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 micr

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.