Yet again our pen bug circuit has hit the paper back with yet another redefined and more powerful circuit, presented here in surface-mount for the first time. So here is the Pen Bug Mk 4. Being one of the most popular circuits, the bug has the ability to be hidden in a marker pen and be totally undetected by an unsuspecting person.

 Our first Pen Bug circuit appeared in More FM Bugs and the Mkll modification was presented in Security Devices. The only difference between these was the antenna arrangement. In the Mkl version the antenna was wound onto a spiral and fitted into the barrel of the pen. In the Mk II version a tank circuit was added and the coil of the tank circuit used as an active antenna.

The Pen Bug MkIII out-performed the earlier models where the antenna was longitudinally wound and coupled between two stages via the emitter of the oscillator.

Firstly, the thought of using the coil in the tank circuit for transmitting an RF signal was a little far fetched, but we used it as an example for the people would were interested in experimenting with our bug circuits.

This antennas designs didn’t live up to the expectations of transmission, and range, so it was finally dismissed from our range of transmitters

Now we have resided back to the conventional antenna, the whip. The whip or straight-line antenna can be coiled, wound longitudinally or folded. The arrangement makes no difference to the type of antenna, only to the signal strength and the overall size.

Even though we used this antenna set up in the previous Pen Bugs we have considerably improved the circuit to the point were it has efficient battery consumtion, this is a common factor where the circuits cause a dilemma.

The size of the Pen bug 4 has shrunken considerably with the now, readily available surface-mount components designed into the layout. These have taken the place of the standard sized components in our pen bug IV.

The circuit has also been reduced in size due to some omitted parts. The improved circuit has allowed the supply current to be reduced but the output power has stayed the same.

From the outset let me say that it is very difficult to get a good range from a bug fitted inside a pen.

As you can imagine, it is practically impossible to produce an antenna with any effectiveness from inside a pen barrel and this limits the range of the bug. We got about 10-15 metres under good conditions and we thought this was a fair achievement.

The main problem with the Pen Bug, or any bug that doesn’t have a long enough antenna length is getting an effective range. Due to the cramped space inside a pen there is almost no room for an antenna and the range of our original pen bug was severely limited.

The Pen Bug Mk IV is designed around the conventional method of using a whip antenna, this has proved to be the most practical and efficient way of transmitting an audio signal. As the range, even with a quarter wave antenna is longer and clearer.

To get the greatest range it is important to peak the tank circuit very carefully so that it delivers the greatest signal into the antenna. To do this you will need the Field Strength Meter MkII as described in this book. You simply adjust the air trimmer to get the maximum output - it is a very simple procedure.

All the versions of the Pen Bug are the smallest in width in our range of bugs, and using commonly available components, here is our version Pen Bug MkIV version.

CONSTRUCTION

Before starting the assembly, it is important to have everything ready with the parts laid out on your workbench for easy identification.

The surface mount capacitors are going to have to be the most important issue, during the parts placement as the values are not marked on the package. The only way of identifying their value is in the order in which they come in the carrier strip.

For a comprehensive guide to soldering surface-mount read the related article in this book ‘Soldering Surface-mount, also read the construction notes for the Micro-bug and the Silverfish, as these will give you further guidance and extra information

Start at one end of the board, the microphone end, but leave the microphone to last.

Each part is fitted as you come to it and everything is easy until you come to the oscillator coil. This is mounted upright on the board and you must sstraighten the ends of the coil so that they fit down the holes in the board. Don't squash the turns together as they may have to be stretched apart during testing, for setting of the frequency. Make sure the enamel is scraped off the leads before fitting by using a sharp blade or file or sand paper. The most conventional way is to remove the enamel by applying a fair amount of solder to iron tip and hold it to the wire to heat up the wire, until the enamell bubbles off, leaving the copper wire tinned .

Continue with the rest of the parts including the 3-10p air trimmer and tank coil. The board is now ready for the microphone, switch and batteries.

You can solder directly to the button cells if you firstly scrape the top and bottom with a blade or file. This will roughen the surface so the solder will stick to it. Make sure the soldering is carried out very quickly otherwise the seals on the cells will be damaged and the cells will leak causing them to be damaged. These chemicals are very corrosive, so be careful.

