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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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!3 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)  comsa421 month ago

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

evilsined made it!6 days 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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yesvin14 days ago

Can I use 3.9pf instead 4pf?

ASCAS (author) 22 days ago

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

sparshkamat25 days ago

hi i am sparsh ,

could make a instructable or a schematic with the 3.5mm jack please ASCAS and send it to me on kamatsv2003@yahoo.com

yhdesai26 days ago

YOU AR ON EFY!!!
http://electronicsforu.com/newelectronics/video/detailvideo.asp?id=2558

hola amigo te puedo pedir un favor grande tu puedes armar un fm Telephone Transmitter :http://devadasemail.blogspot.com/2012/06/fm-telephone-transmitter.html

HMSTR1 month ago

Can i use 50pF trimmer capacitor?

mmalapitan made it!1 month ago

I built this one a while ago and it's working. :) thanks to the author! The circuit transmits but the output in the radio is not that good. But still I can hear my voice. Maybe it's just because of my soldering thing or the battery. AND YES, THE TUNING IS VERY BLOODY. :D

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ace4sia1 month ago

I want to make this but I wanna modify it, instead of mic I wanna use a 3.5 audio jack, and instead of battery I want to use a USB port so I can broadcast it from almost anything (my laptop would be the main goal for this project) but USB port have only 5v, is it possible to modify it so it can be turned on with 5v?

can i use breadboard just to check if it will work?

of course.. just put it this way, breadboards are prototype boards.. you use it before making PCBs

ShannonD12 months ago

The schematic shows C1, C2 as .01uf and C5 as 4.7pf. Part list does shows 10nf,100nf and 4pf. Which one is correct? If the parts list is correct than which capacitor is C1, C2 and C5?

Thanks

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ASCAS (author)  ShannonD11 month ago

0.1uf is equivalent to 100nF and 0.01uF is equivalent to 10nF. Don't mind the 4.7pF, a .7 pf difference barely makes a difference. They are both correct.

jankeifer ASCAS1 month ago

dude, where'd you find a 4pf? built one of this like 10 years ago.

Never mind, I had a brain fart. .1uf is 100pf and .01uf is 10pf.

mine is working but acting weird. i have to position it just right and if i bump it, it loses it. and if im holding it in my hand, i have to move my fingers around and find the sweet spot to make the static stop but then it is crystal clear.

cold solder maybe?

Gabriel252 months ago

It's awesome... I finally have a working FM transmitter circurit!

probably my only problem is that it's too sensitive. How can I change the mic volume?

Also, it has an utterly short range (5-6 meters through air)... Any way to help that?

The values that are different for me(if that helps):
R3 = 105R
C4 = 56pF (not a trimmer, but it works and the transmitting freq. is 100-104MHz)
C5 = 2.2pF

T1, T2 = BC182

NPN, max operating freq.: 150MHz

Could it be the problem? (i put it in because it was the only NPN laying around...)

the coil maybe the problem ;)

Oh, never mind, the range is good, i just tuned onto a frequency that receives the signal, but in worse quality and to a short range...

tomb5 Gabriel251 month ago

Connect a resistor to the mic?

nico bulaon2 months ago
I made it! thanks!
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ASCAS (author)  nico bulaon1 month ago

Awesome!

but how can i put a 3.5mm audio jack??

I'm planning on doing that. Maybe replace microphone with 3.5mm audio jack?

jankeifer tomb51 month ago

you can also use a jack (female) that has the connecting function.. like when you plug a jack in it the microphone will not work and when you remove the jack, you can use the microphone again

replace the mic with a 3.5mm audio jack.

ofersto3 months ago
I used a brown trimmer capacitor, and it's not works. Do I have to replace it with a yellow one?
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ASCAS (author)  ofersto1 month ago

You need a green one or yellow one in order for it to work.

ASCAS (author)  ofersto1 month ago
(removed by author or community request)
ofersto ASCAS1 month ago
Which capacitor I need to use if I want it to be not variable? I want it to be 108MHz
ofersto ASCAS1 month ago
Which resistor I need if I replace the microphone with a mono jack?

which antenna is used for best transmitting purpose

This was impressive. BIG Thanks for sharing the technique on using a screw to make such precise turns in the wire. You just taught an old man something new!

akashverma1 month ago
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