A monitoring circuit for determining the state of a bi-state device

摘要

A monitoring circuit 1 for monitoring the state of a bi-state device such as a hall effect sensor 2 for determining the state of a seat belt buckle of a motor vehicle comprises a regulated voltage power supply 7 for applying a five-volt supply to the sensor through a primary transistor 8 and a current sense resistor 10. A comparator 12 compares the voltage applied to the sensor with a threshold voltage reference for detecting an over current condition in the sensor. A secondary transistor 14 is responsive to the output of the comparator 12 for switching off the primary transistor 8 in the event of an over current condition in the device. A microprocessor disables the secondary transistor for a settling time period. The microprocessor 5 determines from the comparator 12 and the current sense resistor (10) an over current condition and the current being drawn by the sensor. The microprocessor may select one of four resistors R1 to R4 of a current emulating circuit for developing a current through the current emulating circuit (3) which emulates the current drawn by the sensor, and a monitoring circuit monitors the current. A seat sensor comprising a force sensitive resistor is provided for locating in the seat of a vehicle for determining the presence of an individual on the seat, and the force sensitive resistor is monitored by the microprocessor (5).