Abstract: A miniaturized card unit, resembling a standard credit card, incorporates solid state counter, a multiplexer means, and a small battery. The counter may be incremented or decremented in accordance with a direction control signal. A multiplexer in the card unit is releasably engageable with the demultiplexer provided in a card reader unit. Control signals, simultaneously applied to the multiplexer and demultiplexer transfer the count stored in the card unit to decoder driver circuits which drive a display for visually displaying the decimal count stored in the card. A logical circuit monitors selected segments of the display to enable operation of the device controlled by the card reader unit so long as the card inserted therein contains a non-zero count. The logical circuit prevents operation of the device upon the occurrence of a zero count as well as the absence of a card unit.