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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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AmriFyesterday

Hi ASCAS, i'm sorry i made that simple fm transmitter as like yours, but doesn't work, the receiver doesn't receiving the sound from transmitter. For board i am using hole PCB not printed PCB. How to fix this? please reply to my email or any air band.

Thank you.

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AmriFyesterday
ChandanZ6 days ago
Very Good Project!!!
Txer20 days ago
Excellent got this to work ,however I don't want to use elec mic ,how could I use this for direct input from a female jack to plug phono straight in .not interested in headphones ideas .i simply away of making it work with direct sound from phono plugs thanks any help be great thanks .
rpotts2 Txer6 days ago
Just substitute the plug for what you want. Reread the little FAQ at the end
JohnreyA7 days ago

where do you place a 4pf

davideddu8 days ago

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

Thank you!

george172 months ago

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

Txer george1720 days ago
No I used it on a breadboard from maplins.works fine
DebojitK made it!1 month ago

Hi,

I built it but it does not work as such. Some alterations of the spec:

1. coil wire gauge-1 mm ~ 18 AWG

2. bolt dia-5mm. had 8 turns

3. Trimmer seems to be 40 pf because the middle leg got 3 connected wire terminal, dont know the exact values, but 22 pf also gives same result.

4. power wires are little longer -4/5", should not make any issue I think

5. the oscillator transistor's collector and + line are adjacent, so the coil and trimmer parallel connection required some jumper connections.

6. Dis it on a veroboard.

Problem- I can tune within 95-105 mhz but the response area for a particular frequency is very short, also very difficult to adjust. But it highly depends on my two hands' position around the circuit. Even if I touch any part of the circuit it seems to respond but in that case i have to touch that part very faintly. All in all, the frequency is not getting stabilized. Range test is very far idea for me :(

Receiver-cell phone fm receiver with earphone. Adjustable per .1 mhz.

Can anyone assist me please. This is very frustrating!!

Thanks,

Debojit

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Txer DebojitK20 days ago
Hi use 22awg on 5mm drill or even better grab Alec keys they say the mm on each one .wrapp around 5mm, 7 turns .30pf there's your fm radio .
DebojitK1 month ago

Hi the frequency is not getting stabilized. Whenever i am removing the screwdriver or moving myself the signal is lost.

Txer DebojitK20 days ago
Use a plastic one .it will stay tuned

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)  jankeifer3 months ago

+1

Txer ASCAS20 days ago
Ascas how do you do it with direct input.i don't want want to use elec mic ?

Hi,

I am facing the same problem. How did you fix the issue?

For my case I confirmed that there is no dry solderings...

kees.drullman made it!27 days 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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spatuladle1 month ago

where did you read that

jfrancis141 month ago

Within the USA it is illegal to operate on these freqs above 0.01 microwatts without a license.

https://apps.fcc.gov/edocs_public/attachmatch/DOC-297510A1.pdf

davast1 month ago
I dont have 4pf what is the subtitue
I've tried it several times but always failed please help me

Is it possible to use a TRS line in as the input instead of a microphone?

bry.carling1 month ago

The trimmer has three terminals for physical rigidity on the board. Only two need be connected. The one at the top (middle) is one side and the two others that are on the sides are just connected to each other internally (physically) Check with an ohm meter if you are not sure.

Tanmayg1 month ago
The trimmer capacitor has three terminals but why are we using only two? Please help I am new to electronics
gadgettron95 made it!1 month ago
great project! but I couldn't get enough range like u made...
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blake22622 months ago

if you don't have a 4pf cap, can you use 4x 1pf caps in parallel? Or will a 3.9pf do?

you can use 3.9pf but don't use 4x 1pf. Keep that in mind: Your frequency scale will change wildly.

E92 months ago

all these are good but can you send me the length and width of the board

m.hemanth2 months ago
Hi I didn't get a 22 swg wire I tried with inner conductor of a dish( co axial wire) . It seems not working. Help me
davast2 months ago
I do not have 2n3904 There may be a replacement for this transistor, Please let me know a substitute on this transisitor
vricsi94 davast2 months ago

Any general purpose transistor will do it.

Corky1232 months ago
I tried making this but I have some problem. When i turn it on, the sound on my receiver becomes silent. When i tap the mic, I can hear the tapping, it's loud. When i speak or apply audio on the mic, it is still silent. What could be the possible problem? Could it be the mic?

Thanks in advance :)
ziame2 months ago

Is it okay to use a FM variable capacitor instead of trimmer capacitor? since there is no trimmer available.

sheraz142 months ago
what happened with your finger?
george172 months ago

what was the total cost of the project?

Regie made it!3 months 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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DhananjeyanN3 months ago

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

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