| 摘要 |
<p>1,246,264. Intruder alarms. ELLIOTT BROS. (LONDON) Ltd. 29 Nov., 1968 [30 Nov. 1967], No. 54626/67. Heading G4N. [Also in Divisions H3-H5] An alarm system comprises a linear array of geophones 12, alternate geophones (or groups of geophones) being connected to respective circuits 13, 14, adjacent geophones in each circuit being in antiphase so as to minimize false alarms due to distant noise. The system preferably includes a pulse discriminating circuit responsive to the pattern of human footfalls but immune to long bursts of traffic noise, to single heavy bursts and to the footfalls of small animals. After amplification at 15 . . . 17 and possible pulse lengthening, signals above a predetermined amplitude trigger a detector 118 which sets a bi-stable 122 the output of which triggers a monostable 125 via a gate 123. Monostable 125 opens a gate 126 for a period T1. An output from bi-stable 122 also releases a clamp 121 on peak detector 120 so that threshold detector 119 is triggered when the input signal falls to a predetermined percentage of its peak value. This action resets bi-stable 122, which triggers a monostable 128 producing a standard length pulse. Rejection of pulses of excessive duration is achieved by monostable 125 closing gate 126 at the end of period T1. Rejection of pulses of low repetition rate.- Standard length pulses from 128 are fed to a leaky pulse integrator 131 which triggers a Schmitt circuit 132 after a number of pulses at a sufficiently high rate. Rejection of pulses of high repetition rate.- Monostable 130 drives OR gate 127 for a period necessary to set a rechargeable monostable 129 which closes gate 123 and opens gate 124 for a period T2. If bi-stable 122 is set again within a time T2, its output is rejected by gate 123. Gate 124 also drives OR gate 127 which holds 129 in the charged condition as long as 124 is held open by bi-stable 122 being in the set state. A further link 133 allows bi-stable 122 to be reset by 125 if detector 119 should fail to trigger. The loop 124, 127, 129 may be replaced by a differentiator inserted between 122 and 123. Modifications.-In Fig. 3 (not shown), stages 15 . . . 17 are followed by separate detectors feeding a common chain of a limiter, Schmitt trigger, monostable, pulse integrator and second Schmitt trigger feeding an alarm-operating relay.</p> |
