Picture of FM Listening Bug
The on thing every spy needs is a small and well concealed listening device. The FM Listening Bug Kit gives you all you need to build a small, single Transistor, FM transmitter that can operate between 80 MHz and 150 MHz. Allowing you to tun it to an open frequency on an FM Radio and listen to what is going on in the room you leave that bug in. This bug has an effective range of about 100 meters. 

In this Instructable we will cover:
  • The Schematic and some basics of how it works
  • Parts needed
  • Getting the PCB layout onto a prototyping board (pegboard)
  • Laying out the components on the board
  • Tools needed to assemble the kit
  • Soldering everything together.

Step 1:

Picture of
FM Bug.jpg
As you can see in the schematic below this is a very simple circuit and will be an easy build. On thing the Schematic doesn't convey very well is where the Antenna actually needs to be placed. For best results you will want the Antenna soldered to the first turn on L1. The Antenna should be about 2 inches long 

Below is the schematic from our friends at Upverter.com.

Rfmhz7 months ago
It will work I just modified this .but there was a slight error on his schematic that e never showed there .any how nice work thought .you put your antenna in to centre piece hole and on the metal can place the outer earth wire of coax onto the metal can .that my friend is what I call earth is up and ground is what we stand on :-)
Rfmhz7 months ago
It will work I just modified this .but there was a slight error on his schematic that e never showed there .any how nice work thought .you put your antenna in to centre piece hole and on the metal can place the outer earth wire of coax onto the metal can .that my friend is what I call earth is up and ground is what we stand on :-)
roustabout11 months ago

Even if this circuit works, it won't transmit unless the antenna is allowed to pass through the Altoids box. The metal box is a perfect Faraday cage.

I may not be the smartest man but if the wire touches the can will it transmit then? The can is a conductor...

biraj10021 year ago
I flip BC547 transistor as U told that U mistake in the diagrams.But it could transmit.i check all the frequincy but alas. How I know that I made it right or its work. Tell me the soluition
mpilchfamily (author)  biraj10021 year ago

Sorry but this project just doesn't work. I do not have the tools needed to properly troubleshoot and figure out what is going on. When dealing with transmitters even the smallest thing like a bad soldering joint can cause a capacitance issue messing with the whole thing. It could also be my layout. A slightly longer trace then expected by the original designer could be affecting things as well. But it takes a good bit of specialized equipment to troubleshoot the problem with the design. Thousands of $$ worth of equipment.

ckgarside2 years ago
You cant order any less than 1000 variable capacitors, and I really don't wan't to spend $294. Are there any substitutes?
mpilchfamily (author)  ckgarside2 years ago
This has been asked in the comments before. At the time of making this instructable they had them available in smaller quantities. You can use any resistor of the same value you want. There are many to choose from and you may even find a cheaper deal.

Just so you know the transistor is displayed facing the wrong way on the layout. So make sure the flat side of the transistor is facing away from the battery. You may also want to make this on a perf board that isn't copper clad. Then let us know if it actually work after that.

I've gotten mine to transmit but can't tune it to any US FM station.
What resistor do you prefer for this application?
mpilchfamily (author)  ckgarside2 years ago
The ones in the list.Just do a search on the Mouser site to find another option for that same resistor value. Brand and tolerance are not that big of a deal in this circuit.
dont you mean capacitor not resistor ?? capacitor have Farads and resistor Ohms
FoamboardRC2 years ago
Wait, so this thing takes sounds and transmits it on a radio frequency? Then, how do you listen to the sound being transmitted?

Could I replace the microphone with a audio jack so that I can plug it into my iPod and transmit? Thanks.
mpilchfamily (author)  FoamboardRC2 years ago
If you can get it to work it should transmit to an FM station on the radio. Or so the original schematic i found claimed it could. So far i haven't been able to get it working. But you can find all sorts of single and dual transistor FM radio designs online to try.
Then what purpose does this device serve if you can't listen to what being transmitted?
how much will it cost to get the parts at radio shack in the us
mpilchfamily (author)  smiley G.I JOE2 years ago
Not all the carts can be found at Radio Shack. But of the parts you can get you'll sped about $20 and still need to buy other parts. For example the transistor BC547 isn't available at the Shack but they do carry the 2N3904 which can be used in its place. If you buy the part from a parts dealer like Mouser then you'll pay about $0.07 for the BC547 or 2N3905. At radio shack the part will cost you over $2. The Switch will cost you over $4 and thus the prices go when you buy them retail. You are paying for the extra packaging radio shack has on the parts and there 3% to 6% markup.

