This invention relates to an alarm system and has particular, although not exclusive, reference to an alarm system for a vehicle.

Different types of alarm systems are known for use with vehicles, the majority of these being wired into the vehicle electrics and operable to sound an alarm when a particular condition is sensed which may be, for example, physical movement of the vehicle or switching _ on of the ignition circuit. Although such systems function generally satisfactorily, they are relatively expensive to install by virtue of having to be connected into the vehicle electrics.

Accordingly, it is an object of the present invention to provide an alarm system which is self contained and which may be portable so that it can be used in a n_tmber of different environments.

According to the present invention there is provided an alarm system comprising a pyroelectric sensor of the single element type, electrical circuit means operable to receive the output signal from the sensor and provide an amplified output signal, audio alarm means connected to receive the amplified output signal and be energised thereby, and power supply means operable to supply electrical power to the sensor, electrical circuit means and audio alarm means, the sensor, electrical

circuit means, audio alarm means and power supply means being mounted in an opaque but not fluid tight casing, and the electrical circuit means being electrically screened, whereby, in use, the pyroelectric sensor is sensitive only to certain changes in pressure of the ambient atmosphere.

Thus, the present invention provides a self-contained alarm system which is sensitive to certain changes in ambient pressure, such as the opening or closing of a vehicle door, but uses a pyroelectric sensor modified so as not to be sensitive to infra-red radiation which is the normal function of such a sensor. Preferably, the alarm system is in the form of a unit detachably mounted on a mounting plate which is attached, for example, to the fascia panel of an automobile, whereby the unit can be detached from the base plate and used elsewhere, for example in a room to detect unauthorised entry or in a briefcase to detect unauthorised opening thereof. As with all alarm systems, it is necessary to provide the authorised, user with a means for rendering the system inoperative and to this end, a first embodiment may employ a key-operated mechanism, the alarm being rendered operative by means of a push button; it is convenient to use the same key-operated mechanism to allow the alarm unit to be detached from the base plate when such is provided.

A second embodiment may use a remote, eg. radio control device to allow the user to render the system inoperative, while still using the key-operated mechanism to separate the alarm unit from any base plate. In the second embodiment the radio control device may also be used to render the alarm operative.

In addition it is useful to provide a booster unit to be fitted in a vehicle's engine bay to enhance the effect of the alarm by flashing the indicators or sounding a siren which may be fitted if desired.

For use with such a booster unit the alarm unit may be provided with connectors such that it can also receive power from an external source, and/or connectors such that signals may be carried to and from the booster unit in the engine bay system if the alarm is used in 4 vehicles. The portable alarm unit may still be detached and used in other environments. The engine bay system may 'be wired into the vehicle's own battery power supply, and is also wired to the vehicle's indicators, and when the alarm sounds, the engine bay system flashes the indicators and sounds the siren. Preferably where such external connections are to be made one set of connectors may be provided on a base plate and another set on the alarm unit such that when the alarm unit is mounted on the base plate, the sets of connectors engage one another, thus making the necessary connections.

An alarm system in accordance with the present invention, will be described in greater detail, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a block circuit diagram of the alarm system, Figure 2 is a top plan view of the system, Figure 3 is a front view of the system. Figure 4 is a side view of the system, Figure 5 is a section on the line V-V of Figure 3, Figure 6 is a section on the line VI-VI of Figure 3, Figure 7 is a plan view from below of the system, Figure 8 is a section on the line VIII-VIII of Figure 7, Figure 9 is a plan view of a mounting plate for the system of Figures 2 - 8,

Figure 11 is a side view of Figure 9, and Figure 12 is a plan view of Figure 9.

Referring first to Figure 1, this is a block circuit diagram of the basic alarm system which comprises a pyroelectric sensor 1 of the single element type housed in a casing which is apertured in order to expose the sensor element to changes in pressure of the ambient atmosphere. The output signal from the sensor 1 is connected to electrical circuit means 2 which amplifies and filters the sensor output signal and supplies it to an audio alarm 3. The components 1, 2 and 3 receive power from a power supply unit 4 which is battery-operated, whereby the alarm system is entirely self contained. The components 1,

2 and 3 are also mounted on a ^' printed circuit board 5 which is seen in Figures 5, 6 and 8. The sensor 1 and the circuit 2 may both be potted (indicated at 6 in Figure 1) in order to minimise the effects of temperature changes, and are also electrically screened (indicated at 7) in order to prevent false triggering of the alarm system which may be caused by switching of electrical components in the immediate vicinity of the alarm system, the operating of radio communication systems such as used by taxis, for example, etc.

Turning now to Figures 2 - 11, these illustrate the physical construction of the alarm system, with the components 1, 2 , 3 and 4 being housed in a casing 8 to provide an alarm unit. The casing is of two-part, moulded construction, having a base portion 9 and a top portion 11, the two- portions being secured together by bolts 12 as seen in Figure 8. The casing 8 is moulded from a synthetic plastics material such as ABS, for example. The printed circuit board (PCB) 5 containing component 1, 2, 3 and 4 is mounted in the top portion 11 of the casing 8 and batteries for energising the power supply unit 4 are contained in a compartment 13 in the base portion 9 (actual batteries not being shown) and the various electrical components mounted on the PCB 5 are accommodated in further compartments 14 and 15 in the base portion 9 of the casing 8.

