GB2163583A - Signalling system - Google Patents

Abstract

A security alarm system has a number of remote detectors coupled in a loop including a plurality of signal lines and a common return line. Each detector is arranged to provide current paths between a unique binary combination of the signal lines and the return line when activated to generate a corresponding condition of signals in an indicator unit coupled to the loop so as to provide an indication identifying the activated detector. The detectors may each incorporate a shock sensor connected in series in the return line for causing a momentary open circuit when activated, and a latching circuit coupled between selected signal lines and the return line. <IMAGE>

Description

SPECIFICATION Signalling system The invention relates to a signalling system particularly but not exclusively for use in security alarm systems.

Hitherto, the majority of security alarm systems have used a closed-circuit alarm "loop" wherein sensors and detectors are series connected; an alarm condition is signalled when a detector opens the circuit, usually by means of a switch or relay contact opening. When a number of detectors are series connected it is not possible to determine at the central control, which detector may have initiated an alarm condition since most detectors may only open momentarily. Various means are available to aliow contact or detector identification at the central control, but hitherto these means have been relatively expensive and prone to errors. One system currently available makes use of various values of resistors which are placed across the detector contacts so that when the detector opens the resistance value, and therefore the specific detector can be determined.

The primary disadvantage of this system is that variations in detector contact resistance and loop wiring resistances can give errors; also, whilst the "coding" resistors are relatively inexpensive, the associated electronic circuitry at the central control is relatively complex requiring analogue to digital conversion, storage and digital display means.

Another system uses active coding devices which transmit signals back to the control unit which then decodes and displays which detector has or had given an alarm condition. The disadvantages of this sytem is firstly a high relative cost at both detectors and control system and, secondly, a high potential for false alarms or errors due to electrical interference.

One object of the present invention is to enable a number of remote sensors to be discreetly identified and displayed at the control centre by a system highly tolerant to resistive changes in the alarm loop, relatively immune from electrical interference and also requiring low cost circuitry.

According to the present invention we propose a signalling system particularly but not exclusively for use in a security alarm system, and comprising a plurality of detecting or sensing means or the like connected in a control loop inciuding a number of control lines, each of the said means being connected to a different combination of control lines such that when one is activated the signal or signals appearing on the control lines uniquely identifies the detecting or sensing means concerned.

The number of control lines is preferably the binary equivalent of the number of detecting or sensing means used, whereby each of the said means can be uniquely identified by binary conded indicators connected to the control lines.

In one embodiment having seven remote detecting or sensing circuits a 4 core cable is used to interconnect the remote units such that three of the cables are effectively "weighted" in a binary sequence, the fourth cable being used as a common return. This 4 core cable is connected to a control/indicator unit at one end and terminating means such as Zener Diodes or resistors at the other end, whereby under quiescent conditions a stable current flow and line voltage will result. The sensor units may be connected in series with the common return line so that the operation of tamper switches within the sensor housings will cause an interruption of current flow.

An alarm condition detected by one of the remote units which may be, for example, a shock detector and its associated circuitry, cause current to flow from one or more binary weighted signal lines, thus causing reduction of voltage between the signal line and the common return line whereby the identity of a particular remote unit in an alarm or "latched" condition may be deduced by reference to the signal line voltages to which it is connected.

One or more of the sensing units may comprise an inertia-mass type shock or vibration sensor connected to an electronic circuit so that momentary breaks in the electrical continuity of the sensor cause a charge to appear on a capacitor whose instantaneous voltage is proportional to the intensity and duration of physical shocks.

An alarm condition is preferably signalled following the "latching", "firing" or conduction of an SCR or similar electronic circuit and that latching causes current to flow in the binary coded signal lines to which the sensing unit is connected.

If desired a BCD to decimal conversion can be applied to display a decimal number which corresponds to the coded number of the remote sensor unit in alarm condition.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a block diagram of a signalling system as applied to a security alarm system; Figure 2 is a circuit diagram of one of the remote sensing units forming part of the system of Fig. 1; and Figure 3 is a circuit diagram of a control/ display unit forming part of the system of Fig.

1.

The security alarm system shown in the accompanying drawings ailows the identification of up to seven shock- or vibration-sensors preferably of the mass-inertia type, in which high frequency shocks or vibration generated by forceable entry through building structures cause momentary opening of an electrical cir cuit.

