Protective circuit for analog sensor multiplexer number - has photon-couplers contg. LED's which are connected to analogue sensors, and which produce light that is intercepted by photo-transistor

摘要

The circuit operates in conjunction with a multiplexer containing a number of low-impedance analog sensors. The circuit detects and indicates the occurrence of an open circuit in any one of the sensors, which each include a piston-coupler with a LED. The light emitted by each diode is intercepted by a photo-transistor connected through resistors of high value across the related sensor input line to define a network generating a small offset-current. Should an open-circuit occur, the offset current charges the capacitor of a noise-rejection filter connected to each sensor to a high level, with a polarity opposed to the normal polarity established there across by the sensor. As a consequence, the digital value produced by an A=D converter is well outside the valid range, and provides an indication of the abnormal condition.