Introduction: FM Listening Bug

About: I'm a jack of all trades and a master of none. I like to tweak, mod and improvise whenever possible!
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:

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

Step 2: Parts List

Note: A full parts list is attached to this page. Listing the part, manufacture, part number, link to, 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. 

Step 3: Pegboard Layout

When creating a PCB layout from the schematic I kept several elements in mind.
  1. PCBs for the layout won't be available for some time so users need to be able to make this themselves on a Peg board
  2. Becuase a Pegboard layout is needed the traces had to be routed on a single side.
  3. The whole project is going to be mounted in an Altoids Tin to conceal it
With all that in mind i've included several files so you can either lay this out on a Peg Board or etch your own copper clad board. Attached is a zip file containing the PDF and Gerber files needed to produce your own PCB. There is a separate PDF file of the layout without the Ground plain filled in. I've also included the Fritzing and Eagle CAD files if you want to play around with the arraignment yourself.

If your going the Peg board route then print the PDF file out and cut it to size. Overlay it onto the Peg Board and hold it up to the light to get the component holes lined up with the Peg Board's grid. Then tape the layout in place. Be sure the copper grid is on the bottom of the peg board and the layout is taped to the top. Go ahead and take a pointed object like the lead from one of the parts and poke out all the component holes.

Step 4: Tools

Before we start piecing this together lets gather the tools we'll need for this.

  • Soldering iron
  • Solder
  • Needle nose pliers
  • Wire cutters
  • Wire strippers
  • Helping hand/PCB clamp
  • Small flat head screwdriver or Probe
  • Solder braid/pump/bulb
  • Magnifying glass
  • Electrical Tape
  • Small Phillips Head Screwdriver
  • FM Radio
Most of these tools are self explanatory. Of course you need a soldering iron and solder to put the kit together. You'll need a small flat head screwdriver or probe to help break any unwanted solder bridges. You can very easily bridge contacts unintentionally while soldering the traces. especially when you have traces that will be very close together like the ones around the Voltage regulator. The magnifying glass will come in real handy inspecting your work and ensuring there are no unwanted solder bridges.  

Now to start putting everything together.

Step 5: Sizing the Pegboard

If you plan to place your bug into an Altoids Smalls tin you will want to trim the board down. Trimming it to the same size as the layout sheet is all you need. You can use a pair of wire cutters to trim off pieces of the board. Or you can use a knife to score the board where you want to break it and use a pair of pliers to break the board. Scoring along one of the rows of holes will make it easier to break. I recommend the scoring and snapping method.

I used a ruler to line up where i wanted to score the board. Then i took a box cutter and Scored along the edge of the ruler a couple of times. Then i took my pliers and snapped the edge off. Use a file or 100 grit sand paper to clean up the edges. Then drop it into the tin to see how it fits. 

Step 6: Placing the Resistors

We will start by placing the resistors on the board. R1 is the 22 K resistor, R2 is the 47 K Resistor, and R3 is the 330 ohm Resistor. All 3 resistor need to be placed stand on end. So take one of the leads and bend it over 180 degrees as shown in the picture above. Place the resistors in there designated spot referring to the reference diagram above as needed.

With the resistors in place, bend the leads out so they will not fall out when you turn the board over to solder them into place. When working with pegboard like this i like to use the leads of the components to link the traces from one component to another. So bending you leads in the direction of the nearest component they will be linked to is a good idea.

With the resistors in place flip the board over and solder them in place. Don't worry about soldering the ends of the leads down as traces until we get all the components in place. But go ahead and use your pliers to bend them around to where they need to go and cut off any access so its out of the way.

Step 7: Placing the Capacitors

Now we will place the capacitors. Pay attention to the polarity of the Electrolytic capacitor (C1). The silver stripe down the side of the can is the negative side, (its also the side with the shorter lead) and needs to be on the left hand side if your keeping your board oriented in the same way the reference image is. You will need to fully straighten out the leads to fit in the board correctly.

As for the Ceramic capacitors make sure you have the correct ones going into the correct spots. The Capacitors will be labeled on there bags but not on the components themselves. So double check the capacitor and where it goes on the board.
  • C2 - 1000 pF
  • C3 - 10 pF
  • C4 - .022 uF
  • C5- Variable Capacitor
The variable capacitor is the component with the little Phillips head screw in the top of it. The leads of C5 won't be long enough to bend out. But friction should hold it in place for you while you solder the components. The Leads of capacitors C2 will need to be straightened like the Electrolytic capacitor. As you can see C4 is a fairly large capacitor. Feel free to insert the leads on either side of the indicated marks. Also the leads of C5 are a bit wider then indicated on the layout but it will fit. 

