GB2137790A

GB2137790A - Burglar alarm system

Info

Publication number: GB2137790A
Application number: GB08309550A
Authority: GB
Inventors: John Malcolm Morrison
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-04-08
Filing date: 1983-04-08
Publication date: 1984-10-10

Abstract

A burglar alarm system includes a sensor circuit (SC) comprising at least one sensor switch (SW), and a control circuit (CC) to which the sensor circuit is connected, and operable when the system is activated to apply a potential difference across the sensor circuit and to detect a change in the flow of current through the sensor switch. Test means (TC) are provided, operable when the system is in a test mode to reverse the polarity of the potential difference applied across the sensor circuit. A display device (DD), which may be a light-emitting diode, is connected to the sensor switch so as to be responsive to the said reversed polarity to indicate the condition of the sensor switch. A sensor circuit may comprise a number of series or parallel-connected switches, and more than one sensor circuit may be provided. <IMAGE>

Description

SPECIFICATION Burglar alarm system This invention relates to a burglar alarm system of the type in which separate sensors are used for each alarm point, such as windows, doors and the like. If a system has a large number of sensors, activating the system can take a long time. It is necessary to ensure, for example, that all doors and windows are closed and fastened, and the system then has to be activated to check the condition of each sensor. If a fault indication occurs, it is then necessary to deactivate the system, check that particularly sensor again, and then reactivate the system.

It would therefore be desirable if an alarm system could provide an indication at the sensor of the state of that sensor, prior to activating the alarm system. The closing of a window or door, for example, could then be checked or repeated until a correct indication was obtained, prior to the system being activated.

It is an object of the invention to provide an alarm system which provides such an indication at selected sensors.

According to the present invention there is provided a burglar alarm system which includes a sensor circuit comprising at least one sensor switch, a control circuit to which the sensor circuit is connected and operable when the system is activated to apply a potential difference across the sensor circuit and to detect a change in the flow of current through the sensor switch, test means operable when the system is in a test mode to reverse the polarity of the potential difference applied across the sensor circuit, and a display.

device connected to the sensor switch and responsive to the said reversed polarity to indicate the condition of the sensor switch.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 is a schematic diagram of a system having a single normally-open switch in each sensor circuit; Figure 2 is a similar diagram of a system having a single normally-closed switch; Figure 3 shows a system having a number of normally-open switches connected in parallel; Figure 4 shows a system having a number of normally-closed switches connected in parallel; Figure 5 illustrates the application of the invention to a modified alarm system; and Figure 6 illustrates a modification of the arrangement of Figure 5.

Referring now to Figure 1, this shows a schematic circuit diagram of a simple burglar alarm system. A number of separate sensor circuits SC1, SC2 etc, each containing a single normally-open switch SW, are connected to a common control circuit CC. The control circuit applies a potential different across each sensor circuit and is arranged in the activated condition to detect the flow of current in a sensor circuit caused by the closing of a sensor-switch. For the sake of simplicitv the alarm condition detector is not shown, such arrangements being well-known.

Across each switch is connected a display device DD comprising a current-activated device such as a light source or an electro-mechanical indicator, connected in series with a diode D poled in such a way that current does not flow through the display device when the alarm system is in the activated condition. A test circuit TC connected to the control circuit causes the polarity of the potential difference applied across the sensor circuits to be reversed. In a simple arrangement the alarm condition detector may be rendered inoperative when the system is in the test mode. Alternatively the detector may remain operative to indicate the presence of a faulty sensor.

In operation, the alarm system operates normally when activated, the connection of the diode D in each sensor circuit preventing current flow through the display device DD. When the system is put into the test mode the diodes DD become conducting due to the reverse polarity of the voltage supply, and current flows through the display device DD if the sensor switch SW is in its correct normally-open condition. If, however, the switch is closed, then the display device will be short-circuited by the switch and will not operate.

Hence the display device indicates whether or not the sensor switch has been correctly set. Thus whilst checking all doors and windows, for example, prior to activating the system, it is only necessary to check that each associated display device has operated whilst the system is in the test mode.

If the system uses normally-closed switches in place of those normally-open switches described above, then each sensor circuit may be modified as shown in Figure 2. A resistor R is connected in series with the normally-closed switch SW, and the diode D and display device DD are connected in series with one another across the resistor R.

The value of the resistor is such that sufficient voltage will be developed across it by the current flowing through it to activate the display device when in the test mode. In operation in the activated condition the voltage developed across the resistor R is of the wrong polarity to allow the display device to be activated, but the reverse polarity of the test mode causes the display device to be activated if the sensor switch SW is in its normal closed condition. If the resistor R is unnecessary or undesirable when the system is in the activated condition, then a second diode BD may be connected across it so that the diode bypasses the resistor. The voltage drop across the resistor is then replaced by the voltage drop across the diode.

Some burglar alarm systems have a number of.

normally-closed switches connected in series with one another to form a single sensor circuit. The system will then respond to the opening of any one switch, though it may not be possible to identify which switch has opened, nor if more than one switch is opened Each switch in such an arrangement may be provided with a display device connected as described above with reference to Figure 1.

