A Dead Simple, Well Constructed FM Transmitter

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[Angelo] is only 15, but that doesn’t mean his fabrication skills are limited to Lego and K’Nex. He’s built himself an amazingly well constructed FM transmitter that’s powerful enough to be received a quarter mile away.

The FM transmitter circuit itself is based off one of [Art Swan]‘s builds, but instead of the solderless breadboard construction you would expect to find in a small demo circuit, [Angelo] went all the way, etching his own PCB and winding his own coil.

Using photosensitized copper clad board, [Angelo] laid out the circuit with Fritzing, etched a board, and went at it with a drill. The components found in the transmitter are pretty standard and with the exception of the trimmer cap and electret mic, can be picked up in the parts drawers of any Radio Shack. He gets bonus points for using a 1/4 – 20 bolt for winding the coil, too.

The power supply for the transmitter is a single 9V battery, the battery connector being salvaged from a dead 9V. Awesome work, and for someone so young, [Angelo] already seems to have a grasp of all the random, seemingly useless information that makes prototyping so much easier. Video below.

Comments

  1. jpnorair says:

    Lower frequencies can travel a long way, and through walls relatively easily. Here’s a way to get the range much farther: attach a long wire to the ground of your circuit (e.g. 3m).

  2. cheapskate says:

    Nice build! Next step: build a SMD version that fits inside of a 9V battery connector.

    Warning for heavy tab users; the second link will autoplay an ad with sound (when you have a few hundered tabs open it can be a nightmare to find the one that’s playing sound)

  3. freddie says:

    asking here because i couldnt find any resources on google:

    here’s the schematic of the device http://i.imgur.com/ESFks72.jpg

    i get that there’s two amplifier stages to bring the signal’s voltage up, but why is there an inductor at the end? Is the inductor required to make the antenna transmit, and if so, how? Also why does changing the capacity of that last capacitor change the quality of reception, does changing the capacitance change the broadcast frequency, or is that cap have to be ‘in-sync’ with the inductor in some way to produce a higher quality signal?

    • jpnorair says:

      I haven’t looked at any schematics, but from experience I can guess that the large, air-core inductor is part of the antenna, as is the trim capacitor. The purpose of these is to make an impedance match at certain frequency. The large inductor also helps to reduce the required length of the antenna wire.

      • Franklyn says:

        You shouldn’t just reply then. The inductor and capacitor together form a hartley/colpitts LC oscillator that provides the carrier.

        • jpnorair says:

          That’s sounds like nothing more than an archaic name for what we would now consider an elementary LC impedance matching circuit. I know this to be true because every LC circuit is, in fact, an impedance matching circuit.

          • fre_ker says:

            No, that is not the same. You are talking about impedance matching. However in this case the inductor and capacitor forms an LC-oscillator which creates the radio carrier frequency. Adjusting the capacitor will change the resonance of the oscillator and thus the transmitted frequency.

          • Dodo says:

            Q1 amplifies the signal from the microphone. Q2 is the oscillator. The parallel L/C tank sets the frequency the system transmits on. The FM modulation is done in a dirty way. Due to the variable signal applied to the base the junction capacitances in the transistor are changed, changing the resonnant frequency of the tank, and thus modulating the output signal

    • RP says:

      The first transistor is an audio amplifier for the microphone. The second one is the oscillator. The inductor and trim capacitor form a “tank” circuit that resonates at the transmit frequency. The frequency is modulated by the deviation in resonant frequency caused by the signal from the audio amplifier.

    • franklyn says:

      Signals have to be at a high frequency to radiate, you can’t just connect the microphone output to the antenna and broadcast it. The inductor and capacitor along with the transistor form an oscillator that oscillates at a high enough frequency for transmission, this is what is known as the carrier frequency. The premise of Frequency Modulation is that changes in voltage are used to shift the frequency of the carrier to represent the signal, When the microphone signal is applied to the gate it changes the juntion capacitance of the transistor which will shift the frequency at which the oscillator runs thus producing an FM output.

      • Dax says:

        That’s technically untrue. Any signal will radiate, but the coupling from a wire to free air is very very weak.