The top portion 11 of the casing 8 comprises a curved acoustic screen 16 received in a recess in a curved top plate

17. The audio alarm means 3 comprises more specifically a piezo-ceramic buzzer 18 mounted centrally of the top portion 11 of the casing 8 behind the acoustic screen 16. Associated with the buzzer 18 is a Helmholtz ^' resonator comprising resonating chambers 19 and a central aperture 21 in the top portion 11 of the casing 8. The acoustic screen 16 also forms part of the

Helmholtz resonator. The sensitivity of the device is adjustable through an aperture 22 (Figure 2) in the acoustic screen 16, access being gained therethrough to a potentiometer forming part of the electrical circuit means 2. In the first embodiment, the alarm system may be armed by an arming button 23 provided on the front of the top portion 11 of the casing as seen in Figure 3 of the drawings. As best seen in Figure 5, the arming button 23 is in the form of a plunger which operates on and temporarily closes electrical contact 25 on the PCB 5. The terminal portion of the contact 25 has a dimple to prevent the tip from damaging the PCB and to aid movement of the contact.

In order to disarm the alarm system, a key (not shown) is inserted into a keyway 26 which is located alongside the arming button 23 as seen in Figure 3 of the drawings. The key acts on a contact 30 which has two metal strips, each pushed into engagement with a contact on PCB 5 by the key. When both the connections are made, the contact 30 is short-circuited and this operates a switch to interrupt power supply. When the alarm system is armed, an LED 27 is energised, the LED being located

on a corner of the top portion 11 of the casing 8.

In a second embodiment, the alarm system is armed by pressing a button on a radio transmitter (not shown) which may suitably be in the form of a key fob. A receiver located within the alarm unit picks up the encoded signal and operates an arming switch. The LED 27 is energised. Disarming is also by means of pressing the button of the radio transmitter in the same manner. The radio transmitter has a range of less than 15 feet, to minimise the chance of interference with any other nearby equipment.

The casing 8 is detachably mounted on a mounting plate 29 which may be permanently secured to a structure forming part of the environment to be protected by the alarm system, for example the fascia panel of an automobile. The base portion 9 of the casing 8 is provided with two recesses 29 which receive corresponding brackets 31 on the mounting plate 28, .and provided with four spacers 32 which engage the mounting plate when the casing 8 is mounted thereon. To mount the casing 8 on the mounting plate 28, the brackets 31 are located in the corresponding recesses 29 and the casing slid relative to the plate so as to engage the heads 33 of the T-shaped brackets 31 in receiving slots provided by a pair of side panels 34 in each recess 29. In so doing, a mechanical interlock 35 provided in one of the recesses (Figure 6 and 7) is engaged, whereby an unauthorised removal of the casing 8 from the mounting plate 28

is prevented or made less easy. Authorised removal of the casing 8 from the mounting plate 28 is effected by inserting the key (not shown) in the keyway 26 shown in Figures 3 and 6. Thus, in the first embodiment the same key-operated mechanism serves to disarm the alarm and to effect its release from the mounting plate 28. The same key is used to open the battery compartment 13 with which a key 36 (Figure 8) is associated, this keyway only being accessible when the casing 8 is removed from the mounting plate 28.

In another embodiment a power on/off switch may be ^' provided within the battery compartment. Thus, only authorised access to this switch is possible. The switch interrupts power to both the alarm circuits and radio receiver if present.

In use of the alarm system, the casing 8 is first placed in the environment in respect of which unauthorised entry is to be detected, be it an automobile, briefcase, room -or any other area in which sudden changes in pressure of the ambient atmosphere will be effected by unauthorised entry, such as the opening and closing of a door or, in the case of a briefcase, the opening thereof. This placement of the casing 8 may involve mounting the latter on the mounting plate 28 where such is provided, this being achieved, as already described, by locating the brackets 31 in the recesses 29 and sliding the casing with respect to the mounting plate in order to engage the heads 33 of the brackets in receiving slots in the recesses 29, at the same

time engaging the interlock 35. The alarm system is then armed by pressing the arming button 23 which will result in the LED 27 being energised. The electrical circuit 2 comprises a timing circuit which will allow an initial period from the instant of arming for the system to set in order to allow the user to exit the automobile, close the briefcase, etc. without triggering the alarm. After this initial period has expired, any sudden change in the pressure of the ambient atmosphere of the alarm system will be sensed by the pyroelectric sensor 1 which will then produce an electrical output signal which is amplified and used to energise the audio alarm 3. .The audio alarm can only be de- energised or disarmed by inserting the key into the keyway 26. Likewise, the casing 8 can only be removed from the mounting plate 28 if the key is inserted into the keyway 26.

The second embodiment differs from the first in that the alarm system is armed by pressing a button of the radio transmitter. Since this allows the system to be controlled remotely, there is io need for a delay to allow the user to leave the car, shut the brief case etc. Thus, electrical circuit 2 is absent and the alarm may, therefore, be triggered as soon as it is armed. Thirdly, the audio alarm can only be de- energised or disarmed by the user using the button of the radio transmitter.

The audio alarm may be arranged to de-energise after a pre-set period of time.

The alarm system in accordance with the present invention provides an extremely compact and self-contained alarm system which operates in a very reliable manner based on sudden changes in the pressure of the ambient atmosphere so that false triggering is reduced to a minimum, compared to infra-red sensors, tilt sensors, etc. Furthermore, the alarm system can be carried from one environment to another if desired.