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Comment by 998244353 | Hacker News Reader

I wonder why they chose 3.5mm in the first place instead of making an USB thingy. Just because it used to be more universal compared to like 5 different standards for the charging/data port? There are some theories online but no definitive answer as far as I can tell.

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The 3.5 jack device required no digital hardware. It was literally just a magnetic read head wired directly to the jack. This generated voltage just like a microphone. The phone app would then “listen” for the cards data. Incredibly clever and dirt cheap hack for making a phone into a portable payments device.

Universal has got to be it. Afaik, you couldn't plug something into a iPhone charge port and use it with an app; you probably still can't?. Doing it on Android would be a nightmare. Other platforms (which wasn't out of the question at the time), who knows.

But every platform will let you use a plugged in mic, and likely detect that one is plugged in. And if the bias voltage for the microphone isn't enough, you can play a sine wave to the headphone contacts (or one of them) to deliver power.

USB wouldn’t have worked with iPhones, so they would have needed separate hardware for android and iPhone. I’m amazed they were able to do anything with the 3.5mm port though, it seems like some clever engineering

The microphone part of the jack provides a small amount of power to serve as a microphone bias. This can be used to power a few small analog electronics as needed. The mag stripe reader may not have needed anything but a reader coil, though, and the phone can probably decode the "sound" that comes from swiping the magnetic stripe.

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