Introduction: The Ultimate FM Transmitter (Long Range Spybug)

Picture of The Ultimate FM Transmitter  (Long Range Spybug)

Have you ever wanted to broadcast your own radio station within your neighborhood? Ever get curious on where people get those "Surveillance Bugs" from spy and action movies? This small and simple FM transmitter is the toy that geeks have always wanted.

FM transmitters can be complicated to build, that's why I'm teaching you how to make a foolproof FM transmitter. There's no need to buy kits, this tutorial includes the PCB layout and the schematics. It has a range of up to 1/4 mile or more. It's great for room monitoring, baby listening and nature research.

My Experience:

FM transmitters remind me of my early years in electronics. When I was 8, I came across Art Swan's FM transmitter circuit. At the time I had no idea of where I'm supposed to buy the parts, so I recycled mine out of junk. I guess the biggest struggle that you're going to face is finding a trimmer capacitor. I'll give some tips on the last step of this instructable. In a nutshell, I highly recommend this project for everyone and also those who are still new in electronics.

>>>>>>>WARNING: You may experience nostalgia! :D<<<<<<<

Technical Specifications:

- 1/4 Mile Radius Range

-Powered By A 9V Battery

- Lasts For Several Days

- Adjustable 87-108MHz

Please Watch: Celebrating the 1st episode of my new YouTube channel! It's my first time to document a project with videography. I hope you guys enjoy the vid! Please leave a comment below, I would appreciate some advise regarding the video.

Disclaimer: This project is for educational purposes only and is not intended to air/ interfere with present radio channels. Neither site nor I, am liable for careless actions. Please check for the legality before attempting the project within your area. As long as

Step 1: Gather the Parts

Picture of Gather the Parts

All of these are available on any branch of RadioShack! :)



- Copper Clad PCB/ Perfboard

- Solid Gauge # 18 Wire

- Electret Microphone

- ¼" Bolt



- 2N3904 General NPN Transistor (2x)



- 15pF or 40pF Trimmer Capacitor

- 100nF Ceramic Capacitor (2x)

- 10nF Ceramic Capacitor

- 4pF Ceramic Capacitor



- 1M Ohm ¼w Resistor

- 100K Ohm ¼w Resistor

- 10K Ohm ¼w Resistor (3x)

- 1K Ohm ¼w Resistor

- 100 Ohm ¼w Resistor



- A Pair Of Pliers

- Soldering Iron

- Hot Glue Gun

Step 2: PCB & Schematics

Picture of PCB & Schematics

I designed a compact PCB layout for Art Swan's miniature FM transmitter circuit using Fritzing. Use this step as your reference for the assembly.


About The Circuit: These is the exact description of Art Swan, the circuit's Author, "This miniature transmitter is easy to construct and can be picked up on any standard FM receiver. It has a range of up to 1/4 mile or more. It's great for room monitoring, baby listening and nature research"


Download Link:

Step 3: Print the PCB Layout

Picture of Print the PCB Layout

Download the PDF file then print it with your printer's standard setting then cut the printed layout. Be careful when cutting, the tip of my thumb got sliced by the sharp cutter blade.


Download Link:

Step 4: Develop the PCB

Picture of Develop the PCB

I'm using something what's called presensitized PCB fabrication, it's different from the toner transfer method. If you're not familiar with presensitized PCBs, better go with the toner transfer method.


Presensitized PCBs: I expose mine directly to a 10W fluorescent lamp for 5:20 minutes then use a dilute solution of Sodium Hydroxide to develop the exposed PCB.

Here's a separate tutorial for the PCB fabrication:

Step 5: Etch the PCB

Picture of Etch the PCB

Pour Ferric Chloride on a plastic tray then start to etch the PCB.

Step 6: Clean the PCB

Picture of Clean the PCB

Use a swab and Acetone to remove the photo-positive layer/ toner.

Step 7: Solder the Components

Picture of Solder the Components

Use step #2 as your reference. Solder the smaller parts first. Start with the resistors, the capacitors, the transistors, the coil, the antenna then the 9V cattery clip.

