The University of Michigan is on break right now, which means I need my ID card to get into my office building. It got me thinking about how the system is reading my card – Originally, the cards used a magnetic stripe, but a few years ago they swapped those for RFID, so I thought I'd look into how each of those works.
The magnetic stripe consists of a series of zones, where the field points up or down, corresponding to 1 or 0:
This is the same way hard drives store data. A read/write head moves over the zones sensing and changing their field as needed. This is why getting a powerful magnet near your card can scramble the data. The stripe begins with a start pattern that tells the reader how fast the card is moving, so changing speed mid-swipe can also cause an error.
RFID tags use radio-waves, which can be sensed from a distance. The trouble is, producing radio-waves requires power, and it would be inconvenient to attach a battery to every card. That's where RFID tags get clever:
The card reader, which is attached to a power source, sends out a wave that gets absorbed by the RFID tag. That wave carries enough energy to power a microchip that modulates how much is absorbed. The reader then senses how much of the power it sent out is lost over time, giving the series of 1s and 0s needed to transmit information.
When I was growing up, and dreaming of being a scientist, one of my planned inventions (aside from my mother's
"Beam me up" technology) was wireless power. It seems I'm
about 70 years late for that, but there's plenty more to discover!
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