Alarm system

Abstract: An alarm system for use in safeguarding articles in a shop has a strip (12) providing a plurality of alarm stations (16) each associated with an article sensor (24) such as a microswitch. Each station (16) functions independently of the others to activate a remote alarm unit (18) and has its own indicator light (50), to indicate which alarm station (16) has been triggered. Resetting by a shorting plug at the alarm station (16) illuminates a resetting indicator light (52) and the sensor (24) is then resettable during a short time period for which only the alarm station being reset is inoperative. All remaining alarm stations are fully operational throughout alarm and resetting at one alarm station.

Description: The present invention relates to an alarm system. Currently, the type of alarms which are in use in shops and stores to protect products on display are loop alarm systems. In such a system a cable runs from a central alarm unit along the product shelves and back to the alarm unit. Each product is connected to the loop in some manner and there are a number of break points in the loop to enable products to be removed or attached to the loop. If any one break point is disconnected then the alarm sounds and is normally switched off using a keyswitch, thus disabling the whole system. It is then necessary to check each break point in turn and perhaps the whole loop in order to find out which break point has been disconnected. FR-A-2 658 639 discloses a system for protecting against the theft of objects on a support or display. The system consists of an electronic system based on the use of the principle of transmission of data by bus. The system has a detector rail equipped with modules which, after the opening of one or more detectors, transmit data to an exchange. A photothyristor takes this data to actuate an audible or visual warning. A magnetic lock enables one or more detectors to be shunted which has the effect of neutralising the selected points to permit the re-setting of these after triggering or to attend to the maintenance of the objects whilst leaving the rest of the system under protection. The present invention seeks to provide an improved alarm system. According to the invention there is provided an alarm system for protecting products on display in a shop or store, the alarm system comprising:- an alarm unit having an alarm; and a plurality of alarm stations; wherein each alarm station is associated with a respective product, and each alarm station has: an alarm output means connected to said alarm unit; and sensor means associated with each alarm station adapted to sense the presence of said product; characterised in that: each alarm station is operable independent of the other alarm stations of the system; each alarm station is switchable between a first, alarm mode for generating a first signal at said alarm output means to activate said alarm, and a second, re-set mode in which said first signal is absent; and each alarm station is operable on receipt of an alarm signal from the associated sensor means to switch from said re-set state to said alarm mode to activate said alarm; and wherein said alarm system further comprises: a re-set means for each alarm station, said re-set means being actuable at an alarm station following generation of said first signal at said alarm station to apply a re-set signal to said alarm station to re-set said alarm station from said alarm mode to said re-set mode, and being de-actuable to allow switching of said alarm station from said re-set mode to said alarm mode in response to the presence of an alarm signal; and wherein each alarm station has timer circuit means (R6, C2) adapted to inhibit switching of said alarm station from said re-set mode to said alarm mode for a time period after de-actuation of said re-set means thereby to permit re-setting of the associated sensor means. The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a preferred form of alarm system according to the present invention; and Figure 2 is a circuit diagram of a portion of the system of Figure 1. Referring to the drawings, Figure 1 is a schematic diagram of a preferred form of alarm system 10 for use in shops and stores for protecting products such as hi-fi equipment and televisions, on display. The system has a connector strip 12 through which runs cabling 14 connecting a number of sockets 16 with a separate alarm unit 18. The connector strip 12 may be rigidly secured to or formed as part of shelving in the shop or store, or may be in the form of a flexible strip. The strip 12 may be provided with connections at each end to enable several such strips to be interconnected to extend the alarm system over a wider area. Each socket 16 is a four-terminal socket having terminals A, B, C and D which are connected in an associated switching circuit 20, shown in Figure 2. Each product to be protected is physically attached to a microswitch 24 in such a manner that movement or removal of the product from a shelf trips the microswitch. This can be achieved with a simple pressure microswitch on which a product such as a television set is stood. On products such as clothing, for example, the microswitch can be in the form of a clip which has to be opened for removal of the garment, opening of the clip tripping the microswitch. The microswitch is connected to a plug 22 having terminals A 1 , B 1, C 1 and D 1 which can be plugged into the socket 16. The microswitch is a single pole changeover switch with terminals 1, 2 and 