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FM transmitters can be complicated to build, but not this one — it’s about the easiest you can possibly make. And though the science of radio is well understood, there’s a magical, emotional quality about it that we don’t often stop to appreciate. You will not forget the first time you pick up a transmission broadcast from a device you soldered together, yourself, from a few bits of copper, carbon, plastic, and wire.  

I am indebted to Jim and Kat of Sonodrome for first introducing me to that experience, through this very circuit, which I first built on a pre-etched PCB from a kit they offered for sale as recently as 2011.

This design was originally popularized by Japanese multimedia artist Tetsuo Kogawa. The circuit itself is a slight variation on Kogawa’s simplest FM transmitter design, and the method of building it is sometimes referred to as “Manhattan style.” It uses a piece of copper-clad circuit board but, rather than etching the circuit traces through the copper layer, a large piece of continuously-plated board is used to make all the circuit’s ground connections, and small sections of plated board are glued to the surface to form nodes or “pads” that are insulated from ground.  Besides being a convenient way to assemble circuits using minimal tools, this building method encourages you to think about circuits in an interesting way — as groups of connections that are either grounded or “floating above” ground.

kogawa_simplest_transmitter

This transmitter uses ten on-board components and will transmit a monaural audio signal about 30 feet. It is possible to extend that range by adding an antenna, and Mr. Kogawa’s website has more information about how to do that.

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NOTE: Depending on where you live, operating an FM transmitter — even a very short-range one like this — may be illegal without a license. Unless you attach an antenna, it’s very unlikely that anyone will notice or complain about any transmissions you may make with this device. On the other hand, it’s very difficult to predict, before construction is complete, just where on the FM band this transmitter will broadcast. Use due caution during testing, and make sure you understand the law in your area before attaching the battery.

Steps

Step #1: Form the coil.

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  • Strip about 4" of 18AWG solid copper wire and wind 4 turns around the threads of a 1/4-20 bolt.
  • Turn the coiled wire off the bolt as if you're unthreading a nut, and clip each lead to about 1cm.
  • Bend little "feet" on the ends of the leads and adjust them so the coil will stand upright.
  • Holding a pair of pliers in each hand, grab the coil's leads and stretch it evenly along its length until the feet are 12mm apart on center. You may need to even out the coil spacing with a screwdriver or other tool.

Step #2: Cut the board.

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  • Use a straightedge, a utility knife, and the edge of a table to score and snap a 5cm × 4cm rectangle from the copper-clad board. This will be your ground plane.
  • Score and snap a 5mm × 5cm strip of copper-clad board, then score it crosswise at 5mm increments. Snap along these lines, with pliers, to create several 5mm × 5mm "pads." You only need 5, but you may want to make a couple extra.
  • Smooth the corners and edges of the ground plane and the pads with a small file. Be especially careful to remove any sharp copper burrs that might cause cuts on handling.

Step #3: Mount the coil.

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  • Apply a small drop of cyanoacrylate glue to the underside of one of the pads. It doesn't take much. Use tweezers or small pliers to carefully position it in the center of the ground plane. Wait a few seconds for the glue to set.
  • Glue a second pad to the board, above and to the left of the first, along a line running at about 135° with respect to the long centerline of the board, as shown. Position the second pad along this line so that there's about 12mm between the centers of the first and second pads. Wait a few seconds for the glue to set.
  • Solder the coil across the 2 pads as shown. This will be easiest if you pre-tin the surface of each pad, and both coil feet, before applying heat to reflow the solder and join the tinned areas.

Step #4: Add capacitor C3 and resistor R2.

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  • Solder a 0.01μF ceramic disk capacitor (C3) between pad 2 and the ground plane, and trim away any excess leads. It doesn't especially matter where you connect to the ground plane, for this or any other connection in the project.
  • Glue pad 3 to the board somewhere below and to the left of pad 2, as shown. You want enough space between pads 2 and 3 to fit the body of a 1/4W resistor.
  • Solder a 1/4W 27K resistor (R2) between pads 2 and 3, as shown. Trim any excess leads.