As of right now there seems to be a flaw in the circuit design and it doesn't quite work as it should. The bug does transmit but not on any FM frequency that we use here in the US. I believe i know the major problems behind it but don't have the funds needed to make another one to test my ideas.
ianmcmill2 years ago
i just put aside the solder iron, looked at my work. (first time soldering), read the last note and was like...Yeah Thank you very much !

Why are there non-functional walk-throughs here ?
mpilchfamily (author)  ianmcmill2 years ago
The Instructable was compiled as i went through the process of turning the schematic i found into a workable project. Unfortunately when all is said and done the schematic was a bust. But this does give you an opportunity to see what you can do to make it work.

I would hope people would read through the entire instructable before starting the project. I also figured that people would read through some of the comments as well to get an idea of how the projects have turned out for others. That not being the case i did edit the last page here with that note.

Maybe in the near future i'll find the time and money to start investigating the design further and see what i can do to make it work. As it stands now its a none functioning prototype. As so many prototypes tend to be.
Yes you are right of course. And as I am totally new to electronics I would greatly appreciate further investigation. An updated title saying "currently not working" or something similar would be nice. Anyways reading an Instructable completely before starting to recreate it is mandatory.
mpilchfamily (author)  ianmcmill2 years ago
Found a problem.

I have the transistor in backwards. I still need to test it and see if that is the main issue. I'll keep you informed.
oh this is awesome. looking forward to read your results. I don't know if it is important but I think I've read something about the coils. That the two ends should be on the same heigth. That means both end turns should be on on layer or on one even row of pins in the stripboard.
mpilchfamily (author)  ianmcmill2 years ago
The row of holes the inductor lines up with isn't effecting its operation. But i do think building this on a non copper clad perf board may be the solution. Just a matter of transferring the parts to the other board and making sure you have the flat side of the transistor facing away from the battery.
sciencetor23 years ago
Hey, i just completed my circuit, popped in the battery, and switched it on, then started working the dial on my fm radio all the way across, while having the mic next to my laptop running my itunes on shuffle. I worked my way all the way across the dial slowly, but didnt pick anything up. I then picked an empty station and in stead swept the capacitor, hoping to pick it up that way, with no luck. How can i tell if my bug is transmitting, and if it is, on what frequency? Is there a trick to it?(and no, unfortunately i do not have access to an oscilloscope)
mpilchfamily (author)  sciencetor23 years ago
You might want to try a larger battery but you won't know its working till you can hear it on the radio. I still haven't gotten mine to work right either. This came from a schematic i found online.
Does yours show up on the radio at all?
mpilchfamily (author)  sciencetor23 years ago
Like i said, i haven't gotten mine to work either. Do some searching and find yourself a good designs that uses 2 or more transistors. Like i've said in other comments below this is a design i found online. There was no assurance that it was a working design and i don't know enough about any of this to tell if it was or not or fix any design problems.
ok, fair enough, any chance you could link me to the original design online so i can check for any errors?
mpilchfamily (author)  sciencetor23 years ago
Its is among several transmitter designs. I chose it over a 2 transistor design for its possible stability and simplicity. I wanted a small transmitter that would fit in the Altoids smalls tin and could be entered into the Spy Challenge. Here is the site i found it one.

The one under the More stable heading is the schematic i used. You'll see further down there are several 2 transistor designs and the next page offers 3 transistor designs. Those will be much more stable and offer better ranges. 
the only deviation from the original that I can see is that you called for 20/22 AWG wire, while the original calls for .5mm, or roughly 24 AWG. Im not sure if this will make any difference, but i ordered a small amount of 24 AWG magnet wire from my favorite supplier to test just in case, since I got it cheap. one thing it says to watch is the placement of the variable capacitor in relation to the coil, which you appear to have done correctly... so in other words i have no idea. Try building their DIY bug field strength detector?
mpilchfamily (author)  sciencetor22 years ago
Found a problem.

I have the transistor in backwards. I still need to test it and see if that is the main issue. I'll keep you informed.
maboy3 years ago
sorry, I'm new to this sort of thing. So, could you in theory just use an fm radio to pick up on the signal? Or do you have to have a seperate bug pickup device?
mpilchfamily (author)  maboy3 years ago
It's suppose to transmit to a standard FM radio. Since no one else is able to get this design to work it seems its a bad design. I followed the schematic as i found it and transferred the schematic here and did the PCB layout based on the schematic. So as far as i can tell the original schematic never worked right.
mpilchfamily (author)  mpilchfamily2 years ago
Found a problem.