Referring to Fig. 1, the control/identifier unit contains two signailing relays used to interface with the central alarm system of the protected premises. If TAMPER LED indicator lamp lights, the contacts of (TAMPER) relay 'T' will operate. If any sensor indicator light, '1', '2' or '3' lights, the contacts of (alarm) relay 'A' will operate. The control/indicator unit contains three voltage detector circuits which monitor the voltage of the three lines to which they are connected. Under normal (non alarm) conditions, current via R1, R2 and R3 cause End-of-line Zener Diodes ZD1, ZD2 and ZD3 to produce a stable voltage on each line (in this example 6 volts). The voltage detectors will not produce an output on either output if the respective lines are at or near this Zener voltage. If any of the four lines are cut, then these lines will rise in voltage to (in this case) 8V.The voltage detector or detectors produces a signal on their 'T' outputs to cause operation of the 'T' indicator lamp and the 'T' relay.

When an alarm condition exists on any detector, the '1', '2' and '4' outputs present a relatively low resistive value causing lines '1', '2' or '4' (if connected) to go below the Zener voltage. This lower voltage (in this case 3.5V) is detected by the voltage detector and an output appears at the alarm output A, the relative indicator lamp will illuminate and the alarm relay RLA contacts operate. In order to determine which detector is in alarm, they are connected using the well known binary codeddigit technique. The 3 signal lines and associated signal lamps are "weighted" with '1', '2' and '4' values. The 'values' of the illuminated lamps are added to determine which sensor is in alarm. Fig. 1 shows how sensors 1, 6, and 7 are connected to the signal lines. Clearly, where the alarm signalling uses two voltage levels, then a maximum of seven detectors can be identified.However, other systems using three or more voltage levels, or more signal lines could provide greatly increased identification capability. In a practical system two or more sets of voltage detectors are connected into a common housing to provide 14 or more indications on 2 or more separate 4 core cables.

In Fig. 1 the ends of the signal lines are shown connected to a terminating element which determines the quiescent line voltage.

The terminating eiements may be Zener diodes as shown or resistors and need not necessarily be built into the final sensor.

The use of terminating Zener diodes instead of resistors allows a wide variation in detector or loop wiring resistance without causing changes in the quiescent line voltage.

Also, the voltage detector circuits can be provided with a reasonable operate delay in order to avoid responding to induced line voltage transients.

The presentation of the binary coded digit directly to the system user obviates the need for B-C-D to decimal de-coding and display circuits and their inherent extra cost.

It is usual for the enclosures containing the sensors to be protected from tampering by means of electrically closed switches or contacts which open when the cover is removed.

In Fig. 1 these contacts are shown connected in series with the alarm loop neutral return line. In some systems these contacts may be connected to a separate monitoring loop, not electrically associated with the signalling system.

Fig. 2 shows a circuit diagram of a shock sensor detector. During quiescent conditions TR1 and SCR1 are not conducting and no current is drawn by the circuit. Each of the control lines 4, 2, 1 is connected to the return line by a Zener diode which may be incorporated into the final sensor (7). Typically some 3mA will flow through each signal line causing 9 mA to flow in the neutral return line via the normally closed shock sensor and tamper switch SW1. If the tamper switch is operated (by removing the housing cover) or if any line is cut then that line or lines will rise to the source--voltage of the control/indicator unit.

If the unit is subject to shocks typical of a forced entry through any surface upon which a shock sensor is mounted, then the sensor will momentarily open causing current to flow via R1, VR2 and D1 thus charging C2. When the intensity and/or duration of the shocks cause C2 to charge to some 1.4 volts, SCR1 will conduct causing current to flow through the LED, diodes D2, D3, D4 links '4' '2' '1' causing the '4' '2' '1' lines to be around 3.5 V.

Fig. 2 shows all 3 links connected giving a No. 7 coding. VR2 is a variable resistance (sensitivity control) which controls the charging current into C2 and thus determines the amount/duration of shock before 'latching' or conduction of SCR1 occurs. R1 determines the maximum sensitivity of the unit when VR2 is at minimum resistance and also provides a degree of RF interference de-coupling when combined with C1. D1 prevents C2 from discharging rapidly via R1 VR2 when the sensor recloses. R3 provides a controlled discharge path having a relatively long time constant.

