[exelius]

You could do the same with an inductive sensor wired to the headphone jack. This worked for Square to read magstripe data, which is significantly more complex of a signal to parse than simply counting the number of local maxima per second.

Inductive sensors are cheaper, simpler and more foolproof; cyclists have used them for decades to gather both wheel speed and cadence.

[treebeard901]

Slightly off topic: You make an interesting point about Square. In the context of Apple Pay, the move to take away the headphone jack suddenly makes a lot of sense (for Apple).

[exelius]

I don't know about that; the cheap Square reader is a piece of crap that is really just a "starter" product (they're not well-made and fall apart after even moderate use). Anyone who swipes more than 2 or 3 cards a day would be better served by one of Square's commercial readers -- all of which connect via Wi-Fi, Bluetooth or the Lightning port.

Also, Apple would certainly include a Lightning->Headphone adapter that would also be usable for card readers. They just use the line in from the microphone.