Step #5: Add the electrolytic cap, resistor R1, and capacitor C2.

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  • Glue pad 4 to the ground plane just to the left of pad 3. Space the pads to match the lead spacing on your electrolytic capacitor. Solder the electrolytic cap (C1) between pads 3 and 4, making sure the negative (–) lead is connected to pad 4.
  • Solder a 10K resistor (R1) between pad 3 and ground.
  • Solder a 0.01μF ceramic disk capacitor (C2) in parallel to the 10K resistor between pad 3 and ground.

Step #6: Add the transistor.

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  • Bend the transistor's 3 leads, as shown.
  • Glue pad 5 to the board directly to the right of pad 1. Make sure it's close enough to pad 1 that one of your transistor's leads can reach between them.
  • Solder your transistor across pads 1, 3, and 5, as shown. The collector connects to pad 1, the base to pad 3, and the emitter to pad 5. Trim any excess leads.

Step #7: Add the 10pF caps, resistor R3, and the battery clip.

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  • Solder one 10pF ceramic disk cap (C5) across the transistor's collector and emitter, (i.e. between pads 1 and 5), and a second 10pF ceramic disk cap (C4) between pad 1 and the ground plane. Trim any excess leads.
  • NOTE: For more convenient adjustment of the transmitting frequency, replace the 10pF capacitor (C4) between pad 1 and ground with a 20pF variable or "trim" cap. If you use a variable cap, the frequency can be adjusted simply by turning the trimmer shaft with a small screwdriver.
  • Solder a 470Ω resistor between pad 5 and the ground plane. Trim any excess leads.
  • Connect a 9V battery clip to the board, as shown, by soldering the red lead to pad 2 and the black lead to the ground plane.

Step #8: Attach the phone plug.

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  • Unscrew the threaded housing from the tip-shield (TS) mono phone plug and set it aside. Solder a 4" length of red stranded wire to the center "tip" contact, and a 4" length of black stranded wire to the outer "shield" contact.
  • The shield contact has built-in prongs that can be crimped over onto the wires to provide strain relief for the solder connections. Use small pliers to fold these prongs over and crimp the wires beneath them, being careful not to crimp so hard you damage the wire insulation, bend the tip contact onto the shield contact, or otherwise short the 2 connections.
  • Slip the threaded housing over the wires and tighten it onto the plug threads again. Solder the free end of the red wire to pad 4, and the free end of the black wire to the ground plane.

Step #9: Tune it up!

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  • Attach your 9V battery to the battery clip and insert the phone plug into an audio source like an MP3 player or smartphone. Start a song or other easily-recognizable audio track playing, then turn on your radio and scan through the FM band to locate the transmission.
  • TIPS:
    • Start with your receiver right next to the transmitter.
    • A digital tuner with precision down to 0.01MHz may be helpful.
    • Be patient and careful. Scanning is a bit tedious, but if you get impatient you may miss the signal altogether and mistakenly believe the transmitter isn't working.
    • If you scan the entire band and can't locate your signal, try changing the orientation of your receiver's antenna with respect to the board and scanning again.
    • It's best to run your audio source on battery power when you are first isolating the transmitting frequency. If you have to run it from mains power, make sure electrically noisy devices like fluorescent lights, TVs, and computer monitors are not active on the same circuit at the time.
  • You can tune the transmitting frequency by changing the spacing between turns in the coil. Closing the spacing will lower the transmitting frequency, while opening it up will raise the transmitting frequency. You can also use a variable capacitor for tuning (see Step 7).

Step #10: Mount the battery.

