Vehicle anti-theft alarm system

An alarm system for a vehicle or trailer, the system comprising an alarm circuit which is energizable after a predetermined time period responsively to actuation of a control circuit of the system, and switch means for controlling the operating state of the control circuit.

The present invention relates to alarm systems particularly for commercial 
vehicles and/or trailers. 
With hitherto known alarm systems for vehicles and vehicle-trailer 
combinations any attempt by a thief to break into a vehicle or remove a 
trailer fitted with such an alarm system immediately activates an alarm 
(generally the vehicle horn or lights), the intention being to force the 
thief to abandon his attempt. However, when attempting to steal a vehicle 
or trailer the thief will normally choose a time and place most suited to 
his purpose and provided with a reasonable means of escape should this 
prove necessary. Obviously, the thief will, if possible, choose a time and 
place where the activation of an alarm will draw as little attention as 
possible, to allow the thief sufficient time to disconnect the alarm and 
drive the vehicle or trailer away without being caught. 
An object of the present invention is to provide an improved form of alarm 
system with advantages over the hitherto known systems. 
In accordance with the present invention there is provided an alarm system 
for a vehicle or trailer, the system comprising an alarm circuit 
energizable after a predetermined time period responsively to actuation of 
a control circuit of the system, and switch means for controlling the 
operating state of the control circuit. 
Advantageously the switch means comprises a vibration sensor which is 
actuable responsively to vibration thereof to actuate the control circuit 
when the latter is armed, and control switch means for arming and 
disarming the control circuit. 
In one embodiment of the invention the control switch means has a plurality 
of selectable switching states, a first of which serves for arming the 
control circuit, a second of which serves for disarming the control 
circuit, and a third of which serves for actuating the control circuit. 
Advantageously the switch means comprises a plurality of switches and each 
said switching state is set by a combination of switching states of the 
plurality of switches. The switching of the control switch means into the 
second switching state by actuation of the plurality of switches in a 
predetermined manner serves to disarm the control circuit, and by 
actuation of said plurality of switches in a manner other than in the 
predetermined manner serves to arm or actuate the control circuit. 
An alarm system according to the present invention has the advantage that 
it delays the energization of the vehicle or trailer alarm, thus giving 
the thief the impression that no alarm is fitted, and causing panic once 
the alarm is energized. The delay will be sufficient to give the thief 
time to join the normal traffic flow thus adding impetus to the effect of 
the alarm and quickly drawing attention to the vehicle or trailer.

The illustrated alarm system is primarily intended for use on commercial 
vehicles or vehicle trailers to provide protection for the valuable loads 
often carried by these vehicles. 
The alarm system 10 illustrated in block form in FIG. 1 has a control 
circuit 100 which controls the triggering of visual and audio alarm 
circuits 400 and 500 via respective time delay circuits 200 and 300. 
Circuit 300 may alternatively be a photo sensor circuit as further 
described below. 
FIGS. 2a, 2b, 2c and 2d show the system circuitry when positioned as 
depicted in FIG. 4. The control circuit 100 includes control switch means 
as a single pole single throw switch 102 connecting a negative voltage 
supply line 12 of the system to the ganged switching arms of a double pole 
double throw switch 104. The latter has a central "off" position and with 
the switch 102 serves for arming and disarming the system. The switch 104 
has two pairs of fixed contacts but in the present system only three 
contacts 104a, b and c are used. One contact 104b of one pair is connected 
to a positive voltage supply line 14 via a resistor 106. The remaining 
contacts 104a and 104c are connected to the control electrodes (bases) of 
respective transistors 108, 110 of a first switching circuit 112, the 
contact 104c via a diode 114. The emitter of the two transistors 108, 110 
are directly connected to the negative supply line and their bases and 
collectors are interconnected by way of resistors 116, 118 in the manner 
of a multivibrator circuit. The collectors of the two transistors 108, 110 
are further connected to the positive supply line via respective resistors 
120, 122. 
