US 4,991,145 A · Infra-sonic detector and alarm with self adjusting reference

Abstract

A detector and alarm is provided for monitoring air pressure variations in a monitored space, which variations correspond to an intrusion. An infra-sonic detector includes a microphone or the like and a low pass filter to retain only low frequency infra-sonic signals or a bandpass filter to retain a particular range of frequencies between approximately 5 Hz and 12 Hz. The instantaneous infra-sonic signals are compared against a variable reference signal, which is a voltage lagging or delayed variant of the instantaneous signals. In this manner the detector is self adjusting for slow variations in the instantaneous signals but responds immediately to rapid variations to trigger an alarm. In one embodiment the detector only senses negative going air pressure changes to trigger an alarm in order to minimize the incidence of false alarms.

Claims8

Dependent claims

An infra-sonic detector and alarm system for detecting intrusion in a monitored air space comprising:

a transducer for sensing air pressure variations in the monitored space and producing an electrical air pressure variation signal;
amplifier means for amplifying electrical air pressure variation signal;
bandpass filter means for passing only infra-sonic portions of the electrical air pressure variation signal between approximately 5 Hz and 12 Hz;
a self-adjustable reference circuit for providing a self-adjustable reference signal in response to slow air pressure variations due to atmospheric phenomena,
comparator means for comparing the instantaneous infra-sonic signal portions of the electrical air pressure variation signal to said self-adjustable reference signal for generating a trigger signal when said instantaneous infra-sonic portions exceed the self-adjustable reference signal;
and
alarm means activated by said trigger signal.

4 more elements

An infra-sonic detector and alarm system for detecting intrusion in a monitored air space comprising:

a transducer for sensing air pressure variations in the monitored space and producing an electrical pressure variation signal;
amplifier means for amplifying the electrical pressure variation signal;
bandpass filter means for passing only infra-sonic portions of the electrical pressure variation signal;
comparator means biased in such a way that it is only responsive to the instantaneous and bandpass filtered infra-sonic signal portions of the electrical pressure variation signal corresponding to a negative going air pressure variation in the monitored space when said instantaneous infra-sonic portions cross a self-adjustable reference signal;
and
alarm means activated by a trigger signal.

3 more elements

Dependencies are inferred from the claim text.

Infra-sonic detector and alarm with self adjusting reference

Life of this patent

Infra-sonic detector and alarm with self adjusting reference

Life of this patent

Abstract

Claims8