and earth lead. The mylar diaphragm has been permanently charged with a static charge and this is one of the secrets of the operation of the microphone.

Micro Bug components

Before starting, place a tiny piece of blu-tack on the top-side of the Micro-Bug 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.

CONSTRUCTION
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 lost.
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 soldering article.
Also, 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 articles and pictures on soldering on Howard Electronic Instruments website.
Also see this ARTICLE on soldering by WELLER. 

Now we can get down to assembly:
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, 10k, 33k, 47k, 100k and 1M.
The next components to fit to the board are the 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 10p!”
 

The colors 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 $8.00 for the set of capacitors $6.00 for the resistors and $4.00 for the transistors plus $2.00 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 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 components.
We build hundreds of items for customers and the assembly workers have said surface-mount devices are easier and quicker to fit.

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 surface-mount technology.
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 conventional components.
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 sides.
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 paper.
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 separate the metallised end caps from their substrate and cause the component to go open circuit.
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 realign it.
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 end.
This is very important when soldering the transistors.

 

The Micro Bug from the bottom. Notice how neatly the parts are positioned and soldered

 

An enlarged view of the underside of the Micro bug PCB. It shows the placement of the surface-mount components.

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