Motorola Corporation

Charles Gerston

Thomas M Fox

Edmund C Golbeck

A body detector (10) detects when a body (12) is absent from a region (26) being monitored. The body (12) has a different dielectric constant than air. The region (26) is configured as a capacitor (20) in which the body (12), if present, serves as a dielectric. The capacitance of the capacitor (20) is measured (28). This measurement influences a baseline value (30). The capacitance is measured (38) a second time. The second measurement is checked to verify that it is within minimum (42) and maximum (46) values. In addition, it is checked to verify that it is within a more narrow range (54) that is defined in response to the baseline value. The second measurement is then used to adjust the baseline value and the process repeats. The range (54) within which the capacitance must stay slowly adapts to changes in the capacitance of the capacitor (20). Abrupt changes in the capacitance cause an alarm to be annunciated.

Body detector

Amplifier circuitry includes a differential pair of transistors having commonly connected electrodes driven by a normally constant current supply transistor. Portions of the output signals developed by the differential transistors tend to vary with variations in the magnitude of a first power supply potential. A control circuit causes the magnitude of the normally constant current source to vary so that the other excursions of the output signal are substantially equal in magnitude to the first power supply potential to provide balanced signals across the load. A feedback circuit stabilizes the magnitude of the normally constant current supply transistors against variations in the magnitude of another power supply potential or in the constant current supply.

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Scottsdale, AZ, United States of America

Charles Gerston