Cards use a simple LC circuit. When a card is presented to the reader, the reader's electrical field excites a coil in the card. The coil charges a capacitor and in turn powers an integrated circuit.
The cards employ a straightforward LC circuit. Upon presentation of a card to the reader, the reader's electric field stimulates a coil within the card. This coil is then charged by a capacitor, which in turn supplies power to an integrated circuit.