Building A Simple FM Transmitter Bug


[Dino] got his hands on an FM transmitter “bug” kit via a friend, and thought it would make for an easy and fun Hack a Week project. The kit is simple two transistor half-wave FM transmitter, which the manufacturer suggests could be used to bug a room, hence the name. After poking a bit of fun at the instructions, [Dino] gets to work building the transmitter, wrapping things up in a little less than an hour.

Once he finished soldering everything together, he takes a few moments to test out the bug and to explain how various parts of the board work together in order to transmit the FM signal. He mentions that adding a dipole antenna would make it easy to extend the range of the transmitter, and briefly teases next week’s episode, where he plans on constructing a similar dual-stage transmitter.

This sort of FM circuit is one of the first few simple projects you would see in a beginner’s electronics class, so if you know anyone that is just starting to get their feet wet, be sure to pass this Hack a Week episode along.

Continue reading to see [Dino] explain the ins and outs of his FM bug transmitter.


49 thoughts on “Building A Simple FM Transmitter Bug

  1. Dino’s work is excellent and I’d subscribe to his blog if it weren’t for the fact that every single hack gets reposted here!

    Not that I’m complaining, but are HaD still looking for an in-house writer? Pay Dino to keep these quality articles coming.

  2. @stormdog – The references I found for Hartley oscillators showed two inductors or a single tapped inductor. This looks like a simple LC oscillator rather than a Hartley.

  3. not a hack… basic electronics from a kit. if he made it do something it wasn’t meant to do that would be a hack. Even if he made an intellegent choice of changed capacitance and inductance values so as to transmit at a different frequency, but just doing some through hole sodering is not a hack

  4. I’ve been doing electronics for 25 years.. and always wanted to build one of these. I’ll have to go see if I can fine a VOX (voice activated) version.. and that dual stage with higher voltage one sounds cool too! :)

    Thanks for the cross post Mike!


  5. It is not a good start, this is an example of someone who feels compulsed to provide web content on a periodic basis, in a feeble attempt to secure his position as a “knowletgebale” provider for his target audiance.

    There is nothing that can be learnt from the video, he casually list a few learning outcomes such that could be derived such as

    classic amplifiers
    fm transmission
    vhf antennas
    positive and negative feedback
    stray capacitance
    crystal locked oscillators
    signal attenuation

    However doesn’t cover any of these topics and likely doesn’t have the technical knowlege to present them in a coherent fashion.

    These are general topics of the study of radio electronics. The majority of these topics do not apply at all to the project at hand. However, by stateing them he gives creadence to the sudo scientifc study that the “hackers” are clinging to and as such places himself as a dominate figure by being a provider of content to his target audiance.

    This is drival, if you were to be really interested in these topics the appropriate action would be to study them, either formally at an institution of higher learning, or informally via the many community opertunities such as ham radio

    This shows the lazyness of Had, that the feed of information from a known source is presented without first taking a critical look at the content for it’s applicable merit to target audiance.

  6. I made this exact kit over a decade ago. It was lousy then, and it’s lousy now. The thing barely worked, and was nearly impossible to tune into.

    Just once I’d like to see someone design a very simple paired radio for simple digital transmissions. Everything out there is either hobbyist quality junk like this, or expensive modules.

  7. @doug That would be spelled “drivel” you dunce, oh and also “knowledgeable” NOT “knowletgable”, can’t anyone spell online these days? In any case, who are you to presume that HAD cares what the heck you have to say? Stuff can be studiedly independently with a book, it doesn’t require any formal or community anything. Many of those topics are far too advanced for most people to care. The idea that the average person can build a radio transmitter out of readily available components is much more interesting. “true hack”? What kind of B.S. is that? You don’t get to define things. In any case, hack in the dictionary doesn’t mean directly anything of the sort that you are referring to.

  8. I apologize Harton if I have offended you by having an opinion, but it would seem to me that the purpose of the commenting system implemented by Had is in place to facilitate the open discussion of opinions related to the articles.

    In response to your varied forms of self-studying, I feel you misinterpreted my intentions. I was saying that self studying is a excellent form of education and there are a variety of methods to go about that, by no means did I mean to imply that my list was exhaustive and I will agree that “books” are a good way to study. The point I was making is that the form of idealism that the original content provider created by saying that buying and building the Kit, that he just demonstrated will teach you this material, is a falsehood.

    That people who are serious or even non serious have a multitude of other viable options. The point was that the claim made in the video demonstration is not dis-akin to saying if you own a toaster you are practically a mechanical engineer, since the device uses concepts related to thermodynamics, statics, mechanics, and dynamics of material, structural analysis and proper design rules.

    Additionally I never referred to the ideological matter of a “true hack”. I do feel fairly confident that this would fail any definition of the word hack. As for immense technical accomplishment that has provided us the technology for a average person to be able to create a basic radio devices. This hack misses the mark by a long shot, for an interesting personal educational experience spend some time researching foxhole radios .

    My understanding of the origin of the word hack comes from the M.I.T. artificial intelligence laboratory where the students were said to be spending copious amounts of time hacking away at their computer terminals trying to make the computers do things that they weren’t programmed to do. And later similar efforts were likewise called hacks.

