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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george172 days ago

what was the total cost of the project?

george172 days ago

hey ASCAS if i did this on a breadboard would i have to modify the circuit?

Regie made it!5 days ago

Hi, I got some few questions.
1) What would be the difference if a red trimmer capacitor (20pF) was used?
2) Should the coil be mounted near the PCB level because ours is at higher level than the trimmer capacitor? (see pictures). Also, we used a #18 enamel coated solid wire. It's a bit different than the one you used.
3) Any specific details about the antenna since we used stranded wire (or should it be a solid wire?) Comparing it's length, ours is a bit shorter than the one you shown although its 7 inches long.

We made this one and managed to transmit a signal but it was a bit noisy and the volume level was pretty low when playing music through phone and laptop (both through its speakers or ear phones) but when voice is transmitted, the volume level was pretty descent. We tuned it at the lower end of the FM range to avoid using commercial FM signals.


Thanks!

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can someone show me how you would do it on a breadboard

eugene_g made it!9 days ago

Works like a charm, surprisingly clean! Thank you!

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this looks really cool but I'd watch out what you say about being at all radio shacks, the one near me has mostly pre-built stuff and at&t phones :(
yhdesai made it!22 days ago

MADE IT ON PERFBOARD

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lkay524 days ago
EXCEPTIONAL instructable! great instructions, great pics and the PDF's are ALWAYS welcome! thanks!
Kaiser IVE25 days ago

hello.. :D.. i followed the schematic above except that i designed my own pcb layout and after soldering all the components and supplying it with power.. I am hearing a lot of the noise rather than the sound.. what could possibly be the problem?? thank you..hoping for your kind response.. :D

Kaiser IVE25 days ago

hello.. :D.. i followed the schematic above except that i designed my own pcb layout and after soldering all the components and supplying it with power.. I am hearing a lot of the noise rather than the sound.. what could possibly be the problem?? thank you..hoping for your kind response.. :D

Kaiser IVE25 days ago

hello.. :D.. i followed the schematic above except that i designed my own pcb layout and after soldering all the components and supplying it with power.. I am hearing a lot of the noise rather than the sound.. what could possibly be the problem?? thank you..hoping for your kind response.. :D

WcDony27 days ago

Hi how could this be made to operate below 88.1 Mhz, and could it work with a variable capacitor from 6-160 pF?

tevaz1 month ago

where do you get the pcb?

http://www.radioshack.com/2-sided-copper-clad-pc-board/2761499.html#.VMv9Yaaj1hQ

frankieg70827 days ago

Can I substitute the 4pF Ceramic Capacitor for another capacitor? my radio shack doesnt have them

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?

ASCAS (author)  jankeifer1 month ago

+1

(removed by author or community request)
ASCAS (author)  DELETED_ASCAS 11 month ago

Sorry, I forgot. If you print it you'll find out the size.

ondesimo1 month 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.

Rfmhz ondesimo1 month ago
Nah pal doesn't matter .how I do it is hold one strand of wire against the screw part other hand on other part of strand of copper turn then again and so on .remember the half a loop at one end doesn't count and the right hand side half a loop doesn't count .when you have done this make sure the both legs are equal in line when the hole is facing you .to do this keep drew in place it flat on table on the loose end press down till it touches the table and turn it proud and do the other leg you'll see they match in line perfectly
kushalverma1 month ago
what will be the frequency if instead of trimmer capacitor I use 4pf ceramic capacitor
100mhz or above
100mhz or above
100mhz or above
srevankar11 month ago
Can i use inductor( which look like resistor) instead of that Coil?
You can use a 1muh resistor .get a frequency counter
You can use a 1muh resistor .get a frequency counter
Rfmhz1 month ago
In a nut shell?.come on pal you seriously think I'm going to grab my soldering iron and attempt to build this ?.i don't know if your trying to market or something here .no were in this earth is that a coil for transmitter ?and secondly you can use any trimmer capacitor on those ive built dozens of those .iadk your self how many components do you think it takes to be stable on fm ? And secondly by sounds of things when beginners go to attempt to build this will mostl likely be below the fm band so you need to clarify what length of coil is needed and well measured and also the need a frequency counter
jw.1 month 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!

cheeseyjazz1 month ago

whats the range?

ASCAS (author)  cheeseyjazz1 month ago

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

FoamboardRC1 month 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 ?
kushalverma1 month ago
I'm able to listen audio but with huge disturbances ....what to do now ??
YassarH1 month 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

ualbuquerque7 months ago
(removed by author or community request)

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)


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