The output of the transistor 108 is applied via a resistor 124 to a second 
switching circuit 126 which includes two transistors 128, 130 whose 
control electrodes (bases) and collectors are also interconnected via 
resistors 132, 134 in the manner of a multivibrator circuit. Respective 
resistors 136, 138 serve as collector loads for the transistors 128, 130 
whose emitters are directly connected to the negative line 12. 
The base of the transistor 128 is connected to the switching arms of the 
switch 104 by way of a diode 140, to the negative line 12 via the 
collector-emitter path of a transistor 142 to whose base the resistor 124 
is series connected, and to the positive line 14 through a series 
combination of a diode 144 and resistor 146. The junction between the 
diode 144 and resistor 146 is connected to the negative line 12 by way of 
a normally closed inertia switch 148 which may be in the form of a mercury 
tilt switch or pendulum switch. 
The control circuit 100 controls the primary delay circuit 200 through a 
transistor 202 of the latter, the control electrode (base) of the 
transistor 202 being connected to the output of the transistor 130 through 
a resistor 150. The delay circuit 200 includes a schmitt trigger which in 
turn controls the visual alarm circuit 400 by way of a transistor 228. The 
schmitt trigger comprises transistors 204, 206 and resistors 208 to 216 
and actuates the visual alarm circuit when the control circuit 100 
switches the transistor 202 into its conductive mode. However, for reasons 
hereinafter explained the schmitt trigger is connected to the collector of 
the transistor by way of a timing circuit which delays triggering of the 
schmitt trigger for a period of between 0 and 15 minutes after the 
transistor 202 is rendered conductive. 
The timing circuit includes three series resistors 218, 220 and 222 and a 
shunt capacitor 224 connected to the junction of resistors 218 and 220. 
The capacitor 224 is further connected via a diode 226 to the collector of 
transistor 202 which in turn is connected by way of a resistor 228 to the 
positive line 14. 
With the transistor 202 off the capacitor 224 is charged via diode 226 
almost to the potential of the supply line 14 and the voltage on the 
capacitor 224 is applied to the base of the transistor 204 to maintain the 
schmitt trigger in a first switched state and thus the visual alarm 
circuit 400 deactivated. When the transistor 202 is switched on by the 
control circuit 100 it reverse biasses the diode 226 and allows the 
capacitor 224 to discharge. Eventually the voltage on the latter drops 
sufficiently to switch the schmitt trigger into its second state, thus 
activating the visual alarm circuit 400. The discharge time of the 
capacitor 224 can be varied by means of the resistor 222, the latter being 
a variable resistor, and lies in the range 0 to 15 minutes. 
The secondary delay circuit 300 is similar to the primary delay circuit 200 
and serves the same function with respect to the audio alarm circuit 500. 
However, since the input transistor 302 of the secondary delay circuit 300 
is connected via resistor 230 to the output of the schmitt trigger of the 
primary delay circuit 200 so as to be triggered thereby, the time delay 
before the audio alarm circuit 500 is activated following switch-on of 
transistor 202 is equal to the sum of the time delays set by the two delay 
circuits 200 and 300. 
In normal use of the vehicle and/or trailer on which the system is 
installed the latter is disarmed. This is effected by first closing switch 
102 to connect the ganged switch arms of switch 104 to the negative line 
12 (switch 104 being in its "off" state), and then closing the switch arms 
to contacts 104a and b of the switch 104. This holds the transistor 108 
permanently off by short circuiting its base-emitter junction and thereby 
prevents the transistor 202 from being switched into its conductive state. 
By virtue of its connection to transistor 128 via the transistor 142, the 
transistor 108, when non-conductive, holds transistor 128 off. This last 
holds transistor 130 on, which in turn holds the transistor 202 off. The 
audio and visual alarm delay circuits 200 and 300 are therefore rendered 
inactive. 
However, if either of the switches 102 and 104 is tampered with by switch 
102 being opened or switch 104 being switched to either of its two other 
switched states, the alarm system will be armed or triggered. If switch 
102 is opened while switch 104 remains with its switch arms closed to 
contacts 104a and b the bases of both the transistors 128 and 108 are 
connected to the positive line 14, the transistors thus switching the 
transistor 202 into its conductive state, to sequentially activate the 
delay circuits 200 and 300. 