        At 60 Hz, your antenna has to be about 4000 miles in lenght, but once you do, it will radiate. Same thing for speech. At a couple thousand hertz, if you set up an antenna about 60-80 miles long, you could technically transmit plain speech.

    • qwerty says:

      It’s not a two stage amplifier. Only the first transistor (left) amplifies the audio signal coming from the mike while the second acts as an oscillator. By injecting the audio signal into the base of the oscillator it changes operating frequency so that we have frequency modulation. The LC at the end is part of the oscillator and determines the operating frequency.

  4. DainBramage1991 says:

    Very nice build.
    That being said, I think the young man should check the laws of whatever country he’s in (assuming USA here, so FCC) to insure that he’s transmitting legally. I would hate to see a promising young hacker get into undeserved trouble because his project transmitter was a bit too powerful.

    • qwerty says:

      Those things are sold as toys everywhere just as the iPod transmitters. He should only pay attention not to transmit away from the FM commercial radio spectrum, as just above it we have civil airband.

      • Jan says:

        Those toys would be completely illegal in most of Europe, for example. Transmitting without license in a commercial band will get you tracked down and (heavily) fined. Most countries don’t have exemptions for these low power devices like in the US.

      • Tony says:

        The transmit frequency can be set to be a little above or below the FM band, the gaps between the coil windings determines that.

        An interesting point that was made in the original designer’s kit notes is that you can set the frequency to be between stations, and therefore won’t be picked up by the average digital radio – they only scan in jumps, eg 10,15, 20 so you broadcast at 17.5.

        • dutado says:

          You can’t hide WBFM transmitter. Most FM receivers have really bad selectivity.
          Even setting the receiver 100 kHz away may result in a satisfactory reception.

          • DainBramage1991 says:

            Also, not everyone is confined to regular off-the-shelf FM receivers. I have one that is accurate down to a single Hz, and another that is precise down to 10 Hz. Both cover the FM broadcast band, as well as other bands. I can pick out a spurious transmission with ease, as can many thousands of other people.

            It’s far better to simply operate legally, than try to hide illegal operations.

    • gregkennedy says:

      In the US, devices like this typically fall under FCC “Part 15″ rules for unlicensed, intentional transmission. IIRC, you can actually go farther with AM than FM, because the measuring rules are different – AM limits are based on input power to the antenna (which gives you some freedom in antenna design), while FM simply measures field strength post-transmission. Quoting Wikipedia here:

      “On the standard AM broadcast band, transmission power is limited by 100 milliwatts of DC input power to the final RF stage (with restrictions on size, height and type of antenna), or, alternatively, under 15.221, if the AM transmission originates on the campus of an educational institution, the transmission can theoretically be any power so long as it does not exceed the field strength limits stated in 15.209 at the perimeter of the campus, 24000/fkHz µV/m.

      Unlicensed broadcasts on the FM broadcast band (88 to 108 MHz) are limited to a field strength of 250 µV/m at a distance of 3 meters from the antenna. This is roughly equivalent to 0.01 microwatts. Emissions must be kept within the 88.0 to 108.0 MHz band under Part 15 rules.”

  5. Hirudinea says:

    Nice, wonder what he’ll come up with next week, I’m sure he’ll be on here again.

  6. Me says:

    Nice! I remember when circuits like this were very common in electronics magazines. It’s good to see it here!

    Something I have been wondering for a while is if one of these simple 2 or 3 transistor FM transmitter designs could be adapted for APRS. It would have to have it’s frequency raised (144.39 in North America) and it would have to have it’s modulation changed to NFM instead of WFM. I suspect range wouldn’t be a problem due to altitude.

    I’m pretty sure that wouldn’t be too hard for someone with just a bit more knowledge than myself. What I don’t know is if thre frequency would be stable enough. I’ve thought about digging up one of my old magazines and trying it but I’m not quite knowledgable to do the conversion yet.

    I’ve wanted to see someone do this for quite a while now. I think it would be useful for things such as near space balloons. I met someone at a maker ‘show and tell’ sort of meeting once. He was ranting and ranting about how much he hated ham radio. His main gripe was that he felt it was too heavy and too expensive when he used to put APRS in a balloon. He switched to Zigbee and loved it.

