Electronic protection and sensing apparatus

Claim 1

A sensing system comprising a sine wave oscillator, a bridge network coupled to the output of said oscillator and having a positive output channel and a negative output channel and including means for balancing the bridge when said system is operated, an antenna coupled to said bridge and said oscillator, said antenna causing said bridge to produce an output signal when its electrical field is disturbed, a first two-channel differential amplifier coupled to said output channels of said bridge, a demodulator coupled to the outputs of said first differential amplifier and to the output of said oscillator for comparing the outpuT of said oscillator with an output signal from said bridge and producing an error signal therefrom, a filter coupled to the output of said demodulator for filtering the output error signal therefrom and producing a DC output signal, a plurality of differential amplifiers in cascade coupled to the output of said filter and receiving said DC output signal therefrom, the output of said plurality of differential amplifiers being coupled (1) to display means for providing a visual representation of the magnitude of said DC output signal, and (2) to control circuit means for producing a control signal from said DC output voltage, said control signal being usable to carry out a control function, said display means including a ladder circuit connected to the output of said plurality of differential amplifiers and having one portion for generating signals responsive to positive error signals and a second portion for generating signals responsive to negative error signals, and a series of lamps coupled to said ladder circuit and including a center lamp which represents a null condition, a first group of lamps on one side of said center lamp for glowing when a positive error signal is present and a second group of lamps on the other side of said center lamp for glowing when a negative error signal is present, a first auxiliary lamp associated with said first group of lamps and a second auxiliary lamp associated with said second group of lamps, both auxiliary lamps representing an emergency condition, said control circuit including a first differential amplifier having its input connected to the output of said plurality of differential amplifiers and having its output coupled both to said first auxiliary lamp and through an OR gate to an amplifier chain coupled to, and operating, means for turning off apparatus protected by said system, and a second differential amplifier having its input connected to the output of said plurality of differential amplifiers and having its output coupled both to said auxiliary lamp and to said OR gate and to said amplifier chain for operating said means for turning off said apparatus protected by said system.