    This demonstration of a simple simplex radio system shows no likeness to the MIT example.

    Drival is absolutely a colloquial terms and I will stand by my right to spell it how appropriate within my discourse community.

    The true strength of the English language is its ability to change with the current methods of communicating, (citation is available but not provided.). Therefor it is my responsibility, as it is everyone else’s who speaks English, to be constantly redefining the language to best match current rigours of society.

    “knowletgebale” (sic) was a typo I apologize for any distress it caused you

  9. Used to build these when I was 12 (>30 years ago), with RF transistors desoldered from TV sets….
    Problem with this sort of design is:

    1. it is coupled to the antenna, banging the antenna of movement causes the sound to be transmitted, or the whole thing to generate a frequency sweep of a large part of the band it is operating in.
    2. The harmonics of this design are horrendous, and in some cases in excess of the Freq. of the oscillator transistor
    Really you need to be carful with this, since it can and does inter fear with the emergency band
    3. Poor temperature stability.
    4. Massive increase of range , by changing the transistor, voltage, range in excess of 6 miles is easily possible, and with it the swamping of communications in nearby bands.
    5. poor bandwidth/ modulation control if the amp section starts clipping

  10. I am an electronic novice and I agree with most of the comments that the video/link posted is probably too low quality to warrant the name “hack”. I have assembled many kits in my time and I don’t feel any wiser for it. It’s easy to follow directions to assemble a complex piece of electronics equipment. It’s a much hard thing to understand what you’re assembling.

    The only reason I watched the video was for an explanation of how the FM transmitter worked and I found that to be too minimal to be of any use. What I would have liked to see is a more in depth description of how a Hartley oscillator worked and a segmentation of the circuit in terms of function and a description of each. For example, are most of the other capacitors in the circuit there to change DC to AC? Why 22pF for the first capacitor? Is the capacitor tied to the base of the second transistor there to shunt high frequency noise to ground? etc. etc. These things may be obvious to veterans, but an explanation to newbies, such as myself, would be invaluable.

    Speaking of which, does anyone know of a good in depth analysis for a simple FM transmitter circuit? Perhaps a blog or paper? (and if so, maybe that would make a good HaD article/post?)

  11. @Rachel no matter what your desires or need in a data radio are, like it or not, the problem is having spectrum where it’s legal to operate such radios in. Hacking WiFi dongles might be the most obvious approach as they are unlicensed consumer devices.

  12. i don’t like to be negative, but i have to agree with some of the points that @doug has made (and i couldn’t care less about a couple of spelling mistakes, this isn’t a bloody spelling bee for christs sake!).

    there are many levels of knowledge that people have in various aspects of technology, so educational material at every level is welcome, but i think this video only contains the barest of educational value at any level. it is simply a guy soldering a kit!

    what is slightly annoying is that this guy seems to be positioning himself as some kind of ‘guru’. there seems to be a (very welcome) explosion of people offering free educational material on the web at the moment, much of which contains very valuable material. this doesn’t.

    its laughable that he previews the next week’s episode: soldering another kit!??!?

  13. The definition of a hack is held by the person, yes very basic, I see something I could dig up enough parts to do with the children and have fun. Blasting it as “not a hack” is subjective, and not productive. Where are your hacks? Blasting “not a hack” posts is a waste of time also, just people looking for someone to argue with… and you took the bait, as did I.

  14. @Sebastian Simulators aren’t as versatile as they appear to be. There might be a problem in the modeling of transistor or in the way the simulator chooses time unit for the simulation. The later might be adjusted, depending on the simulator used.

  15. @doug

    Thank you for pointing out that this is not a hack. Your knowledge of syntax is uncanny.

    Where’s all your projects with detailed explanation of theory and such? I’d love too see them as I’m sure we’ll all be humbled by the glory of your all encompassing knowledge of all things.


  16. @twopartepoxy

    I’m no EE… I’m self taught and still learning and I’ll be the first to admit that. I learn something from every PROJECT I build.

    Any perception that I am trying to “position” myself as a “guru” is purely the interpretation of the reader. My intent is to inspire people to make things.. not just electronics. I feel I have done so many time as I frequently receive email stating this.

    Why is it laughable that next weeks project is a kit? What’s funny about that? I do plan on modifying it with bigger transistors and more power in an attempt to get more range out of it as I stated in the video. This project was simply to show a basic FM transmitter kit to people that might not know they are available.

    You say you “hate to be negative” and yet you are. I find folks like you very odd… you preface a statement with a denial of the very thing you are doing, thereby giving you false license to be and ass.

    Now go build something, come back here and post it, and I’ll give you my opinion of what I think of it.

    Unless your too busy trolling…..

  17. Just a note, he didn’t buy it. A user sent it to him. He just wanted to put it together and give a brief demonstration of it. There was nothing more or nothing less stated from the video if you had really watched and paid attention to it. It wasn’t particularly interesting, but you’re bashing the guy for assumptions that he isn’t smart or just there to create content for the purpose of garnering attention. Step back and appreciate it for what it is, not for what you think it’s trying to be.

