Instructables

The Ultimate FM Transmitter (Long Range Spybug)

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Picture of 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 made it but but it's not working. i think the problem is with the capacitor i got the one with 3 pins, looking forward to get some help.

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i too made it but its not working any help plz...

what about the trim capacitor..can you please explain the connections

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

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

Arman55922 months ago
Will it get damaged if I'd use 9v 0.6 A ? Or 9v 0.5A ? Or 14v 1 A ? Or 7.2v 0.5A ???
ASCAS (author)  Arman5592yesterday

Nope. If you want to protect it just add a 10 ohm (1/4w) resistor to protect it from the current.

Nope. 7V to 14V will all work, according to the author. I don't know what the minimum amps required is.

ofersto1 month 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)  oferstoyesterday

Yes, the brown one has a different capacitance value.

comsa42 made it!1 month 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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ASCAS (author)  comsa42yesterday

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

I made it! thanks!
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ASCAS (author)  nico bulaonyesterday

Awesome!

victore12 days ago

im having a problem in understanding your PCB and components layout. I dont know which and where should i connect the components.
Can someone post a photo of topview and bottomview of finished product

hello! i tried to make this transmitter but it is not working.

1.can you please tell how to test it.

2.i'm using a breadboard,i hope this might work with it..

Can I use smart phone. FM to receive its signal
Can you use 2 AA batteries instead of the 9 volt battery
fadecomic22 days ago

I'm having a lot of difficulty tuning this transmitter. I can get a really brief transmission, but then the tuning drifts. It also seems to vary greatly with transmitter orientation. I wonder if it's because I used a 0-60pf trimmer. Maybe I need a shorter range, like 10-40pf?

I have the same problem, i can´t tuning D:

ShannonD120 days 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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Never mind, I had a brain fart. .1uf is 100pf and .01uf is 10pf.

Gabriel2522 days 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...)

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...

Great idea! Wonder if I can use it with an electric guitar and a radio (?????)

ASCAS (author)  DangerousTim22 days ago

It's compatible with guitars. The electret mic/ preamp should give enough gain for the guitar's low output jack.

pinomelean29 days ago

You could power it from thise little 12v batteries made up of coin cells and make a tiny transmitter with it.

ASCAS (author)  pinomelean22 days ago

Yes you can! :))

saidmsbah25 days ago

the best one

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Arman55921 month ago
I don't understand the circuit it's so crowded :/

i got some clearer picture of it now. i have removed the 20pf 3 pin trimmer capacitor from the main circuit. I think the problem was with it. now should i mount 33pf ceramic capacitor or 40pf trimmer capacitor or 47 pf capacitor? I am stuck, need help!

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Hope this clears your doubt :-)

Have fun..!

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jagadish661 made it!1 month ago

Perfectly clear Instructable. Had fun in making it.

But the audio quality is not as expected :-(

But still thank u for the instructable.

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hello..i am trying to make this but i can't get something...i use shield cable for transformator or motor. for antenna i use usual cable and for circuit i use from this web..for trimmer capacitor i use 30pf to change 40pf anyone can help me? thx

Arman55921 month ago
akshitubhangale you could use a normal capacitor - the ones you said are all ok . Ideal choice would be variable capacitor with wide range .

thanks for the help. I tried it with all these capacitors but still i didn't find the signal on the radio receiver.

Yes . I have the same problem , I cant find it . It's really hard , like searching for a radio station without any name - in fact , it has no name , so automatic station finding systems might not detect it . You should keep turning the variable capacitor / change the coil shape , search all the stations and do the same multiple times .
Arman55921 month ago
How can I find what frequency it is ? Is there any way to calculate ? Or at least can somebody say , am I supposed to search at MHz or kHz ???
Arman55921 month ago
I MADE IT (first instructable I made) the instructions were clear I just got confused .
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