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.

8 thoughts on “Simple FM transmitter that shows off its own circuitry

  1. 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 ;-)

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

  3. @Larry
    …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.

  4. 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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