FM Bug Detector Kit




Introduction: FM Bug Detector Kit

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

This is for all the spies/paranoid people out there. An essential piece of equipment for any spy is an Electronic Bug Detector. You never know what government agency may be trying to listen in on your conversations or trying to secretly video tape you.

The Electronic Bug Detector is basically a broad-band receiver. It picks up frequencies on the FM band between 80Mhz and 150MHz. If a bug with a sensitive microphone is transmitting nearby, the result will be a feedback whistle on the detector. The frequency range of this device is limited and well under what any real surveillance equipment uses. But it is great for detecting small DIY FM transmitters such as my FM Listening Bug.

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: Schematic

The circuit is basically a broad-band receiver and will pick up all types of transmissions within the aforementioned range. The quiescent current for the circuit is less than 2mA and the output is a Piezo Buzzer.

Note: The circuit will not drive a speaker.

Each stage provides high gain and the signal is picked up by an untuned front end. You can see the front end is untuned as the inductor does not have a capacitor across it. You would think the 20 turn inductor would create a short-circuit to ground. But signals at 100MHz will oscillate in the inductor and can be picked off via the 470p capacitor. Signals of a low frequency will be taken to ground. The 100MHz signal is amplified by the first two stages and the audio component is detected by the diodes on the base of the third transistor. The fourth transistor is an audio amplifier to drive the piezo diaphragm. The inductor across the piezo provides a load for the transistor and creates a high voltage during part of each cycle to increase the volume. The .022uF capacitor removes the high frequency component of the signal. The 47uF across the power supply improves performance of the circuit by preventing signals from the output stage passing to the front end. All of this will run of a small CR2032 3V button battery.

Here is the Schematic from Upverter:

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 SPST Slide Switch $1.37
  • 4x .022uF Ceramic Disc Capacitor $0.16
  • 1x Piezo Speaker $1.30
  • 2x 1N4148 Diode $0.08
  • 3x BC547 Bipolar NPN Transistor $0.07
  • 1x 47uF Aluminum Electrolytic Capacitor $0.12
  • 2x 3.3 K Ohm Resistor $0.06
  • 1x 470 pF Capacitor $0.07
  • 1x Bipolar RF Transistor $0.27
  • 1x LED Circuit Board Indicator $0.38
  • 2x 1 M Ohm Resistor $0.96
  • 1x 220 K Ohm Resistor $0.06
  • 1x Coin Cell Battery Holder $0.60
  • 1x 47 K Ohm Resistor $0.06
  • 1x 32 Ohm Resistor $0.39
  • 1x 2.2 K Ohm Resistor $0.06
  • 1x 10uH Inductor $0.18
  • 495mm (1' 7.5") of 20/22 AWG Solid Wire or Magnetic Wire
  • 1x Grid-Style PC Board (peg board) $3.99
  • 1x Altoids Tin
  • CR2032 Cell Battery
Total cost in parts is just under $12 before shipping. Assuming you already have the wire and Altoids tin on hand.

Step 3: Peg Board 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 file if you want to play around with the arraignment yourself.

If you are going the Peg board route then print the PDF file out and cut it to size. Be sure to cut off the corners from the left side. Overlay it onto the center of 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 in 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.

All the part will fit into the board as they should except for the inductor on the far right side labeled L1. That will be your 20 turn 3mm diameter Coil you'll make from the 20/22 AWG wire. So where the 2 ends of the inductor end up will be determined by how tightly you wind it. We'll cover this more in a later step.

Before we move on you will want to trim the breadboard done to the size of the layout. This way it will be able to fit into the Altoids Tin later on. To do this take a straight edge like a ruler and line it up with the edge of your printout on the board. Using a box cutter score the line across the board several times and your pliers to snap the edge of the board off. Cut 2 sides then re-tape the layout in place and cut the other 2 sides. Once you've cut it down to size you can use a file or piece of 100 grit sand paper to clean up the edges. 

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
  • Hot glue gun and glue
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. The Hot Glue will be used to keep L1 in a tight coil and help secure it to the board. The glue will also be used to help mount the board into the Altoids Tin. 

Step 5: Capacitors

Now we can start piecing this thing together. Always keep your board oriented the same way as the reference image so we can ensure the components are being placed correctly. We will start with some of the smallest components first. In this case we'll start with the capacitors. Be sure to use the reference image above to help guide you in where to place each component.

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. You will need to fully straighten out the leads for it to fit in the board correctly.

Using the reference image above lets take a look at Capacitors C2 - C6. The capacitors themselves may not be clearly marked but there packages will be. The capacitors are as follows:
  • C2-C5 = .022 uF
  • C6 = 470 pF
Place the capacitors in there designated areas and 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 capacitors 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 6: Resistors and Inductor (L2)

We have a bunch of resistors here and all of them should be clearly marked with the resistor color code with the exception of R1 which has the value printed on it. Be sure to use the reference image as needed.

