Tiny FM Transmitter Bugs Rooms

Lucid Science delves into spy-tech once again with this tiny FM transmitter. Their post demonstrates a bit larger version than seen above, using a 9-volt battery and protoboard sized to match which makes for easier soldering. The design uses a microphone, two transistors, enameled wire for the coil, as well as various resistors, capacitors, and a potentiometer. What you end up with is an amazingly clear audio signal that can be picked up with a normal FM radio.

This would make a great project to do with the kids. You can talk about circuit design, practice soldering, and when finished they’ve got an almost miraculous toy to play with. Just be careful what you say around the house, the room might be bugged!

30 thoughts on “Tiny FM Transmitter Bugs Rooms

  1. Thanks once again for linking to one of my projects. The goal here is to show a step by step build process of the basic transmitter to the electronics beginner. The transmitter does work well, but it does have limitations.


  2. I was soldering similar circuit 15 years ago.
    This is realy nice circuit. Only issue is that it changes freq when battery power is dropping.
    It will be interesting to do the same with smd components, smaller microphone and may be quartz. I bet it can be size of microphone you have used.

    This could be nice contest to build fm transmitter from smd elements found on 3.5′ floppy drive.

  3. @Richardf
    dont worry … transmitting devices occur naturally and if you can change the batteries on them every 9 weeks it would be a big help :3
    feel free to read your personal information aloud and use the NATO phonetic alphabet :3

    you can hide these things anywhere! … but FM might not be the best band to use seeing that most people have a receiver for it XD

  4. Chajtek: Assuming it wouldn’t take up tooo much room on the board, you could always use a joule thief type circuit to keep the voltage semi-stable until the very end of the battery’s life.

  5. Reminds me of the ones Radio Shack used to sell, only this one is probably a bit better, you know…because of the love.

    The Shmack ones I used to bump up the power a little and put larger batteries on ’em and simply drop em in the grass in front of my house at night.

    One on either side of my property, you guessed it, each tuned a bit apart on the band and tuned in on separate radios spaced apart in my room for a decently realistic stereo image of cars and pedestrians passing by.

    Good times!

  6. smd versions of this are pretty simple, but it’s easier to go up in frequency with SMT parts. In general the L and C components are larger at FM frequencies. These are (were) fun, but it’s even more fun to use them as beacons to be located with directional antennas. Just replace the microphone with an oscillator of any kind and put it in your car. You can track it with a small radio.

  7. The transmitter doesn’t have to be that stable because of FM’s capture effect. It can drift quite a bit before it causes problems. I’d guess it could drift a few 100khz either way with no problems.

  8. There was a really wonderful series of books I read as a teenager, the best of which was called “14 FM Bugs to Build” by Colin Mitchell.

    Anyone interested in small fm transmitters / bugs should have a look at it.


    My favourite was the 9v battery top voyager (with the first smd I ever soldered):

    And the Amoeba because it fit so perfectly in a tic tac box with it’s 2 AAA batteries.

  9. @RadBrad : Hey long time no see! -great hearing from you!

    Yeah “Probe II SG” is in a closet in “mothballs”.
    I need to bring it out and look into modernizing it!

    Hope you and yours are well! :D

  10. I made a lot of stuff like this many years ago, but it’s the first time I see someone laying components on the schematic as in the second picture at http://www.lucidscience.com/pro-basic%20spy%20transmitter-5.aspx
    I absolutely love this way of letting newbies familiarize with parts. Bravo!
    The frequency drift can be reduced a lot by putting the RF circuit behind a screen, using an additional RF buffer stage, regulating the power supply and compensating for temperature by choosing the right parts. But that would make the circuit much bigger and harder to build by a newbie.
    About 20 years ago I managed to build a FM transmitter using a CB xtal on the main oscillator then tuning the RF amp to the 4th harmonic (27MHz*4=108MHz). You can’t get more than a few KHz of deviation when modulating a xtal on the fundamental freq, but its 4th harmonic had enough juice to be heard loud and clear from a commercial FM receiver. Definitely not HiFi though it was more than enough for bugging.

  11. Back in my day you either built it with wood, or you built it with stone!

    .o(sorry had to do it xD)

    People who were doing this stuff x years ago are a dime a dozen..no offense. Its not even intermediate HAM level.

  12. ah, the joys of growing old.

    I really like to see people doing this stuff and documenting it. I ended up with a pretty state of the art lab, but I started by burning myself with a soldering iron like everyone else.

    I remember running into old guys with silent machine shops in their garages and wondering why the hell they wouldn’t be making stuff with it – they knew how to do anything. CNC gear, full on machining centers, wood working tools and near production level chemistry labs. All gathering dust, and generally with an airplane around somewhere.

    And now my stuff sits around and gathers dust and loses calibration. A few years ago, I gave a kid a 5 digit meter and a 100 mhz scope because he showed me a little data logger cobbled up from a breadboard using a pic chip. He was 14.

    But I could see he ahd the fire.

    My imagination ALWAYS outstripped my skill level, but I guess that’s the way it should be; As soon as I had working neon blinkers I wanted to build a computer; After building a couple of computers I wanted to work on AI; after I could do SMT I wanted to ship masks to be fabbed; and so on through a whole range of technologies. It’s a great life for a hacker, even if my brain could use a couple of defrag passes and a bit of cleanup.

    But even putting serial numbers on white blood cells doesn’t come close to beating the experience of connecting all those parts up based on faith that the schematic meant something, and then hearing my own transmitter on the radio.

  13. Well it works beautifully! Made myself one last night. Couldn’t find a single .22 uF cap in all the boxes of broken electronics I have, so I had to solder 2 #104 caps together. Awesome little device.

  14. J.D. hopefully not all readers on this forum are from US. Do you have blue-tooth headsets in US? It looks like they are against this regulation.

    There should be big distinction between doing thinks for fun with short range. In Europe as long as you are not making troubles it is OK. When you will start making them someone will knock your doors.

  15. @JD
    This secret microphone crap is basically kid stuff. Anyone trying to use this stuff for real is messed up; You know, unless it’s the government doing it.

    Wiretap laws are like sodomy laws – people that are gonna do it are gonna keep on doing it, and once in a while there will be an example made, but otherwise we’ve all long since forgotten what privacy was there for in the first place.

    But in the meantime, the 350 people on the planet with soldering irons who want to play with this stuff are unlikely to use it for more than a day or two. One or two guys or girls will be nuts enough to use it for stalking or spying on auditors, and they’ll get smacked for it.

    But as the Stazi learned, most people are really, really boring. Even dissidents. Building a transmitter is fun, but apart from lonely old guys with scanners, we don’t generally care about other people – half the time, I can’t be bothered to listen to people when they’re standing next to me, let alone want to listen to them when they’re away.

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