To arm the system, switch 104 is switched into its central "off" position 
or to contact 104c. The system can then be activated either by tampering 
with the switches 102, 104 or by undue vibration of the trailer or vehicle 
on which the system is installed. Upon such undue vibration, e.g. when 
trailer and towing vehicle are being connected, the inertia switch 148 
opens to allow the transistor 128 to be turned on via the diode 144 and 
resistor 146. This in turn turns transistor 130 off and transistor 202 on, 
triggering the schmitt trigger of the delay circuit 200 and activating the 
visual alarm circuit 400 and the secondary delay circuit 300 after the 
predetermined time delay. The latter allows time for trailer and towing 
vehicle to be connected together and driven off a few hundred yards before 
the visual alarm circuit is activated. 
The visual alarm circuit 400 has an alarm actuator which includes an 
astable multivibrator 402 having two transistors 404, 406 each of which 
has a respective half of the primary winding of a transformer 408 as a 
collector load. The centre tap of the primary winding is connected to the 
line 14. The two emitters of the transistors 404, 406 are connected to the 
line 12 via the collector-emitter path of a transistor 410 whose 
conductive state is controlled by the transistor 228 of the delay circuit 
200. When the latter is activated by the control circuit 100 and after the 
time delay set by the capacitor 224 the schmitt trigger switches the 
transistor 410 on via the transistor 228 to energise the multivibrator 
402. The emitters of the two transistors 404, 406 are alternatively 
directly connected to the line 12 and the transistor 410 is connected in 
the positive supply line to the transformer centre tap. 
The transformer 408 is a step-up transformer having a turns ratio of the 
order of 40:1. Its secondary winding is connected by way of a series 
resistor 412 and rectifier diode 414 and a shunt smoothing capacitor 416 
to a gas discharge tube 418 of the type having a field coil 420 which is 
energised to fire the tube 418. Power for the field coil 420 is also 
derived from the voltage rectified by the diode 414 by way of a further 
transformer 422 whose secondary winding is connected to the field coil 420 
and whose primary winding is connected through a series gas discharge 
device 424 and resistor 426 to the junction of the diode 414 and tube 418. 
An additional smoothing capacitor 428 connects the junction of the 
resistor 426 and gas discharge device 424 to the line 12. The transformer 
422 has a typical step-up turns ratio of 20:1 and the gas discharge device 
424 is for example a neon tube with a predetermined breakdown voltage. 
The alternating voltage generated by the multivibrator 402 is stepped up by 
the transformer 408 and rectified by the diode 414 to be applied across 
the tube 418 and the capacitor 428. 
The voltage which builds up on the latter as it is charged through the 
resistor 426 is applied to the discharge device 424 until the device 424 
breaks down and discharges the capacitor 428 through the primary winding 
of the transformer 422. The stepped-up voltage pulse thus generated in the 
transformer secondary is applied to the field coil 420 momentarily to fire 
the tube 418. Once the capacitor 428 has discharged sufficiently the 
discharge device 424 switches off, allowing the capacitor 428 to recharge 
for the next firing cycle of the tube 418. 
As an alternative to the gas discharge device 424 the primary winding of 
the transformer 422 can be controlled by a triac which is triggered by an 
astable multivibrator. 
As is described above, triggering of the schmitt trigger of the primary 
delay circuit 200 activates the secondary delay circuit 300 which in turn, 
after a suitable delay set by its timing circuit, activates the audio 
alarm circuit 500. 
The audio alarm circuit 500 has an alarm actuator, basically a varying 
pitch oscillator 502 in the form of an astable multivibrator, connected to 
an audio alarm 544, for example a horn loudspeaker. The oscillator has two 
transistors 504, 506, two capacitors 508, 510 and four resistances 512 to 
518, and is connected by way of a variable resistance 520 to a stabilized 
supply voltage derived from the positive supply line 14 by a series 
resistor 522 and zener-diode 524. The resistances 514 and 516, connecting 
the bases of the transistors 504 and 506 to the resistance 520, are 
preferably variable. 