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  • NOTE: This transmitter design needs very clean, smooth power, which is one of the reasons we choose to run it from a battery. Power from a "wall wart" or other AC adapter is smooth enough for most DC applications, but not for this radio transmitter. Using an AC adapter to power this transmitter is likely to cause the signal to be too noisy to use.
  • Use scissors to cut a strip of hook-and-loop fastener (velcro) tape to fit the length of your 9V battery.
  • Separate the hook and loop sides of the tape, remove the backing from each, and apply the hook (scratchy) side to the bottom of the transmitter board. Apply the loop (fuzzy) side to one of the battery's 2 largest faces.
  • Attach the transmitter to the battery using the velcro during use. When depleted, the battery can be removed and separated from the transmitter for recharging.

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


Comments

  1. chuck says:

    Cool circuit. This has a lot of potential for serious fun.
    Throw a silent rave- with a small transmitter and antenna you can broadcast music to your party. Have your guests bring headphone radios and have a silent dance floor. Bonus points for broadcasting different music styles on different frequencies- it looks super cool to have folks dancing at different speeds and styles.
    Do a pirate drive-in. Use a digital projector and a laptop to show a movie on a conveniently placed wall in an empty parking lot. Broadcast the audio over the radio transmitter, invite your friends and re-live the glory days of drive-in theaters.
    Create walking tours and locational radio art. Build several transmitters on the same frequency. Set them in an area or along a path. Use the individual transmitters to broadcast parts of a story tied to the location, describe sites along a walking tour or broadcast a location specific ‘soundtrack’ to your park or campus. As the listener moves along the path or area they pick up the transmitters in succession. Set up a spatially connected choose-your-own adventure story in your neighborhood where route choices affect the story being broadcast. Depending on how far apart you space them you can get glitchy signal overlaps and noisy resonance and things.
    The FCC doesn’t really care about small hobby circuits like this as long as there are no complaints. With less people listening to radio or watching broadcast television, there’s little chance of causing interference that will lead to complaints. Don’t be rude or stupid, avoid large or weird antennas and don’t try to be the next Radio Caroline and everything will be cool.

  2. Roy Shearer says:

    I made a PCB version of this transmitter and made laser cut housings for it – check out http://www.openthing.org/products/niftymitter/

    Have used them regularly for art installations and within the house as a wireless speaker system, great fun!

  3. David says:

    the law is the same every where, because it is a Federal Law, that says what you can use to broadcast, and that you must be licensed to send voice over the air, would expect that data would fall under the same category. This from first hand knowledge, I built an FM (may be was AM back in the day, think this was before FM) I sold it to a friend and he set up his own neighborhood radio station, and before he knew it the FCC was knocking on his door….lol

    1. willnonya says:

      You assume everyone is in the US.

  4. ibrahim says:

    I want to know how a FM transmitter works. Where can I find information on that.

    1. teiresias says:

      If you happen to see a necropost: the bookstore at ARRL.org is a good place to start.

  5. medyum says:

    thanks for sharing. good video!

  6. Ral says:

    I found some good information on the FCC website concerning low power radio transmission. It looks like that from standpoint of the FCC that this device would fall into the “Part 15 Devices” category and not require a license.

    http://www.fcc.gov/guides/low-power-broadcast-radio-stations

    I would assume that the FM transmitters that you buy in the store to play audio from a media player through your car radio (for cars that do not have an AUX input) would also fall into this category.

  7. ahoule says:

    how do you set the frequency

    1. TerryT says:

      The frequency depends on the inductance of the coil and the value of the 10pf capacitors. The easiest way to adjust the frequency, for example to move away from a strong local broadcaster, is to slightly squeeze or stretch the coil.

  8. S_Hennig says:

    Thanks for reviving some memories. Decades ago I built lot’s of prototypes that way. The only thing I did differently: I used a Dremel to make isolated pads instead of glueing bits of PCB. That way I did not have to wait for the glue to set. Cheap and easy and works up to VHF (well, obviously ;-)

  9. stinker says:

    i better want to know how this circuit is working…

  10. stub says:

    On a stereo jack, the tip of the jack carries the right channel and the ring of the jack carries the left channel. Would it be better to use a stereo plug (tip/ring/sleeve) and solder the tip and ring together in the plug, rather than allowing the mono plug to connect the left channel to ground in the mp3 player’s jack?