    I don’t get why people have to put a full HT in a balloon to do APRS but they can get a more complicated radio like Zigbee on a chip! Something isn’t right about that. An Arduino can do APRS. Combined with a little transmitter like this that wouldn’t be too bad.

  7. Alexandre Detiste says:

    I saw the same thing at the Stasi museum in Berlin

  8. RoboMonkey says:

    Nicely done. I’d suggest using more than one wire color, and testing the connections and functionality prior to applying hot glue. Also for the coil, he should use insulated wire OR coat it once installed.

    But it’s better than I can manage at the moment, well done.

  9. Armani says:

    Anyone want to guess how long the battery will last if constantly transmitting?

    • Greenaum says:

      Well, it’s not particularly high-power. Really doesn’t matter though, nobody’s going to start up a pirate radio station running off the top of a 9V battery. Though it would make you hard for the police to catch, your listeners would have to ring you up and arrange for you to broadcast from a nearby room when they wanted to hear you.

      It’s just a toy!

      • Mystick says:

        Replace the mic with a receiver with an AF stage and you have a microrepeater that could be swarmed in numbers over a large area to increase the output signal footprint to get around the Part 15 rules. You may have some intermod problems, but that could be tweaked with precision placements. Maybe power it with a small solar panel in a small enclosure box.

  10. Whatnot says:

    We’ve seen this before not so long ago, we should now get to the ‘simple analog TV transmitter’ phase.

    • Whatnot says:

      Or even digital terrestrial TV, if chips for such exist. that are cheap and available to the public..

    • NewCommentor1283 says:

      to transmit B&W with NO sound:
      1) get a tv from 1988
      2) extract tuner
      3) open cover of tuner and attatch one wire
      4) power tuner with safe, floating-ground OR ground-referenced-ground !!!
      NOT THE TV’s INTERNAL POWER, it is most likely a death-ground
      4) use a second tv and tune into the black screen
      5) probe around til you can inject NTSC, carefull not to send the DC back into your VCR … tuner might use 18,24, or even 32 volts
      6) enjoy

      • Whatnot says:

        NTSC eh.. that would require a costly import.

        And VCR’s used to have tuner blocks to send out an RF signal to a predetermined channel, which people could tune slightly with a screwdriver. So if you get one of those old VCR’s you should also be well on your way, you have a tuner that can transmit so all you need is a booster, and I’m sure a replaced capacitor can shift to any frequency.

        Thanks for the reminder/idea though. I bet it’s fun to do analog pirate TV these days because nobody has an old analog tuner connected to an antenna, so it would not be noticed except by a few tech freaks.

        • Greenaum says:

          And all the millions of users of digital TV and cellphone frequencies you’d be stomping over. Using a wobbly noisy TV modulator as a source!

          Anyway without wanting to be horrible for several paragraphs, you’ve got the germ of an idea, but in practice your idea won’t work for quite a few reasons. TV transmitters need very stable frequency sources, and what you refer to as a “booster” is really the whole transmitter.

          Lastly, the old analogue frequencies are still very much in use, half now for digital TV, the other half are being sold of to mobile phone companies. They’ve finally finished jiggling the frequencies around in most countries, so that all the digital TV is at one end of the frequency band, while the other end is for phones and other use. So transmissions there would DEFINITELY be noticed!

      • Greenaum says:

        WTF!? How’s that a TV transmitter? “attach one wire”? And where does the VCR in step 5 come from, and what friggin DC? Are you from Mars, or is this a small cutout from a 150-page TV repair manual with no context?

        “death-ground”!?!?!?!

        Shit, there’s two point 4’s!

  11. HackJack says:

    Very elegant design. I like it!

  12. rfengr00 says:

    The old joke about RF circuits needed a fingertip to work was taken literally.

  13. strider_mt2k says:

    I remember simply attaching a roughly tuned long wire antenna to a TV modulator and getting a little transmission distance with it back in the day.
    I loved these kinds of circuits and still do.
    Well done!

  14. zaprodk says:

    Kudos for the nice, small build, but please, get this kid a real soldering iron, that firestick is no good!

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