Abstract: This invention relates to methods for making a credit/debit card purchase without revealing the card number to the vendor of services or goods. The card holder is connected to a data base and provides the card number, plus holder identity verification, to the data base. The data base then verifies whether the card holder is authorized to incur the expense of the purchase, and, if so, provides an authorization indication or code to the vendor; the card number cannot be derived from the authorization information, thus helping to preserve the secrecy of the card number.

Abstract: In an electrical circuit where power supply interruptions may cause significant adverse consequences, a secondary battery is often used to maintain power when the load voltage falls below a predetermined threshold. MOS transistors interconnect primary and secondary voltage supplies with the load. A sequence control circuit assures that only one MOS transistor is on at any time. In one embodiment of the invention, different time constants cause all MOS transistors to turn off before changing voltage supplies. In another embodiment of the invention, voltage comparators, each having a different threshold, cause all MOS transistors to turn off before changing voltage supplies. A capacitor maintains the load voltage during switching and positive feedback assures that the switching interval is brief. This arrangement is particularly advantageous in low power applications utilizing one or more lithium batteries.