  11. Shawn says:

    Great tutorial, but I think you should include HOW the circuit works as well. It would be nice to see the explanation behind the design of the project.

  12. ria says:

    its cheap and easy to build circuit, i will try it.

  13. deojo says:

    Can i remove audio jack, and replace it with electret microphone

    1. stub says:

      I haven’t seen ANY replies from the author here. So I thought I’d chime in. I think an electret mic requires some power. Also the mic signal will be weak and might require some (pre) amplification. But I may be wrong about that.

      1. Elmo says:

        We have tried to put an electret microphone, but the sound quality is not good, but you will hear a significant sound coming out of the receiver .

        1. deojo says:

          please share your result here, if you get way to fix it. :D

  14. Levi says:

    I put this together and for some reason I still can’t find it on the radio and my 9v seems to get hot pretty fast. Any advice would be great thanks!!

    1. John says:

      I imagine that you have shorted the battery’s red and black lead somehow. Use an ohm meter or continuity checker between the terminals of the battery plug (with the battery unplugged). You should see a very high resistance value or an open. If not then check your wiring. Watch for solder shots from the pads to the ground plane.

  15. John says:

    Why use a mono plug? I understand that the transmitter will only transmit monophonically, but with a mono plug you will only transmit one channel from the MP3 player (left or right but not both) loosing part of the music. The better choice is to use a stereo plug and run the red wire to both the left and right pins (black wire to ground as shown). Then you get both channels transmitted!

  16. scott says:

    i built the transmitter as well, how does the amount of turns on the coil affect the signal ….is more better or less

  17. AJ says:

    I’ve built three of them and none of them worked. I triple checked everything and I connected everything correctly. I did use double-sided copper as that was all radioshack had. Is that OK? Troubleshooting?

    1. AJ says:

      When I touch one end of the coil I can sometimes get a clear signal to 107.3. Please help.

      1. Pvt. Parts says:

        Yes I had this but on the transistor E leg, Turs out C2 ground leg did not get solderd well… I did get signals in 89.7 and 99.9 but it is not clear at all :(

  18. Pvt. Parts says:

    Any suggestions on how to make it more clear on sound? when just voice is getting transmitted no problem, but when the music comes in it sounds like a chainsaw

  19. Aditya Vardhan says:

    Where Should I Install The Antennae please tell me

  20. aadil says:

    what+is+the+value+of+the+electrolytic+capacitor.and+also+tell+me+values+of+all+componants.

    1. stub says:

      You can look up the RadioShack parts and get the values from there.

  21. JT says:

    Built on breadboard. Not functional.
    How is carrier wave produced?

    Thx

    1. stub says:

      Seemed like the copper ground plate was pretty specific. Do you think your breadboard is functioning the same way (ground-plate-wise)?

      1. Vish says:

        I want to connect this transmitter to the tv rca output and i tried it but its not working plz help me…..
        Thanx

  22. Dwayne says:

    You should try making an IC type transmitter.

  23. Tibi says:

    I build a lot of these transmitters when I was a kid. Had a lot of fun playing with it baut it sucks that they’re so sensible to changes in temperature and mechanical shock. Every time you touch it or even get close to it, it changes frequency and you have to tune in to find the broadcast. Having a crystal in the equation makes it stable but those circuits are a lot more complicated to build.

  24. can an Inductor of predefined value of certain inductance be used instead of the coil? I couldn’t find an 18 gauge solid copper wire but I have a 1mm diameter copper wire, will that work?
    If an inductor can be placed instead of a coil what value must it have?
    Thank You

  25. willnonya says:

    The text suggest that an AC power source would be too noisy for this, what would you think about using an automotive 12v dc source or USB to power this?

  26. vish says:

    I want to connect this transmitter to the tv rca output and i tried it but its not working plz help me…..

  27. Todd says:

    Could this design be adapted to broadcast in stereo?

  28. Andrzej says:

    I have question to you. How can I connect antena to this circut. Thanks.
    P.S awesome project

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