GB2231660A

GB2231660A - Detecting entry into an enclosed space from air pressure changes

Info

Publication number: GB2231660A
Application number: GB8904827A
Authority: GB
Inventors: David Mainprize
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-03-02
Filing date: 1989-03-02
Publication date: 1990-11-21
Also published as: GB8904827D0

Abstract

A pressure sensor or microphone 19 is used in combination with an electronic circuit in order to detect the movement of a person or intruder within or into an enclosed space which may be a vehicle or building. Changes in air pressure caused by the movement of said person across flexible floors or by the movement of doors is changed into an electrical signal by the pressure sensor. The electronic circuit includes a frequency filter 21 which separate the required signal from other frequencies which may be present. The filtered signal is compared to a reference voltage at 23 such that when a signal of sufficient amplitude and in the chosen frequency range is detected an alarm or other electrical system is activated. The reference voltage is continually adjusted to match the background noise level. <IMAGE>

Description

AIR PRESSURE INTRUDER DETECTOR This invention relates to a methodof detecting the entry of a person into an enclosed space which may be a building or vehicle.

Many methods are in use which detect the entry or presence of a person and particularly an intruder in order to activate an alarm.

One common method is to equip all doors and windows with switches connected by wires to a central control unit. Such a system is difficult and expensive to install due to the amount of wiring required.

Another common system employs micro-wave radar or ultra high frequency sound in order to detect any movement within the space being monitored. Such systems are easy to install but are expensive due to the transmitters and receivers required.

This invention relates to a device which detects the small change in air pressure which occurs within an enclosed space as a door is opened or closed and thereby operates an alarm or other electrical system.

The movement of a door draws air into or out of the enclosed space, wherein the said device is situated, causing a small change in air pressure. The enclosed space may be a vehicle, a single room or a whole building. Similar changes in air pressure may also be detected which are caused by the movement of a foor as a person walks across it, the foor acting as a large flexible diaphragm. The invented device employs 2 suitable pressure sensor or microphone to convert the small change in pressure into an electrical signal. The pressure change persists for only a short time and the resulting electrical signal has an oscillatory transient nature with a predominantly low frequency content. The pressure sensor will also detect other pressure changes which are not required to activate the device.

Low frequencies may be caused by wind turbulence. Audible and ultra sonic sounds are produced by numerous sources and may also be detected by the pressure sensor. The invented device employs a band pass filter such that the device will only respond to a small range of frequencies corresponding to those produced by the opening of a door. This range of frequencies should preferably be in the range liiz to 10 Hz to avoide un-wanted activation by frequencies produced by source other than the movement of a floor or door.

One example of the invention is described with reference to figure 1.

An air presssure sensor 1 is connected so as to provide an electrical signal on line 2 relative to a signal reference line 3. The signal on line 2 is connected to a band pass filter 4. Said filter has a characteristic to pass a narrow band of frequencies and attenuate all other frequencies. The output of the band pass filter is connected by line 5 to the non inverting input of a voltage comparator 6. A reference voltage is connected to the inverting input 7 of said comparator such that the output line 8 takes up a negative or low voltage when the voltage on line 5 is less than the voltage on line 7. The pressure sensor is placed in the enclosed space which is required to be monitored for the presence of a person or intruder. In the absenceof such a person the pressure sensor 1 may detect many extraneous signals.The band pass filter 4 will prevent all signals exept a narrow band of frequencies from reaching line 5.

The reference voltage at input 7 is arranged to be greater than the signal due to background noise at line 5 such that the output 8 remains low or negative. When a door of the enclosed space is opened or closed a significantly larger low frequency signal is produced by the pressure sensor in response to the change in air pressure.

The band pass filter characteristic is arranged to coincide with this low frequency signal such that it is passed to line 5. This increased signal then exceeds the reference voltage at input 7 causing the output 8 to change to a higher or positive voltage.

This change of voltage at the output 8 is then used to activate an alarm or other electrical system.

A similar example of the invention is described with reference to figure 2 which shows the band pass filter in detail. The pressure sensor 9 produces an electrical signal in response to any change in air pressure as descibed in the previous example. The band pass filter comprises of an amplifier 10 in combination with a signal input path through resistor 11 and capacitor 12 and a feedback path through capacitor 13 and resistor 14. This band pass filter arrangement is well known and the components are chosen so as to pass signals of the required frequency but reject all other frequencies.

Signals of the required frequency appear at the filter output 15 and are conected to the input of the voltage comparator 16. As in figure 1 the signal at 15 is compared to a reference voltage at input 17 so as to produce an output at 18 only when the selected frequency signal from the band pass filter exceeds a predetermined level.

A further example of the invention is described with refererence to figure 3. A pressure sensor or microphone 19 produces an electrical signal in response to any change in air pressure as previously described. This signal is connected to the input of a band pass filter 21 by line 20. The band pass filter is arranged to attenuate signals at frequencies outside the range whilst passing the required frequencies to the output at 22. Said band pass filter may also amplify signals of the required frequency or may be used in combination with an amplifier. The output signal at 22 is connected to a voltage comparator 23 which compares the signal level to a reference at line 26 which is the output of a low pass filter 25. The comparator 23 is arranged such that the output changes from negative to positive when the signal at 22 exceeds the reference voltage at 26. A positive voltage at the output 27 is connected via diode 24 to the input of the low pass filter 25 such that under steady signal conditions the reference voltage at 26 is increased to match the signal at 22. Thus the reference voltage is continually adjusted to match the background noise level. A sudden increase in signal, as may be caused by an intruder, will cause an increased voltage at output 27 since the low pass filter cannot resond to sudden changes in signal level. As in previous examples this output is used to activate an alarm or other electrical system.

Claims

1 A device to detect the opening or closing of a door or other apperture associated with an enclosed space, which may be a vehicle or building, or the movement of a person within said space by means of detecting the low frequency changes in air pressure so caused.

2 A device as in claim 1 wherein a pressure sensor or microphone is employed to convert pressure changes into electrical signals.

3 A device as in claim 1 wherein an electronic filter is used to separate a required signal from signals of other frequencies.

4 A device as in claim 1 and claim 2 wherein a threshold reference level is pre-set such that signals above said level cause an output from the device whereas signals below said level cause no output from the device.

5 A device as in claim 1, claim 2 and claim 4 wherein the threshold level is modified according to the prevailing background signal level such that the background signal level does not cause an output from the device but an output occurs due to sudden increase in signal level caused by a person entering or moving within the enclosed space.

6 A device as in claims 1 to 5 which is employed to activate an alarm or other electrical system.