This invention relates to vehicle theft deterrents and more particularly to an addition or adjunct to an existing vehicle anti-theft alarm system. BACKGROUND ART

Motor vehicle theft is an ever-growing problem today and one which results not only in considerable cost and dismay to a stolen vehicle's owner but, in the long term, one which results in higher costs to all holders of vehicle insurance policies and, indeed, to all policy holders in general.

Various devices and systems using discomfort-causing substances have been proposed, one such being disclosed, for example, in the specification relating to PCT Application No. PCT/FR85/00244 of 10 September 1985. In this document, a device using an existing windscreen washer element or elements of a motor vehicle to generate sprays of showers of "chemical liquids such as ammonia, sulphuric acid, and hydrochloric acid, and' dyes --" is installed in a vehicle, apparently with the aim of "- marking, hurting, asphyxiating -" a would-be thief. DISCLOSURE OF INVENTION

However, as it is unlawful to injure seriously even a potential car thief - as with corrosive acids - it is an object of the present invention to overcome the above and other disadvantages by the provision of a motor vehicle theft deterrent device which may comprise a timer capable of being operated by an existing vehicle alarm; a relay actuable in response to the timer; and valve means operable, via the said relay, to release a spray of a pressurised, irritant and/or discomfort-causing substance towards a person unlawfully attempting to start the vehicle or to steal items therefrom.

The valve means may well be a plunger valve adapted to release the substance from a pressurised

container such as a cylinder. The substance itself may -be. a gas or liquid capable of irritating the respiratory system, or the eyes, or both; the substance, however, must be non-toxic and non-corrosive. There again, the substance may be, or include, an indelible dyestuff.

Advantageously, the circuitry and the pressurised container or containers are accommodated in a tampe -proof- metal housing mounted, say, upon the firewall of the; vehicle.

Ideally, the pressurised substance is discharged through one or more concealed nozzles positioned so as to direct the spray or sprays towards the said unlawfully-acting person.

The timer may well be a two-stage circuit timer adapted to activate a secondary circuit to thereby cause a loudspeaker to emit a high-pitched sound painful to the human ear. BRIEF DESCRIPTION OF DRAWINGS

In order that the reader may gain a better understanding of the present invention, hereinafter will be described a preferred embodiment thereof, by way of example only and with reference to the accompanying drawings in which:-

Figure 1 is a simplistic block diagram which shows the interrelationship of the various features of the invention;

Figure 2 is a circuit diagram;

Figure 3 shows, somewhat schematically, a typical layout of the housing; and

Figure 4 is a corresponding top view. BEST MODE FOR CARRYING OUT THE INVENTION

Firstly having regard to Figure 1, the box referenced 1 represents an existing alarm system of a motor vehicle to which the inventive device is fitted; this alarm system is wired to operate a timer represented by box 2. At the termination of a pre-set time period, existing alarm

system 1 having detected the unlawful presence of a person in the motor vehicle, a solenoid operated relay represented by box 3 - aptly termed the "firing box" - opens a plunger valve of a» pressurized container so as to enable the contained substance to be released therefrom and discharged under pressure through concealed nozzle means referenced at 4.

Power to operate the relay is provided by the active bar of the vehicle's own fuse panel so that, even with the 'existing alarm device disabled, the inventive system will complete its operating cycle and release the deterrent substance. The so-discharged substance - as a so-called "atomised" spray - rapidly fills the interior of the vehicle to thereby cause much unbearable discomfort to the intruder who will immediately seek to vacate the vehicle as soon as possible.

The timing device of box 2 is a two-stage circuit timer which also activates a secondary system represented by the "back-up" box 5 to thereby cause speaker 6 to emit a high-pitched sound which is extremely painful to the human ear, thus further adding to the discomfort of the intruder.

Furthermore, in a motor vehicle in which is installed a radio or other sound system, this system is fitted with a proximity magnetic switch which is activatedn by even partial removal of the said system, allowing 12 V. D.C. to immediately energize the solenoid and the loudspeaker or siren. When this radio or other sound system is replaced in its correct position, the alarm is automatically reset.

Referring to the circuit diagram, operation is as follows:- the door switches, or the existing alarm triggering mechanism, triggers IC1 + IC2 on the closing of relays RLl + RL2. This allows 12 V. D.C. from the main fuse bar to pass through RL2 to the normally-closed contact of RLl.

The IC1 timer is set for a delay of from 30 to 60

Ar. seconds to permit resetting if accidently triggered. .When - the IC1 delay time has elapsed, it switches off, allowing ; the 12 V. D.C. to energize the solenoid and loudspeaker.

Timer IC2 is set for approximately 4 minutes by fixed components R6, C7 and C8. When IC2 switches off the 12. V.

D.C. is removed from the solenoid and the loudspeaker : or . siren and the circuit resets to deal with further attempts..

In the event of accidental triggering-off, resetting is by means of SW2 or by the reset trigger of the existing alarm.

Turning finally to Figure 3, the components are accommodated in a tamper-proof metal housing 7 which is able to be mounted upon the firewall of the motor vehicle via the fastening points 8. Ideally, the pressurized container is a steel cylinder 9, tested to 250 bars, and perhaps 300 mm long and, say, 50 mm in diameter. Cylinder 9 is affixed: to housing 7 - with its valve projecting into the latter.- via a bracket 10 and a hose-clamp 11.

Within housing 7, the circuitry is contained -in the box 12 and to cylinder valve 14 is fitted . a .- plunger/reducer fitment 15. The solenoid is represented 'at : 16 and the associated valve 17 has screwed into it an elbow 18, through which the substance is discharged to a hose 19 and in turn to the nozzle means 4 (see Figure 1). Preferably, the solenoid valve is of brass construction, 12 V. D.C, 0-100 psi nominal.

The inventive unit is capable of being recharged simply and rapidly; furthermore, it is also able to reset in the event of accidental triggering-off.

From the abovegoing, it will be appreciated by those skilled in the art that numerous modifications and variations may be made to the invention without departing from the spirit and scope thereof as set out' in- the following claims.