About two years ago I decided I was going to figure out a cheesy inductive power coupling circuit. I sat down one Sunday, started plugging some stuff together and this is what I ended up with:
This is the circuit I brought with me to the "bring-a-hack" dinner at Harry's Hofbrau that Jeri Ellsworth organized on Sunday night. You can see that the LED in the jar is lit- the jar is really just a gimmick and the little parasite works better outside the jar.
The circuit is very simple: the IC in the middle is a 12F683 that I programmed up to be a square wave generator (someday I'll blog that project, but for now, I'll just say that the oscillator frequency is jumper selectable and it can be set to either update constantly when powered or set and forget the output frequency and duty cycle) running at 50% duty cycle and (approximately) 71kHz. The square on the right is a bit of ~28ga magnet wire wrapped a few times around four screws. Power is delivered to it by a transistor- the coil gets driven by the full umph of the 9V while the PIC runs on the 5V regulator to the left.
Efficiency is TERRIBLE. Because I'm driving the transistor hard with a square wave I lose energy into the higher-order harmonics, and then I'm half-wave rectifying the output with the LED. If I made the drive waveform more sine-like and added some joule-thief circuitry to the parasite I'd see much better results. But then, this was never intended to be a huge project, just proof-of-concept.
One last thing, if you're thinking of doing this yourself: every hand-wound parasite is going to have a slightly different optimal frequency. The circuit as I made it was basically an LED, a 100nF capacitor and a coil all in parallel. Depending on the resonant frequency of the LC circuit, you'll see better results at different frequencies.