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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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CollinCoil19 minutes ago

Great job, as a novice hobbyist myself I appreciate all the help I can get when making cool projects like these. Your's was very comprehensive so keep up the good work!

Hi could you please tell me where to place 4pf capacitor and can we use 4.7pf capacitor instead of 4 pf

kaanthegmr made it!12 days ago

This is the circuit that I made.Instead of hand made coil I used a 0.1uH induction coil.I also added a Switch.

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bostroglav17 days ago
nice project!... is it posible to use a pair of BC547 for transmitter?
kees.drullman made it!3 months ago

This is what I designed for the students of my technical school, 21 years ago. It's a dc-coupled FM-transmitter with only one transistor, an electret-mic and a LED in it. I also designed a PCB for it. The range is about 200 meters (depending on several things). It has been build about 400 times and it always works. Wish you succes to build one for yourself !

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ASCAS (author)  kees.drullman22 days ago

Oh wow! Thanks for sharing that. I'll try to build one. This one seems to be much easier too. Creds to you! ^_^

Thanks Ascas (author) ,

I always try to minimize the number of components (smart designing, optimizing) for realizing a project. Here I've done that too.

Wish you succes building it !

Kees (from Holland)

Hi, i am going to make this. What is the Coil in this. is it 16swg ? please publish English Translation. Thanks

ASCAS (author)  SirajH20 days ago

I think it's written in dutch. I guess L1 says "Make an air coil inductor by winding the wire 6 times on a 4mm diameter".

Hi SirajH ,

Sorry for replying a bit late. The coil is made from 'silver-wire' (Ag), with copper (Cu) inside of it. RF-current flows through the silver 'shield' of the wire, which has a better conductance than cupper. The Q-factor of the coil is better then.

Wish you succes building it !

Kees (from Holland)
NihalK222 days ago

from front

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ASCAS (author)  NihalK222 days ago

Mind if you tell me the additional circuit attached to the mic?

NihalK2 ASCAS22 days ago

i don't know shopkeeper gave me when i asked for mic ,

even asked him if it was digital or programmed or something but he told no and he said it was analog

ASCAS (author)  NihalK220 days ago

Try to desolder the mic capsule from that green board.

NihalK222 days ago

is there anyway to check circuit without mic in it

ASCAS (author)  NihalK220 days ago

You can do that by removing the preamp. That means you'll have to cut through it by connecting the audio source to the collector of U1.

NihalK221 days ago

can u please reply

NihalK222 days ago

help me bro please

NihalK222 days ago

here are images

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eugene_g made it!5 months ago

Works like a charm, surprisingly clean! Thank you!

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ASCAS (author)  eugene_g22 days ago

Well done! I admire the cleanliness of your work! :)

NihalK222 days ago

not working please help me

ASCAS (author)  NihalK222 days ago

Hi! What seems to be the problem? Can you send a picture? Any suspicions on why it's not working?

NihalK222 days ago

hi i did it but it dint work , its not even transmitting signal please help me

AbdulKharis1 month ago

Thank you very very much i made it :) ,, it's cool (y)

MrinalK1 month ago

Solid Gauge # 18 Wire is unavailable. What should i do for alternative?

maheshj2 made it!2 months ago

I made it & it works good but the range of transmitter is short......!!!!

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Txer maheshj22 months ago
Make sure your antenna is 7 to 8 inches to get it crystal clear . However you can add a buffer stage to this transmitter for much clearer distance .proves range slightly more .all you do is grab another 2n3904 transistor attach 27k resistor to the bass and other end connects to positive rail .add 100mh inductor connect it to collector and other end to positive rail .and finally grab a 10pf capacitor connect it to the base and other end to your previous inducter were you had your antenna .now move your antenna up to our second inducter .now you have a improved transmitter :-) .
priyo62 Txer1 month ago
can u plz tell me how much turns are required to make a 100mH coil by winding a 18 swg wire around a 1/4" bolt? thnx in advance.
How long are you getting?
Txer maheshj22 months ago
Oh and make sure the emitter leg is touching negative end
Pri_de1 month ago

Hi,
This voltage divider circuit gives only 0.8v to the base.
This way it won't work since the voltage should be well above 0.7v required to forward bias the base-emitter diode.
I don't get how it worked for you?? Please explain.

Pri_de Pri_de1 month ago

Please correct "This voltage divider circuit gives only 0.1v to the base."

hay can i use 3.3pf or 5pf ceramic capacitors then 4pf.

ashuki1 month ago

Back when I'm 12,I still remember the moment my penknife cut trough my nail and you're right,this post does bring nostalgia

davideddu2 months ago

Hello, how can I make it 5V? I want to hook it up to an Arduino to transmit signals.

Thank you!

Txer davideddu2 months ago
5v is so weak you would have to change values of resistors
davideddu Txer2 months ago
Is it fine if I just multiply the values of the resistors in the project by 5/9? Do I need to also change the capacitors?

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.

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