Step 8: Construct the Coil

Picture of Construct the Coil

Strip a solid gauge #18 wire. Use a 1/4" bolt then turn the wire 7-8 times.

Step 9: Adding the Antenna

Picture of Adding the Antenna

Solder a hook-up wire to the antenna pin, it's located on the 2nd transistor's collector pin. Use a maximum of 8 inches an a minimum of 5 inches.

Step 10: Recycle a Battery Clip

Picture of Recycle a Battery Clip

The key to this compact transmitter is the ingenious battery clip.You can get one by dismantling an scrap 9v battery.

Step 11: Glue Them Together

Picture of Glue Them Together

Apply a generous blob of hot glue to hold the clip and the transmitter circuit together.

Step 12: Breadboard Version

Picture of Breadboard Version

Recently, people have been asking me if it's possible to make this project without having to fabricate a PCB. The answer is yes. In fact I built my first FM transmitter on a perfboard. I guess some are new to this, no worries I'm here to teach you.

The answer to this is a "Perfboard/ Prototype board. It's a PCB designed for prototyping circuits. There are three types of perfboards, the one that suits our needs is the dot matrix version. If you're new to this, make magazine has handy YouTube tutorial.

Step 13: Tune the Transmitter

Picture of Tune the Transmitter

Turn on your radio then tune it to your desired channel frequency. You'll get more range from the vacant channels. Don't touch the coil, just turn the trimmer capacitor until you hear a feedback from the radio.

Step 14: Q&A - Tips & Tricks

Picture of Q&A - Tips & Tricks

Where Can I Buy The Parts?

If you live in the US, all the parts that I've used are available from Radioshack. You don't believe me? The parts from my first non-recycled transmitter came from RadioShack, Long Island, NY branch.

Any Alternatives For The Trimmer Capacitor?

Most likely, you'll find one from a scrap transistor radios however you can replace it with a 20pF ceramic capacitor then rely on the coil's adjustment for compensation. Trimmer caps are color coded, only use the Red, Green or Yellow. I used the yellow since it covers a wider range.

Which Wire Should I Use?

Only use solid wires that has a thickness of 18 - 22 gauge. When I was still a novice hobbyist, I was worried about the strict selection of parts. Don't worry too much, eventually you'll learn to improvise when parts are not available.

Why Does The Frequency Change After Tuning?

Let's admit it, tuning the radio is a bloody challenge! If you use a metal screw driver to tune the trimmer cap, chances are you'll end up having a different frequency broadcast the moment you lift the screw driver. This is why non conductive screw drivers are recommended.

Where Can I Find A Non-Conductive Screwdriver?

You'll easily find one from a PC repair shop, or maybe form a hardware store. If you really can't find one, the famous Little Bits kit comes with it.

Is It Possible To Connect A 3.5mm Audio Jack?

Yes, it's possible! All you need to do is to link the common ground then solder a 1K ohm resistor each channel (L & R) forming a junction. Now solder a wire from the junction to U1's collector pin (refer to the schematic). You can now connect your MP3 player! I'll post an additional step regarding this mod.

Can I Hook This On A 12v Car Battery?

This project would still operate at voltages between 7v-14v, so yeah it's compatible. If you want to stay safe and prevent the circuit from burning, solder a 10 ohm resistor in series with the FM transmitter and 12v battery.

Is This Legal?

As long as you don't use it to spy on others and not use a long & huge antenna then yes, it's legal. Just don't use it near an airport. For more info, pleas read the FCC Rules.

Step 15: Don't Forget to Leave a Comment [Stay Tuned]

Picture of Don't Forget to Leave a Comment [Stay Tuned]

Please don't forget leave a comment below. Thank you!


2nd Week - DIY Pocket Spudgun

3rd Week - Homebrew Continuous Tazer

4th Week - Crystal Radio

5th Week - Solar Car (School Investigatory Project)


átháwàít (author)2017-11-21

from where i can get 40pf trimmer capacitor ?? is there any chance of getting from radios or RC cards??