Like before bend the lead out a bit in the direction they need to go. Flip the board over and solder the leads into place. Then route the leads and trim as needed.

Step 8: Transistor and Mic

Now we will place the transistor, mic and switch.

Make sure you have the transistor lined up correctly. As you can see in the reference image the flat side of the transistor needs to be facing to the right. This puts the Emitter on the bottom, Base in the middle and Collector on the top. Bend the leads out a bit and move on to the next component.

Feel free to leave the switch off if you want. It will make fitting it into the Atoids Smalls tin a bit easier. I've left off the switch here so we'll need to bridge the solder trace area where the switch would be. If you decide to use the switch it will need it's leads bent in ever so slightly so they will fit into the board. The leads of the switch won't be long enough to bend towards other components. But friction will hold it in place while you solder it.

The mic will drop into place without any issue. Bend the leads as needed and your ready to solder everything in place.

Step 9: Making Your Inductor and Antenna

Now we need to create the inductor for the circuit (L1). For this you will need a 220mm length of 20/22 AWG sold wire and a 6mm diameter object like a No 2 pencil. You need to use sold wire and not braided wire otherwise the coil won't hold its shape very well. If you use Magnetic wire that would be best. If you are using standard wire strip the first 40 mm of insulation off one end. Then you will make about 8 turns around the pencil. Keep the turns loose so the turns don't touch each other, unless your using magnetic wire then you don't have to worry about it. Leave about 5 or 6 mm at either end of the coil so you can solder it to the board. Put the Inductor in place and solder it in place.

Now take a 50mm piece of the wire and solder one end of it to the 1st turn on L1 near the top of the board. This will be your transmitter's antenna. If you are using magnetic wire then be sure to scrape the coating off the end of the Antenna wire and the turn you'll be soldering it to before soldering it in place. If you are using regular wire then strip the end. Wrap the end into a little hook so you can hook it around the first turn and solder it into place. 

Step 10: Battery Clip

Finally we get to the battery clip. In the area of the GND pad on the reference image we need to create a ground pad for the battery. To do this we will use a couple of leftover bits of wire about 10mm long. Strip the insulation off the wire and create 2 U shaped pieces about 5mm wide in the middle and place them running from top to bottom and side by side in the GND pad area and solder them into place.

Now take the battery clip itself and place it on the board so the battery can be inserted into the right hand side of the board. Notice the 2 tabs in the back of the clip. The need to be towards the middle of the board. When placing the clip it will be a tight fit between it and the coil. Solder the clip in place and we are ready to make the traces.

Step 11: Soldering the Traces

Now starting with one component at a time start bending the leads the rest of the way over to make contact with the components they need to link too. Where you don't have leads to help make your trace start creating solder bridges across the copper pads to the component. It may be a good idea to take some scrap wire and strip it bear to use as a trace from one component to the next. If you don't like the look of solder bridges on a breadboard then use bits of wire to go directly from 1 component to the next.

Take your time and do one trace at a time. Like they always say "measure twice and cut once". In this case check and verify the trace placement twice and solder once.

Step 12: Wrapping It Up

So there you have it an FM Listening Bug. If you plan to place the bug into an Altoids Smalls tin you will want to cover the bottom of the board with electrical tape to prevent any shorts. It would also be a good idea to cover the inside of the tin's lid so nothing shorts out there. 

If you decided to use the switch you will need to measure out where the switch will stick out so you can cut an opening for it in the tin. A 1/4 inch drill bit should be sufficient for making a large enough hole for the switch. For best reception you will want the antenna to be on the outside of the tin so feel free to drill a small hole to allow the antenna to stick out.

We only want the board to sit in the Altoids tin. It shouldn't be permanently mounted since you will have to remove the board in order to change the battery. Of course the board could have been made smaller but then it would be difficult to put together as a kit.

So there you have it a nice little FM Listening Bug you can use to spy on your friends. Pair this with my Electronic Bug Detector Kit and you have a fun little electronic hide and seek game. Create multiple bugs and hide them all over the place. Or you can just use the bug as an FM wireless mic and have fun playing Radio DJ.

NOTE: To date i haven't been able to get this design to actually transmit to an FM radio. I found the original Schematic at this web site and assumed it was a functioning design. I used the More Stable design for this project. Maybe the BC547 transistor needs to be replaced with either a 2N3904 or a 2N2222. The Coil may also be too big as well, but i don't have the available time to troubleshoot the project any further.

UPDATE (10/5/2012): I found a problem in the board layout. The transistor is pictured on the layout backwards. I'll be working on a new prototype soon and make sure everything actually works before updating the instructable. I'll keep you all posted.

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