Solid-state threshold detector

Abstract

An apparatus for providing a warning when the magnitude of a monitored parameter, such as vehicle fuel level, is within a critical range. A charge pump repeatedly charges a variable capacitor having a capacitance progressively changing with the magnitude of the monitored parameter and a reference capacitor having a capacitance equal to the capacitance of the variable capacitor when the magnitude of the monitored parameter is at the threshold of the critical range. After each charge, the two capacitors are discharged at a controlled rate with their voltage charge being monitored by threshold detectors, each of which provides an output when the voltage across the respective capacitor exceeds a specified level. The outputs of the threshold detectors are coupled to a logic circuit which compares the duration of the outputs of the threshold detectors and generates an output when the duration of the output from the level detector monitoring the reference capacitor varies from the time duration of the output from the level detector monitoring the reference capacitor in a sense representing a critical magnitude of the condition. An indicator is responsive to the output signal from the logic circuit for providing a warning of the critical magnitude of the condition.