They're almost a rarity nowadays. Don't worry about the variable capacitor and use a common ceramic capacitor with similar value in its place. If you want to change the frequency, adjust the coil length using tweezers.

burnsd (author)2017-12-03

Would I have to change the amount of winds in the coil if I use 14 gauge copper wire? It seems like it would make a difference, but I don't know how to compensate for the thicker wire. Any advice about how to calculate this would be great. Thanks.

AnkushTheCuber (author)2017-09-17

Are there Any any alternates for the 2n3904 transistor?

BC547 or 2N2222 are the alternatives of 2N3904

AlardE (author)AnkushTheCuber2017-11-11


You can use BC547, 2N2222 as alternatives to the 2N3904.

BUT make sure you check the pinout first.

Some transistor pins are swapped around.

Example: EBC, CBE , ( Emitter , Base , Collector )


See pic's of another Microbug.... ( My PCB Artwork )

1 - Pic is full colour with overlay's.

2 - JPEG of the PC Board.

3 - The PDF of the actual board " true size."

Bryanisthename (author)2017-10-24

Hi guys, I have some doubts here to clarify,
-Are we allowed to use an inductor instead of using a coil?
-Are we able to connect the circuit on a breadboard( those white circuit boards)
-constructed the circuit with an audio jack,
However I am only able to produce an output on my radio when the antenna of the transmitter circuit comes in contact with the radio antenna.

Would really appreciate if someone can help me out :)

Bryan Lim

Geniusboy79 made it! (author)2017-09-29

Works fine. But i did it differently

ankitd54 (author)2017-09-25

Hi bro
How can i assembel this fm transmitter in normal mobile

lsvf17 (author)2017-07-29

Any chance this could interfere with a neighbor's loud radio, if placed very close to it?

ElectroD1 (author)lsvf172017-09-10

It probably could. But if you're in the US, that's illegal, since you'd be interfering with broadcast transmissions.

Edward_Stryfe (author)2017-09-10

Would I be able to wire a 3.5mm jack in place of the electric microphone so I could have a cleaner sounding broadcast?

JadenP9 (author)2017-09-08

Is the 2n3904 transistor used to amplify the input sound, if so cant we use the input source on its own? If not can i use a different transistor?

RogerF22 made it! (author)2017-07-30

It worked! However, the maximum distance was 2 meters. Was it the inductor?

AdamM326 (author)RogerF222017-09-05

Nice work same problem maybe I used higher coil winding or either higher diameter. what do you think.You can contact me on this email:

MRWAY2NICE (author)2017-08-30

First off love ur work very nice , i do have a question if i may ? how well do you it would go in say a power board or double adapter and run it off mains power ,here its 240v im thinking may be power board and run the antenna back along the power lead is this possible ,ok thanks again and keep up the good work , cheers

EugenM7 (author)2017-08-27

Your schematics says that the inductor's inductance = .1uH

By winding an AWG18 (1mm thick) copper wire 7-8 times around a 1/4" (6mm) bolt we would theoretically get an inductor with the inductance of 0.25-0.29uH and not .1uH as recommended by your schematics. Check yourself:


This is a big issue because the LC resonant frequency would vary between 47-225MHz which is outside the desired 87-108MHz freq. interval. Check yourself:


Am I missing something?

wai hin soe (author)2017-07-22

I want to know the voltage of each capacitor

s333b (author)wai hin soe2017-07-24

Doesn't matter as long as the voltage rating is higher than the maximal voltage over the capacitor. In this circuit the highest voltage that occurs is 9V from the battery, since all ceramic capacitors have ratings for much higher voltages (often times 500V for the wired ones) you can use any of these. The trimmer capacitors should also be ok with 9V. If you can't find trimmer capacitors you could just use a similar sized ceramic one and bend the coil until it has the right frequency.

wai hin soe (author)2017-07-15

Do the ceramic capacitors have voltage? Some capacitor have F, V. (e.g.4.7uF,50V)

UtkarshVerma (author)2016-12-09

I know it's a silly question and I think I already know the answer but still, please tell me. Does change in Inductance of a coil affect its range? If so, how are both related?

brmarcum (author)UtkarshVerma2017-06-30

Changing the inductance of the coil will affect the circuit's resonance frequency, which for this circuit becomes its transmission frequency. The same is true for changing the capacitance on the trimmer cap. Since adjustable caps are easier to make than adjustable inductors, you hard set the coils of your inductor and then use a trimmer cap.

