Intrusion detecting security alarm circuit - has AC driven network comprising an integrator with adjustable working point controlling amplifier to signal alarm

摘要

The security alarm circuits is tripped by the breaking of its quiescent state criterion when the supervisory circuit signal drives the alarm circuit. The d.c. at terminals (31, 32) of the alarm circuit operates the oscillator with transistor (T31) coil (L 31p) and capacitor (C31) to derive a quiescent state a.c. voltage. The self excitation of the oscillator is due to feedback coil (L 31s), and the operating point of the transistor is determined by voltage divider resistances (R31, R32) while emitter resistor (R33) limits the d.c. current of the transistor. The output of the latter feeds a.c. bridge inductances L32-35 of which coils L32 and L33 form the sensing loops and the other coils act as the primary of a transformer with secondary L36. The voltage of the secondary is applied to amplifier (V) whose output indicates the quiescent of alarm state.