Simple FM transmitter that shows off its own circuitry

[Sean Michael Ragan] built this FM transmitter which shows off its circuitry via a clear plastic dome. The device is electrically identical to one we looked at in September. That version championed a construction method that used small squares of copper clad as solder points which were each super-glued to a large copper-clad platform serving as a ground plane. [Sean] is using a printed circuit board that was laid out by Sonodrome. You can check out their own glass-jar transmitter build where the board artwork is available for download.

One of the tips we enjoyed from [Sean's] step-by-step build is the coil wrapping. He used the threads of a 1/4-20 bolt to guide copper wire as he wrapped a total of four turns. Once the bending is done, just unthread the bolt to separate it from the coil and gently stretch the wire for a 12mm distance between the two leads. Not only is this visually pleasing, but it will help with transmission clarity.


  1. DocDawning says:

    Wow, that looks sooo beautiful. Looks like a fairly simple circuit to build too.. ooooooOOOooo! :)

  2. Very nice looking! I’ll have to dig up my old Ramsey kit and see what differences there are. The schematic on Sonodrome looks simple enough, but I would like to redraw it for clarity. I’ll post back when I do ;-)

  3. dan fruzzetti says:

    I’m so afraid this ability will be lost if an apocalypse occurs a couple generations *after* the digital transition…

  4. Drone says:

    I like the enclosure.

  5. Larry says:

    would think bending the aerial into a fractal, would be better for transmitting

  6. Slanderer says:

    …why would you think that? Without lots of simulation, there is no basis for this.

    Otherwise, while a neat circuit to throw together, it looks pretty shaky. Without a separate transistor to buffer/preamplify with microphone (yes, I know it contains an internal FET buffer), performance can suffer. Although capacitive coupling from the body/metal objects/etc is partially alleviated by being enclosed, the antenna can still capacitively couple to stuff, and without a separate buffer for it, this can possibly shift the frequency too.

    Disclaimer: I’m not hating on this, I’m just saying better FM transmitter circuit topologies exist at this complexity level.

  7. Lee Enfield says:

    So, the coil is 1/4in X 12mm?

  8. Thanks for covering this, Mike, and thanks to your commentariat, here, for the nice feedback. I’m a little intimidated by the Hack a Day audience, honestly, because I know so little about EE.

    Lee: The inside diameter of the coil is supposedd to be 6 mm, and the leg-to-leg distance 12 mm, covering 4 turns of the coil. Even the minor diameter of a 1/4-20 bolt is a bit bigger than 6mm, but it does seem to be close enough for the coil to work pretty well. Without attaching an antenna, it transmits a good 30 feet through open air.

    I hope to add an antenna, at some point, and see what kind of range I can get, although I think at that point I technically need a license to operate the thing. Hope to fiddle with it until I can plug it in in my office and beam my tunes to FM radios all over the house. But not much further than that. =]

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