Hacker News Reader

Comment by marcodiego | Hacker News Reader

I've got an idea!

I remember when CD's were becoming common, most car stereos didn't have a CD player. There was a kludge: "fake" tape which was actually a recorder head on a tape chassis connected to a CD player which transferred the CD player output to the cassette player: https://en.wikipedia.org/wiki/Cassette_tape_adapter

Now, my idea: a tape chassis, with a recorder head, with a built in battery and SD card connector. It could play MP3's directly to the recorder head and I could use old cassette players to play MP3's!

Now, some possibilities: if it is efficient enough, it could be powered by the cassette player motor. No need to re-charge! Another possibility: it could sense the direction and speed of the motor and use it as a UI to play, stop and fast forward/backward; no need of any button!

View on HN

Already exists

https://www.aliexpress.com/item/4000181019563.html

That requires a phone (Bluetooth) and the reviews say it requires charging (not low power).

Close, but not quite there!

They used to exist at least. There's not really a way to get around needing some sort of power.

https://usb.brando.com/usb-cassette-mp3-player_p01122c035d15...

How much power does it need? Could the winding mechanism provide the required power? Got to look up some specs on the maximum torque provided by the winding mechanism and the turning speed.

The capstan is where the maximum torque is, although the very slow rotation would need some gears in order to extract some power from a small dynamo; the tape rolls torque is very limited on purpose to barely keep the tape in tension without breaking it, and they would be unusable for this purpose. Also it wouldn't be easy to keep something in contact with the capstan to extract its energy, since the capstan/pinch-roller pair has been designed to work with tape, not pulleys or gears.

The hardest problem is hence mechanical; a repurposed small stepper motor could work great as generator, and the electronics involved beyond the standard Lithium charging chip would be minimal. But adding the necessary gearbox to this device would likely raise significantly the cost of production, which is probably the reason we didn't see it yet.

That makes sense. I agree that this is probably not cost-effective, but it could still be fun as a hobby project.

> Also it wouldn't be easy to keep something in contact with the capstan to extract its energy, since the capstan/pinch-roller pair has been designed to work with tape, not pulleys or gears.

The force could be extracted using a tape/ribbon similar to a real cassette tape, but one that loops around internal pulleys with a tensioner.

It would be probably not cost effective, but I would still be curious of the maximal power draw you could get from the capstan. I suppose we know the tape speed (2.4 cm/s according to Wikipedia), but we also need the maximal force the capstan can excess on the tape. I wonder if there are some specifications for maximal friction forces within a cassette tape to be conforming, so we can get a good guess at this.

2.4 cm/s translates to 41.7 mN / mW (millinewton per milliwatt). This is not counting for losses for converting the mechanical energy.

edit:

The force requires seems to be excessive. The lowest power draw I can imagine is ~0.1W for a BL device like this (probably generous), and then we would need 4N pulling force on the tape for that (still not counting losses), which is a lot. Anyway, this was a fun exercise.

"it's not working!"

"battery's going out- hit fast-forward for a few seconds to charge it"

TechMoan has a video taking this to its logical conclusion, using almost exactly the adapter you describe (a cassette format mp3 player) in a cassette to 8 track adapter inside a desktop 8 track player.

https://m.youtube.com/watch?v=ppo3IgHWDzA

I really like the idea of using the motor as a combined power source/user interface component! As the proud owner of a 2004 sedan, hacks to get digital audio on the original stereo are a common daydream of mine.

Maybe Bluetooth low energy could fit in that power envelope too?

There are already mp3 players in a cassette form-factor, with output via tape head and 3.5mm jack. I think some of them can receive bluetooth audio, too.

Yeah this has been a thing for at least 20 years.

I think OP is wanting to make something "for fun" rather than purely something to be practical.

The constraints of "senses motor direction" would feel _old school cool_ in a way that tapping "next" on your phone just doesn't!

It would also really drive home how much music fits on an SD card. One vital feature would be to save the current position, so you don't have to fast-forward for two hours again to find your favorite song.

Let's hope that car stereo has a tape counter...

Nice idea... but for those adapters, the sound quality is quite subpar as far as I know. I don't know if this can be sufficiently improved. At least, it is probably better than a FM radio transmitter.

Reply on news.ycombinator.com