Intruder alarm which detects pressure variations in protected space - has microphone and detector sensitive to pressure changes with low-pass filter to eliminate interference and threshold comparator.

摘要

The system detects the opening of a window or a door in an enclosed space. An electric microphone and associated integrated circuit (IC1) and resistors (R1, R2, R3) produces a variable signal. A filter (IC2, R4, R5, C3, C5) eliminates sounds above 1OHz of speech or footsteps etc. The signal is amplified and biassed (IC3, R7, R9, C5) since the pressure wave may be positive or negative according tot he direction of opening and the conditions. The signal is compared with high and low reference voltages in two comparator circuits (IC4, IC5, R8, R13, R14, C6) so that the output changes when the signal passes the fixed limits. ADVANTAGE - Simple installation, no adjustment, favourable performance demonstrated in laboratory tests.