The microphone needs two short lengths of tinned copper wire soldered to it’s lands so that it can be connected to the board with the two short wires that are now on it. The microphone is polarized in the way that the lead going to the case of the the microphone goes to the negative rail of the PCB. The negative rail is the copper strip that runs the length of the bottom of the board and meets the negative terminal of the battery. The microphone must be soldered very quickly otherwise the FET inside the case will be damaged and it will lose sensitivity and produce a lot of back-ground noise, but also as important is the leads previously sodered to it will fall off. These instructions are included with the Silverfish article.

THE CASE

The only thing we haven't provided in the kit is the case.

This has been left up to you as there are so many different types of pens and markers that will hold the project. It is preferable to use an old, dried out marker as this will save you a couple of dollars and you don't have to handle any messy ink.

If you want the pen or marker to write normally so that no-one will suspect the contents, you will have to provide a section up the front to hold the pad of ink and this will have to be sectioned off from the rest of the barrel to prevent the ink drying out.

The main aim is to get a case that will fit the board, batteries and switch. After this you can see how much room you have for the ink.

Next you will have to work out the switch arrangement and it can be either a slide switch mounted inside, a pressure switch kept apart with a pin or a reed switch kept open with a magnet on the outside of the case.

When the pin is pulled out or the magnet removed, the switch closes and the bug is activated.

The Commercial bug (as explained the MkIl article) did not have a switch. Two button cells were fitted into the barrel and the cap screwed on. The bug was then active and would operate for about 8 hours. The idea is to arm the bug before-hand and leave it at a meeting etc. Later you can go back and pick it up.

The bug we saw was actually a ball-point pen and the ink was contained in a refill near the tip. By the time we got it, the ink had run out and the only telltale difference was the larger-than-normal barrel and extra weight. Anyone with a fair degree of intelligence would become suspicious at the extra size and wonder why the pen is so cumbersome.

You could not see the hole at the end for the microphone and no external antenna was used. To get one of our bug to the same size as this it would require the standard components like the microphone the trim-cap and two very small watch batteries to be surface mount components, but this would increase the cost of the kit immencly. A lot of extra skill would have to go into construction as special tools would also be required, so we have chosen the alternative, with something just slightly larger, but much cheaper.

PEAKING

The range of the Pen bug depends almost entirely on peaking or TUNING the output. This is done by placing the Field Strength Meter near the antenna and adjusting the air trimmer until the LED readout produces a maximum reading.

It is essential to be able to measure the output without physically touching any part of the circuit and this is why we use the Field Strength Meter.

You must turn the trimmer on the Pen Bug with a plastic screwdriver as any type of metal implement will change the characteristics of the circuit, and give a false reading. When you see the the three LEDs illuminate in series, you have peaked the output to a maximum. The tank circuit is now drawing the maximum energy from the battery and converting it to electro-magnetic radiation.

Place the board inside a marker pen and test the bug for both range and clarity.

If you are satisfied with the results, fit some small pieces of foam to prevent the board moving around and fit the pen tip. Now you are ready to try it out. Ask someone to use your “Pen” and see if they notice anything different. Don't let them know or they'll want one too!

IF IT DOESN'T WORK

Most of the problems will occur in the tank circuit section as it is important to peak this stage very accurately to get the maximum output.

The first thing to do is test the oscillator section by connecting a Peaker to the collector of the oscillator transistor. If no output is detected, remove the 47p coupling capacitor and try again.

If still no output, go over the layout of the components, making sure none of the tracks have shorted together. Since the board is very compact it is very easy to create a solder bridge or fit a component onto the wrong pads.

Once you are sure the oscillator section is working, connect the 47p coupling capacitor and place your Field Meter near the antenna.

If no reading is detected, the output stage is faulty. Try replacing the RF transistor and check the layout.

Once you can detect a small output from the antenna, adjust the trimmer until the output is a maximum. Don't forget the plastic screwdriver (such as a knitting needle).

Once the circuit is peaked to a maximum, don't touch anything as you fit it into a case or you will upset the output.

It's now ready. Have lots of fun.