3, terminal 1 being connected to both terminals A 1 and B 1 of the plug 22 and terminals 2 and 3 being connected respectively to terminals C 1 and D 1 of the plug 22. The circuit 20 has an EXCLUSIVE NOR gate 30 a first input of which is connected to terminal C of the socket 16 and a second input of which is connected to terminal B. Terminal D is connected to earth through a resistance R3 and also through resistance, capacitance chain R4, C1, the junction of which is connected to a first input of a NAND gate 34. A second input of the NAND gate 34 is connected to a bus line 36 which is connected to corresponding inputs of gates of each corresponding circuit at each other socket 16 and to the alarm unit 18. The output of NAND gate 34 is coupled via a resistance R5 and diode D1 to the junction of a resistance capacitance chain R6, C2, and also to a first input of a second NAND gate 40 whose second input is connected to the output of the EXCLUSIVE NOR gate 30. The output of NAND gate 40 is connected to a SET/RESET latch circuit 42 formed by two NAND gates 44, 46 and whose output is coupled through a further EXCLUSIVE NOR gate 48 to a light emitting diode (LED) 50 and also to a further bus line (not shown in the drawings) which connects with the alarm unit 18. The output of NAND gate 40 is also connected through an EXCLUSIVE NOR gate 52 and a further diode D2 to the junction of the resistance capacitance chain R6, C2. Finally, this junction is also coupled through EXCLUSIVE NOR gate 54 to a further light emitting diode (LED) 53. When socket 16 is empty i.e. with no plug 22 being present, the first input of EXCLUSIVE NOR gate 30 is held high whilst the second input is held low, resulting in the output of gate 30 being held low. In use, when the microswitch 24 is attached to an accessory and the plug 22 is plugged into a socket 16, terminals 3 and 2 of the microswitch are interconnected by the microswitch, thus connecting together terminals C and D of the circuit 20. Terminals A and B of the circuit 20 are also interconnected via the plug 22. The first input of gate 30 is thus pulled low and the second input is pulled high. In this condition, the output of gate 30 is still low. However, if any of the connecting wires between the switch 24 and the plug 22 are cut, or the switch tampered with, or the microswitch 24 tripped then both inputs of gate 30 would be at the same level, as a result of which the output of gate 30 goes high. Because capacitance C2 is normally fully charged, the first input of NAND gate 40 is high and the result of the output of gate 30 going high is that the output of NAND gate 40 goes low, setting the SET/RESET latch 42 and lighting the LED 50 via EXCLUSIVE NOR gate 48. This also trips the alarm at the alarm unit warning personnel that a product has been tampered with. The particular socket 16 in question can easily be identified by the lit LED 50 which, for convenience, can be a red LED. There is, therefore, no need to make a detailed check of the alarm system to locate the product that has been tampered with as this will be readily apparent from the lit red LED 50. The output from NAND gate 40 also causes the output of EXCLUSIVE NOR gate 52 to go high, thus keeping capacitance C2 fully charged and preventing resetting of the circuit which can only happen on discharge of capacitance C2 as is described below. In order to reset the circuit, and turn off the alarm, it is necessary to remove the cause of the alarm to allow gates 30, 40 and 52 to revert to their non alarm states. The SET/RESET latch 42, however, remains latched to maintain the LED 50 lit to indicate that an alarm has occurred. Once the socket at which the alarm has been triggered is located by authorised personnel a shorting plug in which terminals D 1 and A 1 are interconnected is inserted into socket 16, shorting terminals A and D together. This charges capacitance C1 through resistance R4 causing the first input of NAND gate 34 to go high after a time determined by the time constant of the resistance capacitance chain R4, C1. If the second input of NAND gate 34 is also high then the output of gate 34 goes low, discharging capacitance C2 via resistance R5 to reset the SET/RESET latch 42 via NAND gate 44. This extinguishes LED 50 and turns off the alarm. At the same time, LED 53 is lit via gate 54. LED 53 is typically green to indicate that the alarm has been cleared. The second input of gate 34 is controlled via the bus line 36 from the alarm unit 18 which holds the input high or low respectively to enable or disable the action of the shorting plug. If the shorting plug is then removed from socket 16, capacitance C2 charges via resistance R6 and after a time period determined by the time constant of capacitance C2 and resistance R6 LED 53 is extinguished via EXCLUSIVE NOR gate 54. During the reset condition and whilst capacitance C2 is charging (LED 53 is on) the first input of NAND gate 40 is held low, inhibiting the gate from generating an alarm condition. This short time period allows the plug 22 associated with the microswitch 24 to be reconnected after the microswitch has been reset, once the problem which triggered the alarm has been resolved. The benefits of the above described system are that the main alarm is never turned off and need not be accessible. In an alarm situation attention is drawn to the particular alarm station and not a central point where the main alarm is located. Because the main alarm need never be switched off the system is not vulnerable during resetting of the alarm.