The oscillator also has a capacitor 526 connecting the resistance 520 to 
the lead 12. This capacitor 526 serves as a recharging and discharging 
battery for the multivibrator enabling the latter to operate in a periodic 
manner. When the capacitor 520 is discharged the multivibrator is 
inoperative and remains so while the capacitor 526 charges via the 
resistor 520. Once the voltage across the capacitor 526 attains a specific 
value, the multivibrator becomes operative and applies an oscillating 
signal to the base of transistor 528 via a coupling capacitor 530. 
However, once the multivibrator begins to operate capacitor 526 discharges 
via the transistors 504, 506 until the voltage across the capacitor 526 
drops to a level at which the multivibrator ceases to operate. The 
charging cycle of the capacitor then recommences, the period being 
determined by resistor 520. During discharge of the capacitor 526 the 
frequency of oscillation of the multivibrator gradually drops, the centre 
frequency preferably being about 1250 Hz. 
Resistors 532, 534 serve as a bias chain for the transistor 528 whose 
collector is coupled by way of an amplifier including transistors 538, 540 
and 542 to an audio warning device 544 for example a horn or siren. 
The emitter of the transistor 528 is connected to the collector of 
transistor 328 via a resistor 546 so that while the circuit 300 is 
inactive the transistor 528 is maintained in a non-conductive state thus 
preventing the device 544 from being sounded by the oscillator 
multivibrator. However, once the schmitt trigger of circuit 300 switches 
on the transistor 528 via the transistor 328, the device 544 is energized 
by the oscillator 502 to provide a repetitive audible signal of varying 
pitch. 
The transistor 328 can alternatively control a power transistor or other 
switch in a positive supply line to the oscillator 502 so that the latter 
is energized or unenergized depending on the conductive state of the 
transistor 328. In this case the resistor 546 connects the emitter of the 
transistor 525 to the line 12. 
FIGS. 3a and 3b show one form of layout for the system. The latter is 
preferably in the form of a self-contained unit, all parts being mounted 
in a metal housing of robust construction, with that portion covering the 
tube 418 being of unbreakable glass or plastics. The unit is preferably 
located on the trailer in a relatively inaccessible place to deter 
tampering. It is also advantageous if the location of the unit is such 
that light from the tube 418 is not visible to the driver of a vehicle to 
which the trailer is connected. Thus a thief would be completely unaware 
that an alarm system had been activated until the audio alarm 544 was 
sounded. 
A further advantage of the alarm system is that once activated, the 
switching off of transistor 202 does not immediately deactivate the visual 
alarm circuit 400, since the schmitt trigger of the primary delay circuit 
200 is not switched back into its first switched state until the capacitor 
224 has recharged sufficiently through the diode 226 and resistor 228. The 
timing circuit of the secondary delay circuit 300 acts in a similar manner 
thus providing an additional delay before the audio alarm circuit is 
deactivated. 
Although the secondary delay circuit 300 of the described embodiment is 
triggered by the schmitt trigger of the primary delay circuit 200, it may 
alternatively be directly connected to the output of the control circuit 
100. 
The secondary delay circuit 300 may be replaced by an optical sensor 
circuit which monitors the operation of the tube 418 when it is flashing 
and in the event of a malfunction of the latter will immediately activate 
the audio alarm circuit. Thus the blacking out of the tube 418 will bring 
the audio alarm circuit immediately into play. 
The two supply lines 12 and 14 are connected directly to a battery 20 which 
serves as an independent power supply of the system, and by way of 
connectors 22 to the vehicle battery 24. The lead 14 includes a forward 
biassed diode 26 which prevents shorting of the connectors 22 from 
affecting the system. The supply voltage is generally between 6 and 30 
watts. 
Finally, the switch 104 may be in the form of a combination or key switch 
to provide additional security.