Instructables

The Ultimate FM Transmitter (Long Range Spybug)

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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

 
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Step 1: Gather The Parts

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

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MISC:

- Copper Clad PCB/ Perfboard

- Solid Gauge # 18 Wire

- Electret Microphone

- ¼" Bolt

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Transistors:

- 2N3904 General NPN Transistor (2x)

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Capacitors:

- 15pF or 40pF Trimmer Capacitor

- 100nF Ceramic Capacitor (2x)

- 10nF Ceramic Capacitor

- 4pF Ceramic Capacitor

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Resistors:

- 1M Ohm ¼w Resistor

- 100K Ohm ¼w Resistor

- 10K Ohm ¼w Resistor (3x)

- 1K Ohm ¼w Resistor

- 100 Ohm ¼w Resistor

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Tools:

- A Pair Of Pliers

- Soldering Iron

- Hot Glue Gun

Step 2: PCB & Schematics

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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.

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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"

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Download Link: https://docs.google.com/file/d/0BwP5mrDBOvNYaHFnME...

Step 3: Print The PCB Layout

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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.

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Download Link: https://drive.google.com/file/d/0BwP5mrDBOvNYaHFn...

Step 4: Develop The PCB

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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.

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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.

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Here's a separate tutorial for the PCB fabrication:

Step 5: Etch The PCB

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

Step 6: Clean The PCB

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

Step 7: 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

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

Step 9: Adding The Antenna

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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

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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

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

Step 12: Breadboard Version

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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.

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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

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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

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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]

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

Schedule:

2nd Week - DIY Pocket Spudgun

3rd Week - Homebrew Continuous Tazer

4th Week - Crystal Radio

5th Week - Solar Car (School Investigatory Project)

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ondesimo2 days ago

Mine is not working. The only difference I have done is that I used a AWG 22 wire for coil, still 8 turns. Does this greatly affects the circuit. I also used a trimmer capacitor with 3 legs. Just used two legs for my circuit. Need an urgent help.

jw.6 days ago

I have made it but I could not get it to work. I used all the components as per the circuit except I used 22pf trimmer capacitor, if that makes any difference. Since, I could not get 20pf or 40pf trimmer capacitor. Advice please!

whats the range?

ASCAS (author)  cheeseyjazz7 days ago

Please read the guide before commenting. It's roughly a quarter mile, depending on your antenna and trimmer capacitor.

FoamboardRC10 days ago

Nice job, but the schematic is poorly done. I can't tell where parts are supposed to go. The picture lists a name, and points to 1 hole. Each component goes to 2 places, and the transistor 3. Try clearing it up. Thanks!

sir do you have a clearer schematic??? i cant identify where to put the parts... it kind a mixed up..

can you tell what method is used to produce fm here
please i really want to understand the process
you dont need to explain the whole thing just a hint and i will figure out the rest.
i studied analog modulation in 4th semester but cant recall it right now ?
srevankar115 days ago
Can i use inductor( which look like resistor) instead of that Coil?
kushalverma17 days ago
I'm able to listen audio but with huge disturbances ....what to do now ??
kushalverma18 days ago
what will be the frequency if instead of trimmer capacitor I use 4pf ceramic capacitor
ASCAS 121 days ago

how long is the pcb please reply fast

YassarH24 days ago

hi

i wonder if i can change the frequency range from this circuit. i want to make it for radio communication in my community. and it works with 159 - 150 MHz. so which component should i change to attain the desired frequency.

thank you

ualbuquerque6 months ago
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Yes, the design is simpler, but the microphone sensitivity of this Instructables design is much better.

I used to build these transmitters 35 years ago... but for the oscillator, i used a transistor better suited for high frequencies.

I also used Germanium transistors and FETs to reduce the supply voltage. (to run it on one or two button cells)

This way, you can create real spy gear, in terms of size.

If you want more frequency stability, i would suggest a design with Varicap.

With SMD components, the transmitter was no bigger, than the button cell.

Good designs can be found here. (Very good description of the circuits)


Sir the link u mentioned for good designs......i made the three transistor design but its range was much less than the two transistor design......i got greatest range from voyager mkll which is shared in the link u mentioned...... I m not a professional as u all r.....also wanted to know how to match antenna with the output.....would be glad if u help me out.....i have also tried to make the ultima designmentioned here but couldnt get transistor 2n3563 substituted with a bf199.....still its range was very less less than 50 mts

Hi

With frequencies around 100 MHz, you need to observe certain rules in the component layout. Otherwise you will waste energy. In the link above, many of these rules are pointed out.

Stuff like short leads on the components in the tank circuit, diameter-length ratio of the coils, use of silver plated wire for the coil(skin effect) and so on.

Read the text in the link carefully.

If you want to optimize the power output, you need to tune the circuit with a "Grid Dipper" for example.

In these links, you can get a idea of how strange it gets at higher frequencies.

http://www.eeweb.com/rf-design

http://www.embedded.com/design/system-integration/...

RF design isn't as easy as Arduino circuits for example. There is now quick and easy way to learn it. Even for me with a Ham license, it's still funny sometimes, how RF circuits behave. Without any measuring equippment, it's pure luck.

Thnx for the link.....but i cant find such meter out here

You can build one yourself. (Enter "grid dipmeter" into google)

You will find versions with vcuum tubes (where the name comes from, grid is the tube equvalent for basis or gate in semiconductors), bipolar or fet transistors.

