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.
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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.

Step 2: Parts List

Picture of Parts List
Note: A full parts list is attached to this page. Listing the part, manufacture, part number, link to Mouser.com, quantity and price (as of the publishing of this Instructable).

Parts List:
  • 1x 22 k Ohm Resistor $0.07
  • 1x 330 Ohm Resistor $0.06
  • 1x 1000 pF Ceramic Capacitor $0.06
  • 1x Omnidirectional Microphone $1.29
  • 1x 1.0 uF  Aluminum Electrolytic Capacitor $0.32
  • 350mm (1 foot) of 20/22 AWG Sold copper or Magnetic Wire
  • 1x BC547 Bipolar NPN Transistor $0.07
  • 1x Coin Cell Battery Holder $0.60
  • 1x .022uF Ceramic Disc Capacitor $0.16
  • 1x 47 K Ohm Resistor $0.06
  • 1x 10 pF Ceramic Capacitor $0.08
  • 1x SPST Slide Switch $0.37
  • 1x Variable Capacitor 9.8 pF to 50 pF $0.48
  • 1x Dual mini breadboard $2.19
  • Altoids Smalls Tin
  • CR2032 Cell Battery
Total cost in parts is less then $7 before shipping. Assuming you already have the wire and Altoids Smalls Tin on hand. 
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Rfmhz6 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 :-)
Rfmhz6 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 :-)
roustabout10 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?
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