TR1 is connected as an emitter-follower and provides an impedance match for SCR1. The relatively high input impedance of TR1 base assures a more linear discharge rate for C2.

C3 is an RF interference de-coupling capacitor connected in close physical proximity to SCRI. R4 ensures discharge of C3 during the off or re-setting period. C4 acts as a reservoir capacitor to provide a current source to prevent SCR1 (when on) from being turned off by short duration breaks in the line supply, particularly during shock conditions on other sensors on the line.

Following an alarm condition the sensor may be reset by means of a supply interruption for a time greater than the time that C4 can maintain SCR1 in the conducting condition.

Fig. 3 shows a circuit diagram of the control/indicator unit.

During quiescent conditions the voltages at input '4' '2' and '1' will be around 6V determined by the terminating Zener diodes ZD1 and ZD3 (Figs. 1 and 2). If any sensor tamper switch operates or line '1' is cut, then the output will rise to 8 volts. Current will flow via R3, D1 and R6 chrging C2. When C2 reaches 5.5 Volts, voltage comparator IC1 (D) will cause its output to go 'low' causing the Tamper LED to illuminate and TR1 to cut-off.

Tamper Relay RL'T' is then de-energised.

Capacitors C2 and C3 provide a delay preventing operation of either relay due to short duration breaks in the signal line current due to the momentary opening of the sensor contacts during shock conditions.

If any sensor is caused to 'latch' then the respective lines to which it is connected will be taken to around 3.5 volts. If, for example, the '1' input line was taken to 3.5 volts, then the output of voltage comparator ICI (A) will go 'low' causing the '1' weighted LED to illuminate. Current for the 3 signal LED flows via D4 and D5 which provide a 1.4 volt source of current causing TR2 to conduct thus shunting the base current to TR3 and causing it to cutoff. Alarm relay RL'A' will then de-energise.

R4 and C1 form a de-coupling filter circuit to prevent false operation due to voltage transients or RF interference carried on the signal lines.

Claims (12)

1. A signalling system comprising a plurality of detecting or sensing means connected in a control loop including a plurality of control lines, each of the said means being arranged to affect the electrical condition of a different combination of control lines when activated such that the signal or signals appearing on the control lines identifies the activated detecting or sensing means.

2. A system according to claim 1, wherein each detecting or sensing means is assigned a respective binary number and is arranged to modify the signal or signals appearing on a combination of the control lines corresponding to the said binary number.

3. A security alarm system comprising a plurality of detectors at spaced remote locations wherein each detector is coupled to a plurality of control lines connecting the detectors to an indicator unit, each detector being so connected to the control lines as to generate signals on a respective characteristic combination of control lines when activated, and wherein the indicator unit is arranged to produce an indication in response to the said signals identifying the activated detector.

4. A system according to claim 3, wherein the control lines include a plurality of signal lines and a common return line, the detector being connected in parallel with each other between the signal lines and the return lines.

5. A system according to claim 4, wherein each detector is arranged to provide current paths between a unique combination of the signal lines and the return line when activated.

6. A system according to claim 4 or claim 5, wherein at least one of the detectors has a sensor element connected in series in the return line.

7. A system according to claim 6, wherein the sensor element is a shock sensor which is normally a short circuit but'is arranged momentarily to interrupt current in the return line when activated.

8. A system according to claim 6 or claim 7, wherein the sensor element is coupled to a latching device for generating the said signals on a combination of control lines characteristic of the detector containing the sensor element.

9. A system according to claim 5, wherein each detector is connected to the signal lines such that the combination is a binary number characteristic of that detector.

10. A system according to any of claims 4 to 9, wherein at least one of the detectors includes tamper detecting means arranged to create an open circuit in the return line.

11. A system according to claim 10, wherein each signal line is coupled in the indicator unit to one terminal of a supply via a respective resistance element and the return line is coupled to the other terminal of the supply, and wherein each line is connected (a) to a voltage sensing circuit responsive to a drop in the potential difference between the respective signal line and the return line, and (b) to a common tamper sensing circuit responsive to an increase in the potential difference beyond a predetermined normal level.

12. A security alarm system constructed and arranged substantially as hereinbefore described and shown in the drawings.