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Comment by insane_dreamer | Hacker News Reader

> The cards have no battery and aren’t self-powered in any way. Instead, the reader on the gate emits an electromagnetic field that powers the card just long enough to complete the transaction.

Brillant!

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This is how all passive RFID works. It's pretty cool. There's even systems that are both passive and active. For example, as a Disney adult, I'm familiar with their magic bands which do have a battery but also work passively. For tying ride photos automatically to you the battery matters because you're either moving too fast or are too far from the camera for passive to be consistently effective. But for simply using it as payment or a ticket you're pressing the device up against a reader and no matter how old the battery is it'll continue working.

At a first glance passive RFID would be a perfect match for ride photos, the whole mechanism is ridiculously fast so speed of the ride would not matter much (the technology works at highway speeds just fine) and can work over ~15 meters.

But there are two somewhat significant issues with the radio layer in this application: the geometry of tag antenna is somewhat critical (making it into a band would be problematic) and the back scatter is attenuated by salt water blobs (ie. people) almost perfectly.

> The cards have no battery and aren’t self-powered in any way. Instead, the reader on the gate emits an electromagnetic field that powers the card just long enough to complete the transaction.

Brillant!

AT&T showed this in television commercials back in the 1980's.

"Can you go through a toll booth without stopping? You will. And the company that will bring it to you is AT&T."

The 90s https://en.wikipedia.org/wiki/You_Will

Lots of those things happened (maybe not faxing from the beach), but AT&T didn't bring us very many of them.

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