Self-balancing alarm system

Claim 1

A detection circuit for use in an alarm system comprising: a security loop having a plurality of sensing means adapted to be located at respective entries into a zone under protection; voltage producing means; alarm means; a responsive circuit adapted to respond to a change in condition of said security loop to enable said alarm means from a disabled condition; said security loop connected between said voltage producing means and ground to provide a predetermined voltage level at an input to said responsive circuit when said security loop is uncomprised; said responsive circuit including means for linearly balancing a change in voltage at said input from said predetermined voltage level which may be bi-directional to maintain said alarm disabled except when said change equals at least a threshold change in voltage from said predetermined voltage level or said change occurs more rapidly than it can be balanced; said balancing means comprising a pair of amplifiers connected together in negative feedback fashion, delay means connecting the output of one amplifier to an input of the other amplifier, and the input of said one amplifier comprising said input of said responsive circuit; two comparators, each said comparator having a pair of input terminals, means providing a reference voltage at one input terminal of each comparator, the other input terminal of each comparator being connected to said output of said one amplifier, said reference voltage at said one input terminal of each comparator differing from the voltage level at said other input terminals by a substantially equal positive and negative value; said reference voltage providing means comprising two independent sources of a constant voltage potential, voltage divider means connected to each independent source, and each said one input terminal connected to a respective one of said voltage divider means; one independent source of a constant voltage potential comprising a regulated source of D.C. potential, and said voltage producing means including said regulated source; and said other independent source of a constant voltage potential comprising an unregulated source of D.C. potential, a transistor, the base of said transistor connected to and controlled by said regulated source, while the collector being connected to said unregulated source, and said other independent source of voltage potential being derived at the emitter of said transistor.