The Resistors go in the board as follows:
  • R1 - 32 Ohm Resistor
  • R2 - 47 K Resistor
  • R3 - 3.3 K Resistor
  • R4 - 220 K Resistor
  • R5 - 2.2 K Resistor
  • R6 - 3.3 K Resistor
  • R7 and R8 1 M Resistor
Place all the resistors and Inductor (L2) where they need to go and bend the leads out in the direction of a nearby component they need to connect to. Flip the board over and solder the leads in place. As before route and trim the leads as needed referring to the reference image.

Step 7: Transistors, LED, and Diodes

Next we want to place the Transistors and Diodes.

Make sure you orient the transistors correctly. Check the reference image and have the flat side of the transistors facing to the left of the board. This lines up the transistors so the Emitter is on top, the Base is in the middle and the Collector is on the bottom. Q4 is the C1674 transistor and the leads will need to be bent out to fit the board. The BC547 transistors go to Q1 through Q4. 

With the LED and Diodes its important to pay attention to which lead is the Anode and which is the Cathode. On the LED one lead is longer then the other indicating the Anode. So your short lead is the cathode and should be on the bottom side of the board. The Diode's cathode is indicated with a white line around one end of it. That line should be facing the top of the board.

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 rout the leads and trim as needed.

Step 8: Switch and Piezo Buzzer

Now we can place the switch and the buzzer.

The leads of the switch are spread a bit wide to fit in the board to you will need to bend the outside leads in a bit to make it fit. The buzzer should have no problem dropping right into place. The leads of these 2 items won't be long enough to bend over for routing but they should hold the items in place for soldering without any issue.

Step 9: Coil(L1) and Antenna

Now its time to make our L1 Coil. For this you will need 355mm (1' 2") of the 20/22 AWG sold copper wire wire and a 3mm diameter object such as the ink cartridge of a disposable ball point pen. You don't want to use strand wire because it won't hold it's shape as well. Leaving 5mm or 6mm of wire off the end and start making 20 turns around the ink cartridge. Keep the turns nice and tight. You may want to use some dabs of hot glue to help hold the coil together. When your done strip the ends off the coil and place it on the board. You may notice the finished coil is anywhere from 40mm to 50mm long. Depending on how tight you kept the coils. This is much longer then the size indicated on the layout and reference image. But that's OK since there is plenty of room across that right hand side of the board. So the coil can be placed anywhere along that side. Just be aware of what traces need to be soldered to what end of the coil.

The antenna is a simple peice of 100mm long wire.

Put the antenna and coil in place and solder them as needed.

Step 10: Battery Clip

Finally we add the battery clip. Before we can solder the clip in place we need to create the 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 Pad1 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 clip from the middle of the board. You'll notice 2 tabs on the back side of the clip that keep the battery from going out the back side. Make sure those tabs are facing to the outside of the board. 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

Now you have an FM Bug Detector. 

Before mounting the board you need to do some work to the tin. Cover the bottom of the tin with electrical tape to prevent the board from shorting out on the tin. You will need to drill a couple of holes in the side so that the switch and LED are able to stick out. Use a 1/8" bit for the LED hole and a 1/4" bit for the switch. You'll need to drill 2 3/16" holes side by side for the switch and cut out the metal between them. The holes will need to be placed about 10mm up from the bottom of the tin. The LED hole should be about 18mm in from the back of the tin and the Switch holes will start about 22mm in from the front of the case. Drill the LED hole with a 9/64" drill bit. With the holes all set place a few dabs of Hot glue in the bottom of the tin and place your board into the glue.

So there you have it a nice little FM Bug Detector you can use to find FM Bugs. The antenna can be stored in the right hand side of the tin and pulled out as needed. Pair this with my FM Listening Bug and you have a fun little electronic hide and seek game.

In the future i hope to have full PCBs made up so i can start selling these kits.

Spy Challenge

Runner Up in the
Spy Challenge

Pocket-Sized Contest

Participated in the
Pocket-Sized Contest

Remote Control Challenge

Participated in the
Remote Control Challenge

Make It Real Challenge

Participated in the
Make It Real Challenge

Hurricane Lasers Contest

Participated in the
Hurricane Lasers Contest

1 Person Made This Project!


  • Puzzles Speed Challenge

    Puzzles Speed Challenge
  • "Can't Touch This" Family Contest

    "Can't Touch This" Family Contest
  • CNC Contest 2020

    CNC Contest 2020

12 Discussions


4 years ago

What software was used for the pcb layout?

Can this be used for tire pressure Montoring system? I believe they operate at 315 MHz? I assume most are binary; either send a signal or no signal.

Could the bug and this receiver function as a remote control circuit? Seems like you could make a DIY remote control device this way.


Reply 6 years ago on Introduction

No not really. Quite a bit more copmplicatd than this. But there are plenty of hobby RF transmitter and reciver kits out there for that sort of thing.


7 years ago on Introduction

Did you actually put antenna inside metal box? I don't think you will get anything with that cage around antenna


Reply 7 years ago on Introduction

The antenna is only stored in the box. You pull it out when doing a sweep.


Reply 8 years ago on Introduction

Not for the Pegboard version. Leaving the ground fill out of the Pegboard layout and reference images makes it easier for beginners to keep track of what component goes where. If you download the PCB files for etching your own board the ground plane is there.