Claims: An alarm system (10) for protecting products on display in a shop or store, the alarm system comprising: an alarm unit (18) having an alarm; and a plurality of alarm stations (16); wherein each alarm station is associated with a respective product, and each alarm station has: an alarm output means (16) connected (14) to said alarm unit (18); and sensor means (22, 24) associated with each alarm station adapted to sense the presence of said product; characterised in that: each alarm station (16) is operable independent of the other alarm stations (16) of the system; each alarm station is switchable between a first, alarm mode for generating a first signal at said alarm output means (16) to activate said alarm, and a second, re-set mode in which said first signal is absent; and each alarm station is operable on receipt of an alarm signal from the associated sensor means (22,24) to switch from said re-set state to said alarm mode to activate said alarm; and wherein said alarm system (10) further comprises: a re-set means for each alarm station, said re-set means being actuable at an alarm station (16) following generation of said first signal at said alarm station to apply a re-set signal to said alarm station (16) to re-set said alarm station from said alarm mode to said re-set mode, and being de-actuable to allow switching of said alarm station (16) from said re-set mode to said alarm mode in response to the presence of an alarm signal; and wherein each alarm station (16) has timer circuit means (R6, C2) adapted to inhibit switching of said alarm station from said re-set mode to said alarm mode for a time period after de-actuation of said re-set means thereby to permit re-setting of the associated sensor means (24). An alarm system as claimed in Claim 1 wherein each alarm station (16) has a second timer circuit means (R4, C1) adapted to delay switching of said alarm station from said alarm mode to said re-set mode for a time period after actuation of said re-set means at said alarm station (16). A system as claimed in Claim 1 or 2 wherein each said alarm station (16) has a visual alarm indicator means (50) operable in response to receipt of said alarm signal to indicate an alarm. A system as claimed in Claim 3 wherein said re-set means is actuable at said alarm station (16) to deactivate said indicator means (50) with switching of said station into said re-set mode. An alarm system as claimed in any of the preceding claims wherein each said alarm station (16) has a respective visual re-set indicator means (53) actuated in response to said alarm station switching from said alarm mode to said re-set mode. An alarm system according to any preceding claim further characterised in that the sensor means is a plug connector (24,22) engageable in a socket (A, B, C, D) of the associated alarm station (16). An alarm system as claimed in Claim 6 wherein said re-set means is a plug engageable in said socket bracket (A, B, C, D) of said alarm station (16). An alarm system according to any preceding claim further characterised in that the sensor means comprises a garment gripping clip. An alarm system according to any preceding claim further characterised in that the sensor means comprises a microswitch.

Inventors: DALTON PAUL WILLIAM, DALTON, PAUL WILLIAM
Applicants: VOLUMATIC LTD, VOLUMATIC LIMITED
Application year: 1992
Main classification: G08B13
IPC codes: G08B13