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SO SMALL IT FITS IN A MATCHBOX By GREG SWAIN This sneaky FM bug is small enough to hide in a matchbox. It's super-sensitive and can be picked up on an ordinary FM receiver up to 100 metres away. We're not suggesting for one minute that you use this little FM wireless transmitter to listen in on private conversations or to engage in industrial espionage. That would not be nice. Instead, this unit is mainly intended as a fun project for the budding electronics engineer. Provided that you can handle a soldering iron and follow a simple wiring diagram, this project can be assembled in about half an hour. All the parts, including an electret microphone and the batteries, are mounted on a small PC board which then slips inside a matchbox. The only indication that something devious is going on is an external wire lead for the antenna. A miniature slider switch on the board lets you turn the unit off when it is not in use to save batteries. When the unit is turned on, signals picked up by the micro14 SILICON CHIP phone are used to frequency modulate a transmitter stage and this then broadcasts the signal to an FM receiver. In practice, the transmitter is tuned for an output frequency of 100MHz which is right in the middle of the commercial FM band (88-108MHz). You do this simply by adjusting a slug in the coil that forms part of the transmitter tuned circuit. In addition to its obvious application as an FM bug, this unit also has more serious applications. For example, it could also be used as a baby monitor or to monitor a telephone when you would otherwise be out of earshot. By the way, this circuit is super sensitive - so much so that it has no difficulty picking up sounds in a room even when the microphone is fully enclosed in the matchbox. In This larger than life size photo shows just how simple the FM bug really is. Power is supplied by two 1.5V cells. fact, if you get too close to the microphone, the circuit will be overloaded. The circuit also boasts excellent frequency stability and has a power output of about 3mW from a 3V supply and 20mW from a 9V supply (roughly twice that of previous designs). How it works Fig.1 shows the circuit of the FM Wireless Microphone. It uses three NPN transistors, all of which are low-cost BC548 types. The transistors are not critical however, and BC547 and BC549 types will work just as well. A 3V battery supplies power to the circuit via on/off switch St. When the switch is closed, power is supplied to the electret microphone via resistor Rt. Any sound signals in the room are now picked up by the microphone and AC-coupled to the base of transistor Qt via a .01µF capacitor (Cl). Qt functions as a common emitter amplifier with bias set by the ON r.t C7 10pF R6 47k' C6 33pF T 3-9V: ...I... ELECTRET MICROPHONE L1 : ZA-2599 L2 : PCB PATTERN B EOc VIEWED FROM BELOW MATCHBOX FM TRANSMITTER Fig.1: signals picked up by the microphone are amplified by Qt and used to frequency modulate VHF oscillator stage Q2. Q2's output is then fed to Q3 which drives the antenna. lMO feedback resistor (R2) connected between its collector and base terminals. The amplified output appears at the collector and is coupled to the base of Q2 via C2 (O. lµF). Q2 functions as a VHF (very high frequency) oscillator stage. 11 and C4 (33pF) function as a tuned collector load for Q2 while C5 provides positive feedback to ensure that the stage oscillates. A low inductance is used for 11 and a high capacitance for C4 to improve stability and reduce the effect of stray capacitance. This VHF oscillator stage is fre- quency modulated by the audio signal on Q2's base. Thus, when ever a signal is picked up by the microphone, this stage produces a corresponding FM signal. An adjustable ferrite slug in 11 allows the oscillator to be tuned across the FM broadcast band as indicated earlier. The RF (radio frequency) output of the oscillator is taken from the emitter of Q2 and coupled to the base of Q3. This stage also functions as a common emitter amplifier. It drives the antenna via an output resonant circuit consisting of coil 12 and capacitor C7 (lOpF). 12 actually forms part of the copper pattern on the PC board, while C7 tunes the resonant circuit to 100MHz. If you intend operating the circuit at 90MHz, then C7 should be increased to 12-15pF for maximum efficiency. Note that in this circuit, the antenna is isolated from the oscillator tuned circuit. This ensures greater stability since the oscillator frequency is no longer affected by movements of the antenna. Power for the circuit is derived from two 1.5V N-size cells. These give a supply voltage of 3V but you CAPACITORS D D D D D D No. 2 1 1 2 1 1 Value 0 .1µF .01µF .001µF 33pF 10pF 5.6pF IEC 100n 10n 1n 33p 10p 5p6 EIA 104K 103K 102K 33K 10K 5 .6 Fig.2: keep all leads as short as possible when installing the parts on the PC board. See text r~ orientation of Lt. RESISTORS D D D D No. 1 2 2 1 Value 1MO 47k0 4.7k0 4700 4-Band Code (5%) brown black green gold yellow violet orange gold yellow violet red gold yellow violet brown gold 5-Band Code (1%) brown black black yellow violet black yellow violet black yellow violet black yellow brown red brown brown gold black brown NOVEMBER 1989 15 PARTS LIST 1 PC board, code ZA-1 431 , 50 x 31mm 1 DPDT miniature slider switch 1 pre-wound coil with ferrite slug, DSE Cat. ZA2599 1 electret microphone insert 2 1 .5V N-size batteries 1 7 50mm-length of insulated hook-up wire (for antenna) 3 BC54 7, BC548 or BC549 NPN transistors (01-03) Capacitors 2 0. 