  18. @doug

    To have such critical opinions I would argue it is safe to assume you have some experience.

    Send us a link to a few of your projects so we can learn from your work… but first make sure to ask your parents for permission to post online.

  19. This was my first project when I started my electrical engineering degree. I really enjoyed it as everything else I had done up to that time was just LEDs and repairs. I liked that our professor made us calculate the proper values for the capacitors and resistors. The only things we were told was Vin, the transistor polarity, and we had a variable resistor for adjusting frequency. It really helped instill what each component does. The FM transmitter project is a great intro to electronics and EM spectrum.

  20. It’s nice that hack-a-day provides this comment and discussion area. I think that the comments and discussion are more useful when they are used to provide further information and links, and rather less so when they merely claim that the projects are trivial or stupid.

    It’s certainly true that these little bug transmitters aren’t really all that good. They’ve really only got one transistor in them, aren’t crystal controlled or phase locked, provide no real audio input processing/limiting. It’s not really a surprising (or useful) comment to bring that to anyone’s attention, especially with more verbiage than the entirety of the original presentation.

    What they are is a fun introduction to soldering and radio. If that doesn’t interest you, then you could surf on to the next hackaday story, it doesn’t really require any special commentary.

    In the interest of taking my own advice, here’s a link to homebrewing the same kind of transmitter, using just one transistor (no real need to get a kit with a pcb):

    It inspired me to build one:

    And my construction inspired at least one other cool followup, an SMT version:

  21. While it’s just only my opinion, and I’m not directing my comments to Dino in particular, because the world is full of persons who think they can really learn science by assembling something. I have my doubts that someone can learn what capacitance is, and what the function of every capacitor in a circuit is, by simply placing it where a schematic or the silkscreen on a PCB directs. Use any active or passive component as an example other than a capacitor nu suspicions would be the same. Heck one would even learn the difference between an active or a passive component.

    I wouldn’t expect every builder of electronic circuits to be an EE, as I never took my education that far. Thing is without without “book learnin” support by lab experiments, and really studying operational circuits they construct; on really doesn’t stand a chance of trouble shooting a circuit they built if it doesn’t work. The shade tree mechanic method of blindly trying fixes until something finally work is not trouble shooting.

  22. Of course just soldering a component in place won’t teach you about the component BY DEFAULT!!
    You have to have the desire to learn what it does. Assembling a kit might just spawn the desire to learn more and so lead to gained knowledge and the ability to learn how to troubleshoot.

    That being said, what’s your point?

  23. Expertise requires both skill and knowledge. One gains knowledge through study, skill through practice. Both are important: hacks require both. Any given story probably has more of one than the other. I dont see how that is a problem.

    But honestly Doug, if you feel strongly about this, would not a better course of action be to create your own project to your own specs, and link it here?

  24. For myself I stated the point I hoped to make in my first sentence. Addressing those claim that by simply building they can learn anything practical about electronic circuits. In the event they those they influence, lived off in their own world, I wouldn’t bother to comment to it all. Thing is they don’t, and their erroneous conclusions, cost others time, money at best endanger others at the worst. That situation, isn’t limited to electronics and crosses over into most every DIY area.

  25. The oscillator looks to be neither a Hartley or a Colpitts. I suspect the data sheet suggested looking them up for the builders unfamiliar with oscillators. Having a harm time discovering if this oscillator does have a common name.

    Another thing I’m having a hard time discovering is, what is a “half-wave FM transmitter” as that is what this is described as in the write up. I never heard or read the term before, but that doesn’t mean it isn’t valid so searching the web, and print references here I went, finding nothing. Unless the reference is to a half wave antenna? BTW; a dipole IS a half wave antenna.

  26. @D_
    I have the same kit, and built it out yesterday, then went through it and tried to figure out how it all worked. The best I could find on the oscillator was that it was just an LC Oscillator. Its fed by a feedback capacitor that is connected across the Emitter and Collector of the final transistor which is connected to the oscillator circuit. This varies the current on the Base Emitter junction at the resonant frequency. To keep the LC Oscillator going.

    The half wave part of the product description, I would believe to be that the whip antenna that comes with it, is half the wavelength of the 100mhz signal it is tuned to, so a 150cm piece of wire. The instructions list a quarter wave option, that uses coaxial cable in a dipole configuration.

    As for the educational value of kits. I’ve been frustrated a few times, as some kits are sold with the description, “learn about X and Y!” and in reality, if you are lucky they give you enough names, that you can look it up on the internet. Some false advertising on their parts. But I guess like anything, it is what you make of it, so if you want to know, you got to investigate it yourself and realize kits are good for getting you some practice building kits, and feeding you some concepts to further investigate on your own.

  27. If anyone is interested in what each of the components may be used for specifically, I’ve the same kit as dino and jrspruitt and am writing a series of posts about it starting with: to get to more recent ones.

    And it’s thanks to Dino and his tinkering on this that I decided to practice soldering and work on this project instead of procrastinating! Thanks Dino :).

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