FM Listening Bug

Intro: 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:

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

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. 

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.

Tools:

• 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.

Comments

[Radio Engineer]

User @nationalprojectcoordinator is quite right in his comment -- Indeed, the schematic has been copied by the author incorrectly from the source and the antenna is incorrectly attached to RF ground on the schematic in this project -- such an antenna would not be able to radiate radio waves. (I have attached the original correct schematic to this post as a picture -- please see below. In it the antenna is attached to a different point).

However, looking at the photo of the soldered board, the antenna appears to be attached almost where it should be -- between the 7th and the 8th turn of the coil as per the original schematic in the source. On the original schematic the instructions are to attach the antenna one turn down from the top end on the coil. On the photo in this project the antenna appears attached only half-turn from the top (not quite a full turn). However, from practical experience, this is not the trouble with this project -- even without an antenna at all, when this circuit is placed within 20-30 cm (one foot) from an FM receiver the latter should be able to receive its transmissions, if the former were working. The problem most likely is that the oscillator in this transmitter does not start -- no RF frequency is generated at all which might be due to a number of possible reasons, of which the most common is that the DC operating point of the transistor is not set correctly.

Like the user @nationalprojectcoordinator, I am a vintage radio engineer (35+ years of experience) and I spotted the error with the antenna on the schematic almost immediately. However, from past experience, I think that this is the least problem with the schematic, the main problem is that this circuit is not quite suited for replication by novices. Why?Because usually it is difficult to get such simple oscillators like this one to work without actually having detailed notes on how to troubleshoot the finished circuit. At the very least one must have the measured DC (bias) voltages and currents from the working circuit provided by the author. Contrarily to the expectations of some novices in radio, simple oscillators are notoriously difficult to make them work -- more complex circuits, with XTALs and specialised IC, are much easier to replicate and get to work. Here is what an experienced hacker would have done when replicating this simple circuit: she/he would routinely replace all DC bias resistors for the transistor (R2 and R3) with variable ones in order to be able to try and pick the correct operating DC point for the transistor. Then also replace any components in the feedback loop with variable ones (e.g. C4) in order to try to get the oscillator to start. Only after having made the oscillator to work, would the hacker replace the variable components with fixed resistors and capacitors as shown on this schematic (this is not about the tuning capacitor C3, which should be kept variable in order to be able to tune the transmitter to the required frequency). In addition to this, an experienced hacker would have at her/his disposal an LC meter to measure the capacitance of the capacitors and the inductance of the coil (such instruments of lately have become rather affordable and may cost between $20 and $40 on some well-known internet sites). The value of the inductance of coil L1 should ideally be provided by the author because it can vary quite considerably with the size and the distance between the turns, so providing just the physical dimensions usually is not sufficient as guidance on how to replicate this project (an experienced hacker would be able to calculate what this inductance should be from the values of the other components -- this is the reason why on many schematics this information is missing: it is assumed that the readers would do math themselves). In contrast, a schematic using a specialised IC and a Xtal would normally not need all this (the DC operating point of the transistors in the IC would be guaranteed by the internal components inside the IC, the frequency would be determined by the Xtal, not by an external coil and capacitor forming an oscillating circuit like in this project, the external components would not need to have exact or accurate values and even approximate ones would do -- this is why radio projects with IC are usually easier to replicate). A hacker might also have a frequency counter and a receiver tuneable within a broader band than just FM -- these would enable the hacker to more confidently determine whether the oscillator is working at all (due to incorrect or inaccurate values of the components used, or maybe simply due to their mutual position on the board, the oscillator might be working on a different frequency, which cannot be received by an FM receiver)

For the above-mentioned reasons I recommend to readers of this, otherwise immaculately-documented, project to look elsewhere and find other schematics suitable for replication by novices. Also, if the readers wish to complete any project in radio, whether a transmitter like this, or a receiver like the other project of this author, I recommend that they:

(a) try to read around and understand the schematic in detail, its principle of operation and some basics of how to debug it (very few radio circuits work the first time without additional trimming and calibration -- these circuits are "analog" and they require trimming and calibration by definition and by their very nature, this is in contrast to "digital" circuits).

(b) invest a small sum in buying an LC meter (an instrument to measure capacitance and inductance, may cost $20...$40), you will also need an instrument to measure the basic DC parameters of transistors (some AVO-meters, a.k.a multimeters, do this as well). Also you may consider buying a frequency counter/meter (looks like a walkie-talkie and costs $30...$60, it will tell you with certainty whether your transmitter is transmitting and on what frequency), and a wideband SDR receiver would be a good tool to have and a nice toy to play with (SDR=Software Defined Radio, may cost between $10 and $40 for a popular type called "RTL SDR", it receives a much wider band than just FM and would be the ideal tool for testing this project. When coupled with a free software called "GNU Radio" it can be used as an oscilloscope and a spectrum analyser -- for $10...$40 SDR is a great little tool! Disclaimer: the use of this software will require certain knowledge of electronics, e.g. understanding of DSP, digital filters, mixers and complex I-Q signals, etc).

Keep high spirits and perceiver! Radio is a great hobby, start with something small, which has been already successfully repeated by many others, then move on to more complex projects (which may have simpler schematics, like this one). You will be rewarded by the fruits of your labour.

---

(Below is the original schematic for this transmitter -- it looks very simple, however keep in mind that in radio simple circuits may be more difficult to get to work than those with more complex IC. Simple radio circuits may need additional tuning, trimming and calibration in order to make them start doing what they are intended to do -- for this you will need at least some basic radio instruments and skills in troubleshooting radio circuits, as mentioned above.)

[nationalprojectcoordinator]

The antenna must be connected to the collector of the transistor (the 'hot' side of the oscillator tank circuit) in order for the oscillator to properly radiate beyond the tank circuit. Your circuit is incorrect in that it shows the antenna connected on the battery side of the inductor (coil) which effectively is radio frequency (RF ground). I am a retired electronics engineer with 40 years of experience. Your circuit definitely needs to be reworked.

[cocosoft]

Sorry but I for one cannot understand why one would post an instructable that looks good and appears as though the writer knows what they are doing and then waiting to the very end to inform everyone that it does not work. Do you really think everyone is going to maintain an interest till such time as you have ironed out the obvious bugs- at least say it doesn't work at the beginning. Give us all a break and don't waste our time by doing this.

[ckgarside]

You cant order any less than 1000 variable capacitors, and I really don't wan't to spend $294. Are there any substitutes?

[streetj58]

Thrift stores old junk radios and electronic devices.save thrift store from throwing it away

[mpilchfamily]

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.

[ckgarside]

What resistor do you prefer for this application?

[mpilchfamily]

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.

[Cekpi7]

dont you mean capacitor not resistor ?? capacitor have Farads and resistor Ohms

[streetj58]

Santana needs to be tuned.instol a trimmer capacitor needs to be used if you want to use a standard radio to listen.without a scope to check frequency it will take awhile to tune.also ask you friends and look for sales on used junk like CD players for parts.thrift stores for couple bucks can buy items to rob parts 3.00 dollars is what I spent

[Rfmhz]

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 :-)

[Rfmhz]

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 :-)

[roustabout]

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.

[joel.williams.5249]

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

[biraj1002]

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]

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.

[AJ_Smoothie]

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]

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.

[AJ_Smoothie]

Then what purpose does this device serve if you can't listen to what being transmitted?

[smiley G.I JOE]

how much will it cost to get the parts at radio shack in the us