Electronic monitoring system with malfunction indicator

Abstract

A contactless motion detector responsive to the approach of a ferromagnetic element, including an oscillator with a sensing coil, has an output lead connected to one input of a first Exclusive-OR gate having another input connected to a square-wave generator whose pulses appear in the output of that gate with or without phase inversion depending on the output voltage of the motion detector. These pulses are fed, via an amplifier, to one input of a second Exclusive-OR gate whose other input receives the output pulses of the square-wave generator directly but with a delay compensating a lag introduced by the first gate and the amplifier. In the absence of a malfunction, the inputs of the second gate are energized either cophasally or antiphasally to produce either a low or a high output voltage of substantially constant magnitude fed to a load; in the event of a malfunction interrupting the pulse transmission to either input of this second gate, its output becomes a pulsating voltage. Such malfunction can further be indicated by an integrator connected in parallel with the second gate to the amplifier output. A conductor connecting the output of the square-wave generator to the corresponding input of the first gate is preferably led inside a housing around the motion detector, in front of its sensing coil, in order to let a damaging physical force aimed at the detector disrupt that conductor for giving rise to a malfunction indication.