1 µ.F monolithics or ceramics (C2,C8) 1 .01 µ.F ceramic (C1) 1 .001 µ.F ceramic (C3) 2 33pF ceramics (C4,C6) 1 1OpF ceramic (C7) 1 5.6pF ceramic (C5) Resistors (0.25W, 5%) 1 1 MO (R2) 2 47k0 (R4,R6) 2 4 . 7k0 (R1 ,R3) 1 4700 (R5) can use a 6V or 9V supply if you want longer range and space is not a consideration. The current consumption is about 11.5mA. Construction Construction is straightforward. A complete kit for this project is available from Dick Smith Electronics for $16.95 (see panel), so obtaining the necessary parts is ncit difficult. All you have to supply is the matchbox. The batteries are secured by soldering their terminals to loops of tinned copper wire which in turn are soldered to the copper pads on the PCB (see Fig.2). Tin the ends of the batteries first by scraping away some of the plating and then applying a hot soldering iron. Fig.2 shows how to install the parts on the PC board. Install the two wire links first, then the resistors and the capacitors. These parts should all be pushed flat against the PCB before soldering their leads. The three transistors can now be installed. Push them down onto the board as far as they will comfortably go and be sure to match the transistor cases to the outlines on the wiring diagram. Solder their leads, then install the slider switch. The switch mounting lug adjacent to the edge of the board should be snapped off using a pair of pliers (see photo) otherwise it will foul the side of the case. Above: the square section at the top of coil L1 should be trimmed off, preferably before mounting it on the PCB. Orient the coil as shown. Left: the circuit fits neatly into a matchbox. 16 SILICON CHIP Similarly, the square section at the top of coil 11 must be cut off to provide clearance for the matchbox lid. Use a sharp razor blade for this job but be sure to remove the ferrite slug first. Install 11 so that the highest shoulder on the plastic former is adjacent to the switch. The electret microphone is mounted on the board using two short lengths of tinned copper wire (see photo). To do this, first solder the leads to the pads on the back of the microphone, then bend them so that they match the holes in the PCB. The microphone can then be installed on the board with its positive terminal adjacent to R5 (4700). You can set the transmitter output to virtually any spot on the FM band but we recommend a frequency of about 100MHz. Don't forget to turn the unit off when it is not in use (the switch is shown here in the 'on' position). It's quite easy to identify the microphone terminals. The negative terminal is the one connected to the metal case. Now for the batteries. These are mounted on one end of the PCB and are secured by soldering their terminals to loops of tinned copper wire. These wire loops in turn are soldered to the copper pads on the PCB. Be sure to install the batteries the right way around and note that they face in opposite directions to each other. Finally, connect a 750mm-long antenna (use insulated hookup wire) to 12 on the copper side of the board. Of course, you can use a shorter antenna if you wish but note that this will give a reduced range. Testing A commercial FM receiver is required for the test and adjustment procedure. Set the volume control to the usual level, then turn the receiver on and tune it to 100MHz. Now turn the FM transmitter on (just slide the toggle switch towards the centre of the board) and adjust 11 using the insulated alignment tool. When the transmitter is tuned to the correct frequency, you will hear a corresponding whistle in the receiver. If you now speak, you should be able to hear your voice via the receiver but note that it will probably sound quite distorted due to overloading and feedback effects. To fully test the unit, slip it into the matchbox with the antenna lead emerging through a hole in one end of the drawer. It can then be taken into a room where other people are speaking (or the TV is going). Any conversations should now come through loud and clear on the receiver. That's it! Don't forget to move the slide switch to the off position when the unit is not in use otherwise you'll quickly flatten the batteries. De-bugging Now, what if you suspect that your room has been bugged by one of these FM wireless microphones? How do you detect it? It's quite easy. All you do is turn on your FM radio and wind the volume control well up. You then tune slowly right across the FM band. If at any time you come across a whistle, you have been bugged. The whistle will be caused by acoustic feedback between the microphone in the bug and your radio's loudspeaker. It's then a matter of ferreting the little blighter out. You do this by moving your FM radio around the room. As you get closer to the bug the whistle (or howl) will become louder. [§;I Where to buy the kit A complete kit of parts for this project is available from Dick Smith Electronics stores or by mail order from PO Box 321 , North Ryde, NSW 2113 . Phone (02) 888 2105. The kit comes with all parts (including two 1.5V batteries) and sells for $16.95 plus $2 .00 p&p (standard mail only) . Quote Cat. K-5006 when ordering . Note: copyright of the PC artwork for this project is owned by Dick Smith Electronics . NOVEMBER 1989 17