An Especially Tiny And Perfectly Formed FM Bug

It used to be something of an electronic rite of passage, the construction of an FM bug. Many of us will have taken a single RF transistor and a tiny coil of stiff wire, and with the help of a few passive components made an oscillator somewhere in the FM broadcast band. Connect up a microphone and you were a broadcaster, a prankster, and probably set upon a course towards a life in electronics. Back in the day such a bug might have been made from components robbed from a piece of scrap consumer gear such as a TV or VCR, and perhaps constructed spider-web style on a bit of tinplate. It wouldn’t have been stable and it certainly wouldn’t have been legal in many countries but the sense of achievement was huge.

As you might expect with a few decades of technological advancement, the science of FM bugs has moved with the times. Though you can still buy the single transistor bugs as kits there is a whole range of fancy chips designed for MP3 players that provide stable miniature transmitters with useful features such as stereo encoders. That’s not to say there isn’t scope for an updated simple bug too though, and here [James] delivers the goods with his tiny FM transmitter.

Gone is the transistor, and in its place is a MAX2606 voltage-controlled oscillator. The on-chip varicap and buffer provided by this device alleviate some of the stability issues suffered by the transistor circuits, and to improve performance further he’s added an AP2210 low-dropout regulator to catch any power-related drift. If it were ours we’d put in some kind of output network to use both sides of the differential output, but his single-ended solution at least offers simplicity. The whole is put on a board so tiny as to be dwarfed by a CR2032 cell, and we can see that a bug that size could provide hours of fun.

This may be a small and simple project, but it has found its way here for being an extremely well-executed one. It’s by no means the first FM bug we’ve shown you here, just a few are this one using scavenged SMD cellphone parts, or this more traditional circuit built on a piece of stripboard.

17 thoughts on “An Especially Tiny And Perfectly Formed FM Bug

  1. Would an evil government entity living in 2018 and bent on oppressing it’s people not use something a little more sophisticated than a bunch of components soldered to a coin cell holder in a design copied off of a HaD article?

  2. A project I have long wanted to see is a simple, easy to build and inexpensive transmitter that could be used with APRS. I often think of this when I see these FM ‘bug’ projects. Could one serve as the basis of such a project?

    A few things would have to be different though. It would have to be narrow banded, using about 5KHz deviation as opposed to the 200KHz allotted to FM broadcast band channels. and the frequency would have to be higher, 144.390 MHz (in the US) to be exact. Finally, it would have to be much more stable.

    The idea would be to use this in an unattended beacon or maybe even on a balloon. This makes a requirement for what might be the biggest change of all. It must be stable! There will be nobody there to twiddle the trimmer every few minutes in order to keep it on frequency.

    I hadn’t heard of the MAX2606 before. Could this be the part that makes an APRS beacon with the simplicity of a toy FM bug possible? I’m excited now to find out!

    1. You could also go for a custom ground overtone crystal. 5th or 7th is possible. The oscillator gets a little more complicated as it needs an LC tank to to select the right overtone of the crystal, but 5kHz mod frequency (34ppm) should be possible by pulling of the crystal even with an overtone unit. They have lower pull range.

  3. You should apply to HaD. Maybe they’ll hire you to write all the disclaimers stating what you should or should not do, the safety equipment you should be wearing, and to remember to breath while reading so you don’t pass out.

    1. I think that breathing reminder is a perfect idea. Even though it’s very very very very unlikely, it’s still possibly possible. Think about all the lives we can save. I am so excited … need to breathe…

  4. Oh, puhleeez, Clemens. Somewhere along the line you lost your sense of fun. This is probably the most dangerous form of mental illness. Get back into the real world with the rest of us. Start off by deliberately littering. Throw an empty soda can out of your car on a residential street. One with children on it. Yes, be a bad example. Do NOT go back to pick it up. The next day you can be creative, but do something equally horrible. Soon you will laughing at jokes like the “socially irresponsible” people do. You will soon be using the term “society” in casual conversations less than 75 times in one day. Weeks or months down the road, you will say something that actually makes another person laugh.

  5. Also an interesting idea: build a miniaturized downconverter that takes GPS signals and retransmits them on the high FM band, so that it can be used indoors with a modified SDR/FM receiver. This also means it can provide location data which only works inside a building, for thwarting third party tracking (any additional data can be generated from Wifi triangulation)

  6. I used to build these FM bugs in my early days of hacking, about 40 years ago.
    Today there’s no point building these as a module from china sends HI-FI stereo FM, some even with RDS, on the FM band.

    Some of them even a mile around.

  7. Back in the ’70s, we used to take the transmitter from a “Mr. Microphone” and add an output transistor to it. It boosted the power and decreased the body capacitance effects on the frequency. Not quite as small as this, but pretty good (and inexpensive) for the time.

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