GuShH's DevBlog

A simple FM Bug for beginners

For months I’ve been looking for a simple FM BUG project, the ones online require inductors which you either have to acquire or build, if you don’t have a LCR meter it becomes rather hard to get the circuit working, specially if you’re a beginner without an oscilloscope! – Sometimes they don’t even tell you which inductance is required and you have to calculate an estimate, which is the main reason why many high frequency RF projects fail in the first place.

From all the projects out there I’ve only seen one which didn’t require an external inductor since  it simply used a pcb / trace inductor, however the board was big and the circuit itself had lots of stability issues, etc. I wasn’t going to waste my time with it.

My first FM BUG was based on one of the many schematics out there, it seldom worked. It was microphonic (due to the air core inductor) but the electret capsule itself did not modulate the output at all, needless to say it was very unstable and it never worked properly.

This circuit on the other hand performs pretty well, even if you’re manipulating the board or touching the coax it will stay within the tuned frequency (unless you touch the transistor or timing capacitor!). The power is very low, so don’t expect great distances, specially in populated areas!

This is the schematic:

No surprises there, it’s very similar to other schematics so therefore I won’t go into the working details. Actually I might argue this one is a bit on the low efficiency side, but at the moment I don’t have the equipment to further improve it’s design.

But let’s not focus on that and instead let’s talk about the inductor / antenna, it’s based on a 12cm long 50-75Ohms coaxial cable with one of it’s ends soldered (mesh and core are joined together) this is a big plus, we don’t have to use a flimsy air core inductor and we don’t need a lengthy wire antenna either – Great!

This tiny ghetto transmitter is guaranteed to work first time, as long as you double check all connections and you make sure your transistor is properly placed and in working order. I used the BF199 because it’s got a low capacitance and it’s ideal for this type of application, but you may use a 2n2222 or similar general purpose NPN BJT. I know, the schematic calls for a BF259 which is what I used in my simulation, but believe me it works beautifully with the BF199.

I didn’t have a 8.2pF capacitor so I used a smaller one, which naturally led the transmitter to work on the upper FM broadcast band, but hey — better than nothing!

The sound quality is not the best, this FM Bug could clearly benefit from an audio amplification stage. Another idea would be to scrap the electret and use an mp3 player instead. I reckon R4 could be ignored in that case but you might want to use a resistor in series with the input capacitor.

Another important component is the variable capacitor which forms the tuning circuit, I used a 4.5-20pF trimmer type capacitor, it’s the first one I found in the junk box, it was a bit flimsy and it required lots of patience to tune but I eventually got it to the point I wanted.

I recommend you get a ceramic screwdriver for the tuning or you can improvise one with a piece of plastic, it’s important because otherwise your body capacitance will affect the transmitter!

For those interested I also included the PCB layout, it’s very easy to etch your own PCBs so you should definitely give it a try! – Remember to keep the component leads to a minimum, we’re working with high frequencies, any parasitic capacitance, etc. will modify the behaviour of the circuit one way or another!

Click here to download all pertinent files for this project: fmbug_docs. The exports are not in perfect quality due to the PDF printer driver I used, but hey at least I’m providing the PCB layout, other projects leave this as an “exercise for the reader” (that’s because they never actually built the damn thing!)

One improvement over the current layout would be to get rid of the header for the power connection and use a button cell with a holder instead, another one would be to use a 3.5mm connector instead of the electret being soldered directly to the board.

And in case you’re wondering, here’s the BOM:

• 1x BF199 or similar RF NPN BJT.
• 1x 8.2pF Capacitor.
• 1x 2.2nF Capacitor.
• 1x 0.1uF Capacitor.
• 1x 50nF Capacitor.
• 1x 100 ohm resistor.
• 2x 4.7k resistor.
• 1x 5.6k resistor.
• 1x 3-30pF variable capacitor.
• 1x 3V source (I used 2x 1.5V AAs with a battery holder).
• 1x 12cm long piece of 50-75Ohms coaxial cable to serve as the inductor/antenna.
• 1x electret microphone (double check the polarity!)

Standby power consumption is ~5mA. During normal operation you should expect peaks of 10mA but on average 6mA is about right. L1 would be lucky to see peaks higher than 15mW!

If you want to increase the transmission distances you may use a metallic enclosure and solder the negative of the battery to the enclosure itself.

Also keep in mind my design was not meant to be used as a concealed spy bug, if you really wanted to build a proper spy bug you’d have to use SMT parts, these are generally not available to the beginner and therefore I went with through-hole components instead. Plus I’m against spying people without their consent…

That’s all for now, hopefully you’ll build and enjoy this tiny FM “BUG”, I know I did 🙂

Cheers.

Oh by the way… check your local laws regarding this type of device. I can’t be held responsible if you interrupt your neighbours favourite station!