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Circuit Bodging: FM bug transmitter

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Maxwell13.3 April 2010

Everybody who has seen a James Bond movie, or anything involving spying of any sort, has heard of the concept of a bug. A small electronic device is placed in a person of interest’s room, and the agents in the other room can listen in on what’s going on. Evidence is gathered, and the story can move on.

Of course, in reality things are a little dif-ferent. The microscopic devices used in Hollywood are slightly more advanced than what we can achieve on our bread-boards. Unless you have a very steady hand or access to an smd soldering oven you’re not going to be able to replicate the miniature devices seen in the movies.

Legal concerns

There are a few less frivolous concerns when it comes to radio circuits. Radio frequency transmissions are strictly regu-lated and simply transmitting on AM/FM bands is illegal - something that has shut down many decent pirate radio station in the past. The reason for this is sim-ple: you need a license to use a certain frequency - and these licenses are sold at auction. Because commercial radio sta-tions are also interested in acquiring a license to transmit, these become incre-dibly expensive. As such, transmitting on AM band is illegal in the Netherlands. Luckily, there is a provision in the law for

low-power FM devices, for instance to lis-ten to your iPod in the car without having to fit a new stereo system.

Please note that the power allowed for these devices is severely limited. The cir-cuit we have featured has a range of about 20-30 metres, but more powerful devices step over the low-power boundaries and as such are not permitted.

Listening in

Before we can even start to think about transmissions, we need something to transmit. In our case, we’re looking to transmit audio. In figure 1 we have a simple one-transistor amplifier that takes

Circuit Bodging

FM bug transmitter

Many people have their interest in electronics start with radio. As much as radio is taken for granted by

most people, the idea of sending information through the through the air at the speed of light remains

enticing to anyone who thinks about it for longer than a few minutes. As such, we submit to you this

simple yet effective FM transmitter.

Author: Ben Allen

its input from a condensor microphone. A condensor microphone (figure 2) is a special type of microphone in which the diapraghm (the moving part of the mi-crophone) acts as one of the plates of a capacitor. As the diaphragm moves, the capcitance changes.

Figure 1. The condenser microphone and surrounding circuitry. Figure 2. A number of electret condenser microphones.

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Maxwell13.3 April 2010

39

Part list:

R1: 22k

R2: 1M

R3: 10k

R4: 47k

R5: 470

C1: 223pF

C2: 100nF

C3: 102pF

C4: 5.6pF

C5: 27pF

C6: 6-45pF trimmer

L1: 1µH

ANT1: simple antenna

T1, T2: BC547

Using a microphone

The circuitry in figure 1 is fairly self-explanatory. One thing to take into ac-count is that the condenser mic needs a voltage to be applied across the device for it to operate. Unlike dynamic micropho-nes, condensers don’t create an output by themselves. We are, in essence, measu-ring and amplifying the change in the de-vice’s behaviour as opposed to measuring an output from the device itself.

Turning it into radio

The interesting stuff happens in figure 4. First of all, note that the input does not have a DC filtering capacitor. This is be-cause the microphone amplifier circuit already features this capcitor, C2. If you intend to use the circuit to broadcast so-mething other than the output from the microphone you should add the 100nF capacitor in series with the input. C6 and L1 provide an oscillator, the fre-quency of which can be adjusted with C6.

When tuning the circuit, tune your radio Figure 4. The RF modulator.

to 90MHz and attempt to recieve a signal whilst adjusting C6.

Because the capcitance of T2 is in the same order of magnitude as C6, the idea is that as the transistor amplifies the in-put, this capcitance influences the oscil-lator, causing the frequency to change - Frequency Modulation.

This is presented, once again through a DC filtering cap, to the antenna, which broadcasts the signal.

Power supply

The power requirement of the circuit is very simple, as shown in figure 3. A 3-5V supply is enough, so a series connection of 3 AAA cells can power the circuit easi-ly. The large 470µF capacitor C7 provides smoothing in the case of external power supplies.

Final Notes

Making an FM transmitter out of discrete parts is notoriously unreliable. A simpler, but less interesting way is to use ICs to provide modulation, as all they require is an input signal and an antenna, and in some cases a crystal for the oscillator. If your goal is reliability as opposed to expe-rimentation, an IC such as the MC2833 or the MAX2606 might be more to your liking.

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