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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

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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Can I mod it for frequency band 118 - 136 MHz (Aeronautical Com band)

As a pilot I can't imagine what use it would be on aeronautical freq.? Probably illegal? Interfering with these bands could be disastrous.
As a Pilot, it is recommended that one may carry an additional handheld radio to facilitate comm in case of a radio failure. Those handhelds off the shelf are sometimes way too expensive.

Disastrous nature depends on the nature of the user. Many transceivers are sold online and at pilot shops, and can be easily bought for 300 to 500 dollars, or maybe less for the refurbished ones.

Now, how about a receiver on the said band (not a transmitter). Any tips? :)
another major problem, is that most aero frequencies are AM, this is an FM unit

can the antena/wire copper?

FarhanH1 made it!5 days ago

i made it but its not working... i think maybe the transistors are placed the wrong way.. as in your pictures they are facing the inductor coil. plz help me... Thanks ;(

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fritend19 days ago

Is there someway I can modify this so I can fine tune the frequency easier?

i made it ;)
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i made it :)
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Ludgy9 days ago

I built this using smd parts but can't get a crystal clear signal.
The radio picks up the signal but it's very garbled and noisy.
Going to try a different inductor and antenna length and will report back.

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Adam222212 days ago
Instead of using a 4pf can I use a 5pf because that's the only one I can find and also instead of using a 40pf trimmer capacitor can I use a 50pf trimmer capacitor
Hackerbr13 days ago

Hi, can I replace the 2N3904 General NPN Transistor by a BC547B? ThankYou!

GERALDB223 days ago

Sir, Im in the Philippines and I am only a student.. Please help me..

I can use 100pf trimmer capacitor for 30pf capacitor... Because no available component in my city.. I recycle the 100pf trimmer capacitor for my old radio....
Thank.. You.. sir

Put a 47pF capacitor in series with the 100pF trimmer.

ahhh.. Thank you sir... :) : ) :)

Polymorph14 days ago

Use a cheap RTL-SDR to check the frequency as you tune this.

kees.drullman made it!4 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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Hi there kee.drullman! I'm interested in building your version of this. I know most of the components are obvious but what specifically did you use for L1 inductor? Can you tell me the value of the component?

it says 6 turns with 4mm diameter. Play with spacing to get it in band with the trimmer capaciter at about halfway.

ASCAS (author)  kees.drullman1 month 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)  SirajH1 month 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".

so say i want to make it a personal radio station just for fun (yes i know i could just build a better, more powerful curcuit) can i extend my entenna over 8 in. would that help the range

Thanks, very amazing project! And of course non expensive.

But what about if I want to scramble the audio and de-scramble at the receiver in order to use it as title says "spy bug"??? Thanks
Yar MuhammadT2 months ago

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

same problem, cant find 4 pf in my countrie

MichaelL6225 days ago

I am noticing some inconsistencies. The photo of your board does not match the pcb layout. I noticed your photo shows the 10k resistor and the 10nf cap opposite of the pcb layout (next to the variable cap). there is not mention of the 4pf cap on your pcb layout either.

HI, i am trying to build one but i cant find the 4 pf cap in my countrie , is that really necessarie? (srry for the bad inglish)

i hope he corrects that

WilliamG2316 days ago

and ..... i cant find a 4pf ceramic capacitor in my country , can i use a 5pf? or any alternative?

WilliamG2316 days ago

Hi

can i put a mp3 plug insted a microfone?

2illickans16 days ago
Please help me with this project .I dont have a pcb board so can I do this on a Breadboard
MichaelL6219 days ago

Built exactly as the plans call and it just doesn't work. Transmits but the sound is distorted and weak.

AJMansfield21 days ago

Now this is just with using through-hole fabrication, just imagine how small you could make this using surface-mount parts...

H2Okee21 days ago
You could extend the range with a more efficient antenna.

Nice Project....

Can anybody share the results of noise and range???

I really want this circuit but my minimum requirement is 30 feet rang and NOISE FREE COMMUNICATION, so please share the results with me.

DwayneS24 days ago

nice! I built this exact thing back in college in 1998... good to have the plans here again if I need them.

2illickans24 days ago
will I be able to make this project using a breadboard .

This is a very good instructable for making small pc boards. As such it is great for beginners to electronics. Someone who wants to learn. The wire is just silver coated hookup wire. Copper will work as well. The difference in conductivity is too low to be of any concern. Just use the correct size. The parts including the trimmer cap are available from Mouser.com or digikey.com. About 2 bits. :) 25 cents. As well as a dozen other suppliers in the US. I know there are also electronics suppliers in the UK. Other countries you'll have to look.

I don't know about shipping from mouser to other countries, but I can order small packages for shipping of less than $5. I would look on UK or Australian Ebay.

However; if you just want an FM transmitter this is a cheap one that should be at least as good as this one. http://www.canakit.com/mini-wireless-fm-transmitter-kit-ck105-uk105.html

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