Bluetooth earphones are quite nifty and very popular. In terms of convenience, if not sound quality, they're a big upgrade from their wired predecessors. You can listen to the audio of your choice without the wire catching on powerful machinery/door handle.
Perhaps the biggest downside of such products is the battery life. Since everyone likes their earphones to be as light as possible, they tend to be equipped with a piddly little 150mAh lithium cell that might get you 8hrs playback, at least when new.
Charging them frequently requires any standard micro USB, but, what if you don't have one handy? Sure you have one at home, but if you're at home, you probably don't need bluetooth earphones anyway! No, you're out and about, maybe on a 7hr flight on a 757 owned by an airline that did not tick the USB box on the last refit form. What do you have? Well, probably an earphone outlet, in your phone, in the aircraft seat arm, in someone else's phone, in the back of the hotel TV - lots of places.
Now, standard specifications on earphone/line outputs aren't as standard as they might be. This is because earphones themselves are wildly variable. So anywhere from 8-600 Ohm impedance. This means that at the low impedance, the amplifier must maintain the relative modulated voltage at reasonably high currents. Overall, there might be 0.4W available from a 1.7VRMS output. Even better, you don't care about signal quality, and things like aircraft probably have a lot of reserve power to account for potentially hundreds of simultaneous listeners. So you can probably cheat with a lower impedance circuit.
Next you need an opamp rectifier, tiny DC-DC regulator and you're away. A 5V DC charging signal from the audio output of anything.-- bs0u0155, Apr 22 2020 // an opamp rectifier, tiny DC-DC regulator and you're away. //
You don't even need that. You've got AC ... first, a small transformer to jack up the voltage, then a germanium full-wave rectifier, a capacitor, and a 78L05.
You could almost fit it all inside a fat 3.5mm jack plug.
[+] for the concept of harvesting "free" energy.-- 8th of 7, Apr 22 2020 Apple, Samsung, and Google all have wireless Bluetooth earbuds that fit in nifty little pocketable charging cases. Airpods, Galaxy Buds, and Pixel Buds respectively. Might consider giving them a look.-- 21 Quest, Apr 23 2020 // A 5V DC charging signal from the audio output of anything//
What, I should put on high fidelity cans to charge these newly circuited earbuds?-- wjt, Apr 23 2020 //fit in nifty little pocketable charging cases.//
But the cases just have a slightly bigger lithium cell in them. Kicking the can down the road if you ask me.
//I should put on high fidelity cans to charge these newly circuited earbuds?//
No, this is the clever part, you could be wearing your wireless earbuds while they're plugged in and charging. To outsiders, you'll look like you're wearing conventional wired earphones plugged into the audio out of an aircraft seat. But, YOU will know they're REALLY wireless because you're listening to the bluetooth audio of your phone. Although admittedly your wireless earphones would still have an actual wire, but YOU will know it's doing something different.
I'm sure the boffins in R&D could even add functionality by working out some kind of audio pass-through. If that works, the marketing dept. will have a field-day selling the lightweight battery-free model.-- bs0u0155, Apr 23 2020 Great link. From that: "the vast majority of mobile phones do not offer a standardized power and analog data interface. In this paper we show that it is possible to augment the ubiquitous headset jack with exactly this functionality"
Apple removes headphone outlet.-- bs0u0155, Apr 23 2020 //You might be 15 or 20 years late with this idea.//
I invented the turboprop in 1990, so at least the timing is moving in the right direction.-- bs0u0155, Apr 23 2020 The patent on the wheel's expired some time ago. If you can re-invent that and get a patent, you'll clean up.
Have Allison actually paid out anything to you for the turboprop yet ?-- 8th of 7, Apr 23 2020 //Did you really bring some innovation to turboprop design?//
I was 8. I thought it was a natural progression of the turbojet- turbofan line. So for that reason, propfans, turboprops, geared turbofans, all derivative and not really patentable...-- bs0u0155, Apr 23 2020 Of course not, because you'd be a minor, and your parents would have to co-sign the application.-- 8th of 7, Apr 23 2020 random, halfbakery