Abstract: A method of conducting an economic transaction includes the steps of wirelessly receiving a card-transmitted signal, where the card-transmitted signal includes user account information and an indicator of an amount of elapsed time between a recognition of a vibration applied to a wirelessly-transmitting credit or debit card and a transmission of the card-transmitted signal from the card. The method further includes determining a last detected vibration at a point-of-sale device, calculating an amount of time elapsed since the identification of the last detected vibration, and comparing the amount of time elapsed since the identification of the last detected vibration with the amount of elapsed time between a recognition of a vibration applied to a card and a transmission of the card-transmitted signal from the card.