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The spinning heads of the tape player apply torque to make the tape move by the reader head. But that torque is limited. If the tape reaches its end, or the friction becomes too large, so that the spinners have a hard time spinning the spools inside the cassette, then most modern cassette players have a function to spit out the cassette, or reverse direction. A cassette internal power generator would use that torque to produce power, the power output is proportional to that torque. So, one would have to figure out the allowable torque the cassette internal generator can take from the spinners, before the player thinks "end of tape" and spits out the tape. There is probably a spec out there for that. Once you'd know that available torque from the spinners, you could then estimate the power produced by the cassette internal generator. That would be the power available to the Bluetooth circuitry. Maybe it would be enough power, don't know :-) You might not even need a battery. Cassette goes in, spinners start generator, generator powers & boots Bluetooth circuitry, stays like that until you hit stop. But first figure out the torque available from the spinners, it all depends on how much torque is available.
I know timing is late but perhaps this might help someone -- It seems that Heibaige has something that might work for this purpose, I have not yet ordered it but is the only one I have seen claiming to charge while playing. https://www.amazon.com/HEIBAIGE-iTape-Cassette-Bluetooth-Receiver/dp/B079JPQJ4M
I have been looking into adding a Bluetooth adapter to my existing car stereo. I have done some research and found a few, but they all require you to charge them first. I don't want to waste an outlet that I could be using to charge my tablet and my phone (for wifi hotspot)
I was thinking it would be possible to use the spinning heads of the tape player in the car to generate a charge and store it in a battery while it is actually being used.
- It should be able to support A2DP streaming audio if possible.
- It should fit the form factor of an existing cassette tape, but could protrud out the access port if requried (i.e. for battery or other electronics).
- It should be able to be self powered (trickle charge the battery from spinning heads of the player)
Is something like this possible?
This picture show my Nexus 7 mounted with a ProClips adapter. USB cables are connected for powering the tablet and phone (not shown, but just out of view of the image). Old school cassette adapter for streaming audio and podcasts. Car is a 05' VW Passat.