As said before, building a somehow working transmitter is possible without deeper understanding. Optimizing it is another thing.

If you're realy interested, try to find a HAM in your area. They are mostly helpful, if you show interest in their hobby. That's how i got into it at the young age of 12. I was first teached the basics of electronics an later that of HF.

With 17 i passed the test for the HAM license successfully.

Ok sir....but here its not like that....its india....noone is ready to help u....anyways thnku....will find some way anyhow
Ok sir....but here its not like that....its india....noone is ready to help u....anyways thnku....will find some way anyhow
Jawaaz t.rohner6 months ago

instead of dipole,,we can use a yagi uda for more range and directivity...what do you say

t.rohner Jawaaz6 months ago

So many possibilities.

If you really want to spy on someone, take a cheap mobilephone, disconnect the speaker and set it to take (your) calls automatically.... probably not possible in the dungeons of the NSA 30000ft below ground level ;-))

And for sure, it's illegal. At least around here.

If you are however interested in wireless communication, get in contact with your local "Amateur Radio Club" aka HAM. There you will find a lot of nice knowledgeable people who will be happy to help you, to get a license. I did it, when i was 17. (No mobile phones at that time...)

Then you can push out some couple of 100 Watts into the air legally. (You are absolutely right, the antenna makes the distance. ERP in the right direction ;-)

My "biggest" VHF amplifier cranked out meager 15 Watts. This is more than enough in a mountain area, like i live in. The HAM clubs put repeaters on mountain tops, that helps to get more mileage in the VHF band.

Jawaaz t.rohner6 months ago

If we cant turn up the power in RF stage of circuit for increasing transmission distance,,changing antenna and using a good directional antenna with high gain will work a lot,,I love Yagi-Uda,,it is best ......(because we can increase the elements for increasing the gain )...also a good coaxial impedance matched cable should be used rather than a simple wire,because a lot of RF power gets wasted in a simple wire.....there are so much to tell and discuss...i build a very small and simple fm transmitter and used a simple wire directly from the circuit as shown and got a range of about 50-100 feet only ,,as I used the coaxial cable and a perfectly matched Yagi Uda with my circuit,,I got a range of about 2km...that is the difference of using a good matched antenna

t.rohner Jawaaz6 months ago

A matched high gain antenna is very important.

That's one of the first things you learn, when preparing for the HAM license test.

I let my license rest at the moment, but i could activate it at any time.

VHF and UHF antennas are relatively small, but if you want a decent antenna in the short wave bands, your property needs to be big enough... Think of a 7 element yagi for the 10 or 20 band...

Jawaaz t.rohner6 months ago

yes true....that will be very big antenna for LF signalling......I am Electronics and Communication Engineer By Profession So I have learned all these stuff in College :)

t.rohner Jawaaz6 months ago

Haha, same here. But in industrial electronics. I had to study the HF stuff besides the main education ;-)

Jawaaz t.rohner6 months ago

I live in a place where all the fm band is free...just two radio stations like at 92.7 and 102.6...rest is all free so I enjoy my channel at any frequency :D ...and I like to keep it illegal :D Why do I pay for something which I can do without paying :D

ASCAS (author)  t.rohner6 months ago

+1

Jawaaz6 months ago

I build the same circuit few months ago,,i doubt about the range you have shown,,it hardly reaches 50 metre radius

Same problem with me sir hardly 30 mts.....
schmitta6 months ago

Make a dipole antenna . Divide 468 by the frequency in MHz (if 100 MHz use 100). You will get the dipole length in feet. Divide that answer in half (for 100 MHz we get 468/100 or 4.68 . Divide that in half for 2.34 feet or or 28 inches) Make two wires this long (28 inches) and attach one to the collector of Q2 and the other to ground. Point the one connect to Q2 up and the one connected to ground down. Your signal will be vertically polarized (like the car antenna) and have a gain of 2. You will get max distance from this antenna. (for experimentation only check Part 15 of the FCC rules).

Can i use any wire while making dipole??
annyan9041 month ago
Can i get a three transistor design schematic please

I am building this circuit and am having problems trouble shooting can anyone give me a run down on the stages of the circuit so I can test what's going on the only thing I understans is the LC tank circuit that broadcasts the frequency and I have that working I have access to a lab with an Elvis and an oscilloscope just need to know what parts of the circuit are doing ie the modulator primarily

comsa42 made it!4 months ago

So I decided to build my own transmitter and it turned out.. erhmm.. different.

I used a 5x7cm perfboard and I used up a majority of it. As you have said, tuning this thing is super difficult! It's even harder when your radio only has a "scan" button; no real tuning... Anyways, I have confirmed that this thing works a couple of meters, but nothing beyond that. Thanks for posting!

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When working with RF signals, using prefboards is not a good idea, the boards have a lot of noise and "leaks", i suggest you to make a PCB, if you cant, do it with traces and pads, use virgin pcbs and use de copper as a groud frame, and make the "pads" with little cuts of other pcbs, and make sure you make all conections as short as possible, That should make it work

ASCAS (author)  comsa423 months ago

Awesome! I guess you'll need to have a longer antenna and a yellow colored trimmer capacitor.

evilsined made it!1 month ago

Hey made the project. Nice one too. But I can't see to hear the transmission well at the radio. I can only hear me tapping on the mic or low sounds like 'U'. Can you help me out please?

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yesvin1 month ago

Can I use 3.9pf instead 4pf?

ASCAS (author) 1 month ago

Please read step#15 and please don't spam.

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