Low-brake fluid sensor

DETAILED DESCRIPTION The present invention solves this problem by providing a liquid level detector using probes having low currents in the order of 1 to 5 microamps.
The probes are coupled to detector circuits having signal integrators for providing an integration of the signal over a time period to thereby avoid a false alarm due to a momentary shift in attitude of the vehicle or due to vehicle acceleraton.
The detector circuits are each coupled to a single signalling device such as a lamp to provide an alarm signal when the fluid falls below a predetermined level.
The present invention further provides for compensation of the circuitry over a wide range of temperatures such as in the order of -40.
degree.
C.
to +125.
degree.
C.
This is achieved by providing a reduced supply voltage to the probe and detector circuits and adjusting the detector threshold.
This has the effect of preventing a high potential from being applied to the detector circuits when the resistance of the fluid increases due to a reduction in the temperature.
Thus, the circuit prevents false alarms due to reduced temperature in the fluid.
The system is made failsafe by the construction of the circuitry to provide a signal if the probe or probe lead is broken.
This is attained in one embodiment by grounding the container in which the probe is inserted.
In this embodiment, the probe is effectively series connected through the fluid to the container.
In a second embodiment, designed to accommodate a nonconductive container, two probes are fixedly mounted on a nonconductive member and effectively series connected through the fluid.
If a probe or probe lead in either embodiment is broken, the resistance becomes effectively infinite and the signalling device provides an alarm signal.
A self-check circuit is operative to test a substantial portion of the remainder of the circuit by activation of the lamp to indicate proper circuit operation in response to external inputs.
The self-check circuit is further coupled to a monitoring system such as the standard automobile monitoring system to provide a signal in response to a difference in pressure in the master cylinders