I have the transistor in backwards. I still need to test it and see if that is the main issue. I'll keep you informed.
Akolpacke3 years ago
Very thorough and well documented/explained build, great job… I’m anxious to try my hand at putting this together. Only thing that bothers me was your “Note” at the end of the instructable which stated that you had not yet been able to get your design to actually transmit to an FM radio. Did drilling a hole in the case and allowing the atenna to be outside the case solve the transmit problem??? I’m looking to make sure everything works before I sink any time into building one for myself.
mpilchfamily (author)  Akolpacke3 years ago
It doesn't work in or out of the case. And it seems others can't get it working either so i have to assume its a bad design in general. But you can check the site i found it on and try one of the 2 transistor transmitters which will be more powerful and stable.
mpilchfamily (author)  mpilchfamily2 years ago
Found a problem.

I have the transistor in backwards. I still need to test it and see if that is the main issue. I'll keep you informed.
vipin7863 years ago
May be this will be helpful http://sound.westhost.com/project74.htm
mpilchfamily (author)  vipin7863 years ago
Excellent find! Just need to order the diode and see if this design is total crap or if its just me. LOL
This things works perfectly i used it to test my fm transmitter.
did you ever build the tester?
Yea I built this long time ago it worked , you need to connect your probes to analog multimeter never tried with digital one.
This thing will only show if your lc tank is oscillating or not & will not be able to show frequency at which it oscillates.
mpilchfamily (author)  vipin7862 years ago
Found a problem.

I have the transistor in backwards. I still need to test it and see if that is the main issue. I'll keep you informed.
i clicked on the link for the trimmer capacitor and it says i have to buy 1000 of them minimum... is that what you did or is there a cheaper place to buy this capacitor... cause for 1000 of those the price sky rockets to around $200
mpilchfamily (author)  Guccisgizmos2 years ago
That's been a recent change. I was able to order just 1 of them at the time i built this. Be sure to read all the comments before you start this build. There is a problem with the schematic/parts list i used and the finished product doesn't work. Why? We don't know. Don't have enough knowledge and experience in radios to be able to figure that out.
mpilchfamily (author)  mpilchfamily2 years ago
Found a problem.

I have the transistor in backwards. I still need to test it and see if that is the main issue. I'll keep you informed.
The FCC is going to get you for this! (You have to have a special license to broadcast AM, FM, PM, NTSC, PAL, FRS, GMRS, CB, HAM, or any other standardized TV or radio protocol at a distance over 200 feet.)
FRS (,GMRS?) and CB are free to use under certain rules. For FRS 0,5W is the max, which Walkie-Talkies use. And that is quite over 200 feet. Same for PMR446 in the EU (FRS is not legal here, and PMR446 is not legal without license in North America, which cause European tourist to get in trouble when they use their Walkie talkies).
mpilchfamily (author)  VirtualBoxer3 years ago
First of all posting plans for building a transmitter doesn't violate any regulations. Building the transmitter doesn't violate any regulations. Using a transmitter that is over a specified power rating is against FCC regulations. You can broadcast on AM or FM without a license as long as your transmitter is not transmitting over a certain wattage. FCC regulations (AFAIK) doesn't specify a distance only a wattage. Though the distance a transmitter transmits is directly related to its power output. Actually AFAIK AM is completely open for use without a license.

But all of this is a mute point anyway since this design doesn't even work.
also, i swapped out the coil with one built exactly to the original design, no luck
mm4rs03 years ago
Wait, take a closer look at your perf board. You used copper pads. try one without pads. The pads act as small caps.
njrajgelani3 years ago
can i use ceramaic 1.0 uF capacitor instead of 1.0 uF alumenium electrolite capacitor and
instead of 2 pin variable capacitor, 3 pin capacitor
if yes, how???
please answer
mpilchfamily (author)  njrajgelani3 years ago
No on the electrolytic swap. Yes on the 3 pin variable. Yes use 2 of the pin not all 3. Use the Center and one of the outer pins. Maybe your variable capacitor will make this design work. Cause so far it seems no one can get it working.
I'm begainer and fialed in makeing dark detactor for 3 times!!!!!
if this will not work, i'll resign from elctronics.
anyways, thx for answering.

sorry foe spelling mistecks.
mpilchfamily (author)  njrajgelani3 years ago
This is not a project for a beginner. Try something simpler. Something that you can get a good understanding of how it works.
Smtty093 years ago
So do we know yet if this works for sure?
smilgy3 years ago
Is the spst switch just a simple on/off? And can you use a BC547B instead of a BC547? And the last question, what is the .022uF capacitor in pF ? as in on this website http://www.bitsbox.co.uk/cercaps.html
mpilchfamily (author)  smilgy3 years ago
The switch is just to turn it on and off. I left it out in the above build since it makes it easier to put it into the tin and removing the battery is a simple way to turn it off.