To change the range you need to use a larger power supply and make sure your components can handle the power running through them.

champion17 (author)2017-06-08

I made it ..... but it only produces a continuous sine wave sound in the radio .

can anyone help me figure out its problem ?

TranT94 (author)2017-06-06

What do you mean connect to the common ground and solder 2 1k resistors to form a junction?

kurzin (author)2017-06-03

Please dont remove this page till i make my own copy of this transmitter

HAFEZ4 (author)2017-05-19

hi whats the schematics of the pcb when use relly?????how can i use relle instead of the trimmer??? please answer

mr_newton (author)2017-05-18

Fantastic. I was wondering if you were still planning on adding a step for the 3.5mm mini jack mod? Cheers!

Prerna Garg (author)2017-04-14

can i use mobile's radio or it need actual FM radio.One more thing , I didn't get trimmer capacitor ,got a capacitor with three legs can it be used.

what are the specifications of the mic used.

rush 423 (author)2017-04-11


HarveyH7 (author)2016-12-19


Taimee (author)HarveyH72017-03-30

hey! could u plz tell me the size of pcb layout?

ohoilett made it! (author)2016-04-09

Thanks dude. Works great.

Taimee (author)ohoilett2017-03-30

hey! could u plz tell me the size of pcb layout?

Taimee (author)ohoilett2017-03-30

hey! could u plz tell me the size of pcb layout?

KrishnaM48 (author)ohoilett2016-05-02

Pcb layout size of print what?

ohoilett (author)KrishnaM482016-05-03

Sorry, I'm not sure what your question is exactly.

AbdalaM (author)ohoilett2017-03-14

Whatssap me

imi91 (author)ohoilett2016-11-22

im already done make the circuit but im still cannot get the frequency that i want..can u look my circuit if have mistake..tq

ohoilett (author)imi912016-11-25

I found that getting a specified frequency given the author's proposed hardware setup is quite difficult. I end up getting whatever frequency is not broadcasting very strongly in my area. Just try scanning through all AM and FM frequencies and turning up the gain on your transmitter.

falcon74 (author)ohoilett2016-12-26

How does one turn up the gain on this transmitter ?

ohoilett (author)falcon742016-12-26

Increase the power supply voltage would be the quick and dirtiest way. A few others might be to change R2 and R3 to increase the bias voltage at the base of that transistor. Or maybe decrease R4. There are a few others, but again, it comes down to increasing the collector current of the transistors which would happen by change the collector resistance, increasing the base voltage/current, or by increasing the power supply voltage.

HarshitY2 (author)ohoilett2016-11-25

its working or not

ohoilett (author)HarshitY22016-11-25

It worked fine. I couldn't specify a transmit frequency, but I could pick up the signal on a few open channels.

Would u sent me a breadboard circuit degin photo?

kriz0 (author)2017-03-29

May i change 1m ohm 1/4w with 1m ohm 1/2w resistor? And change 4pf ceramic capacitor with 5pf? :)

Taimee (author)2017-03-27

anybody help plzz

Taimee (author)2017-03-27

help plz. i want to know the pcb size

AkhilB22 (author)2017-03-21

I connected the circuit as per circuit diagram but there is no any out put.what the input for the circuit and how much frequency range it is?

AbdulM144 (author)2017-03-11

I don't find 4pf ceramic capacitor

AdamGadut (author)2016-12-02

The schematic diagram included a 0.01pF Capacitor - which is not existent anywhere (unless probably its a DIY capacitor). This capacitor isn't included on the parts list so i guess its a mistake

About This Instructable




Bio: Hi I'm Angelo! I am a 18 y/o college sophomore taking my majors in BS-ECE at the DLSU. I use my course as ... More »
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