The transistor i used in this build is the BC547C which may be part of my problem with getting it tuned. The B type has a lower DC gain then the C so the B may be better.

Are you having trouble sourcing a .022uF capacitor? The original schematic calls for a 22nF which is .022uF which is 22000pF. Its a very small capacitor.
ivanjacob3 years ago
is it possible to change the mic to a input jack??
mpilchfamily (author)  ivanjacob3 years ago
mrmerino3 years ago
If the antenna's inside the tin, won't it be shielded by the tin if you close it? Or do you leave it open? Very neat though, I'll have to try it!
mpilchfamily (author)  mrmerino3 years ago
Drill a small hole in the side to allow the antenna to be out. Yes the tin can shield the antenna and prevent anything from being transmitted.
sciencetor23 years ago
hey this is a great and easy to follow instructable and the parts for my build just came in, but i was wondering, what kind of modifications would have to be made to run this off say, a nine volt battery? i only ask because that would provide for an extremely extended battery life
mpilchfamily (author)  sciencetor23 years ago
Running it at 9V shouldn't be an issue. It will actually increase the transmitter's range. But running it off a pair of AA will give you longer run times then a 9V battery.
Cool! But is there anything that needs to be modified for either of those configurations(resistor values, capacitor max voltages etc.)?
mpilchfamily (author)  sciencetor23 years ago
All components are more then capable of handling 9V. The design is probably better suited for 9V to start with.
awesome! thanks for the input!
also, i have a challenge for you, can you make a tuneable fm transmitter with a standard headphones AUX input that runs on its own power and fits in an altoids can, and can broadcast music from an ipod/mp3 player input, an can be made cheaper than the $40 storebought ones that only run on a car outlet? then make an instructable that is as clear and awesome as this one? that would be pretty darn awesome!
mpilchfamily (author)  sciencetor23 years ago
I'm no radio designer. I found this schematic online. The microphone can easily be replaced by a 1/8" jack. But you will want to find a 2 transistor design and you will need to use all SMD to keep it small enough to stay inside an Altoids tin. The hard part is getting everything laid out on the PCB in a user friendly arrangement.

Keeping it under $40 may be difficult. While your are paying a markup from the manufacture and the retail outlet it may still be cheaper to buy the retail version. Since they get large discounts on parts due to the volume they deal in. Take the microphone for example. one of them cost you $1.29. But if you buy 1000 of them you only pay $0.35 per unit. Drastically reducing the overall cost to manufacture the item.
DoctorDv3 years ago
Great Instructable and congrats on winning the challenge! Would it be possible that you put this and a FM radio in an altoids tin so if you made two, you would basically have walkie talkies? Thanks!
mpilchfamily (author)  DoctorDv3 years ago
Its possible but i wouldn't use this design if you want a decent transmitter. Google for a 2 transistor FM transmitter. They will be more stable and much easier to tune. But they are a bit more complex.
Thestoog3 years ago
I know i sound either idiotic or dumb by asking this but what exactly does it do does it record what is happening in the room or what exactly?
mpilchfamily (author)  Thestoog3 years ago
Its a FM microphone. You hid this in a room and it transmits the audio so you can listen to it on an FM radio.
Thank you :) and is it fairly simple to find out what's its frequency is?
mpilchfamily (author)  Thestoog3 years ago
This particular design can be a bit tricky to dial in. There are much better deigns out there that use 2 or more transistors making for a more powerful transmitter and they are easier to tune.
Hmm i always enjoyed a challenge, yours is still great since it has a very small and practical use, hope you win the spy contest :D and i will try to find some of these designs.
madhops06203 years ago
The instructable doesn't say anything about tuning the bug to the right frequency. I'm assuming this is done by adjusting the screw on the variable capacitor; is that correct?

Other than that, great 'ble. Very well explained, and the pictures are very helpful. Great job! You've got my vote.
mpilchfamily (author)  madhops06203 years ago
Yes the screw in the variable capacitor allows you to tune the transmitter. You have to turn it very slowly. It doesn't take much to send the signal across the dial.