If a system is required to have a number of normally-open sensor switches connected to a single sensor circuit, then the switches may be connected in parallel with one another, as shown in Figure 3. The display device is then connected in the same manner as that described with reference to Figure 1, with the addition of a resistor R in series with the switch and diode combination. Normally-closed sensor switches may also be connected in parallel with one another, as shown in Figure 4. The bypass diodes BD of Figure 2 may be omitted so that the resistors R limit the current normally flowing in the system.

It will be understood that any switch, whether normally-open or normally-closed, will supply information about its state which may be displayed by a suitable indicator.

My co-pending patent application No.

82.05783 is concerned with a tamper-proof alarm system in which identification of any operated sensor switch is possible. The arrangement requires that each sensor circuit has a number of sensor switches connected in series. Each switch has a first resistor connected in series with it and a second resistor connected across it. The system described above may be applied to such an arrangement by connecting the diode D and display device DD in parallel with the normallyopen switch SW and resistor R2 as shown in Figure 5. A second diode DB is connected in parallel with the series-resistor R1, poled in the same sense as diode D.

In operation, the diode D1 and DB are reverse biased when the system is in the activated condition. In the test mode, however, the diodes are forward biased, and so long as switch 5W remains open the e display device will be activated by the current flowing through it. If, however, the switch SW is closed, then the display device will be short-circuited, and will not operate.

If the system uses normally-closed switches, then the arrangement may be changed to that shown in Figure 6. This requires the sensor switch to be either a changeover switch or two switches, one normally-open and one normally-closed, the latter being the usual alarm sensor switch. The normally-open contact is connected between the combination of diode D and display device DD, and the other diode DB. Both diodes are similarly poled.

It will be seen from Figure 5 that in the usual activated condition the diodes are reverse biased, and the display device will not be activated. When the system is in the testmode, however, the switch SW iS closed, the voltage developed across the resistor R1 will activate the display device OD.

If switch SW is incorrectly set, however, then the display device will be short-circuited and will not operate.

The nature of the display device DD has not been discussed in detail. Preferably this should be a light-emitting diode, since this makes the use of an additional diode D probably unecessary. In addition, the light-emitting diode takes only a somali current to operate it, and it is also a small device which may readily be located near to an alarm sensor.

A further modification to the system described above makes rapid checking of the display devices simpler, particularly if they are light-emitting devices. In this modification the reverse potential polarity applied by the test means to the or each sensor circuit is made to be intermittent, say in the form of a square wave of a frequency between 1 and 1 OHz. This will cause the light-emitting diodes to flash regularly if their associated sensor switches are correctly set, whilst still giving no indication if the switches are not set.

The detailed nature of the test circuit has not been described since many suitable arrangements are possible. It may, in its simplest form, comprise a single or double-pole switch through which a supply voltage is applied to the sensor circuits.

The switch may be manual or electronic in form, the form being suficient for a manual test facility, whilst the latter is necessary for a system with automatic testing. Other siotable switching arrangement may also be used.

Clearly, the state of a changeover switch may conveniently be displayed by two devices of elifferent appearance, such as light-emitting diodes of two different colours. Similarly normallyopen or normaily-closed switches could be provided with light-emitting diodes so that, for example, one colour is display when a switch correctly set in the test mode, and another colour is displayed when the switch is activated in the normal mode.

Claims

1. A burglar alarm system which includes a sensor circuit comprising at least one sensor switch, a control switch to which the sensor circuit is connected and operable when the system is activated to apply a potential difference across the sensor circuit and to detect a change in the flow of current through the sensor switch, test means operable when the system is in a test mode to reverse the polarity of the potential difference applied across the sensor circuit, and a display device connected to the sensor switch and responsive to the said reversed polarity to indicate the condition of the sensor switch.

2. A system as claimed in Claim 1 in which the sensor circuit comprises a single normally-open sensor switch, the display device being connected in parallel with the switch contacts.

3. A system as claimed in Claim 1 in which the sensor circuit comprises at least one normallyclosed sensor switch, the or each switch being connected in series with a resistor and having a display device connected in parallel with the or each resistor.

4. A system as claimed in Claim 3 in which the sensor circuit includes a diode connected in parallel with the or each resistor so as to be forward biased when the system is activated.

5. A system as claimed in Claim 1 in which the sensor circuit comprises a plurality of normallyopen sensor switches connected in parallel with one another, a separate display device being connected across each switch contact.

6. A system as claimed in Claim 1 in which the sensor circuit comprises a plurality of normallyclosed sensor switches connected in parallel with one another, a separate resistor connected in series with each sensor switch, and a separate display device connected in parallel with each said resistor.

7. A system as claimed in Claim 1 in which each sensor switch includes first and second series-connected resistors and a normally-open sensor contact connected in parallel with the first resistor, the display device being connected in parallel with the sensor circuit.

8. A system as claimed in Claim 7 which includes a diode connected in parallel with the second resistor so as to bypass said resistor when the system is in the test mode.

9. A system as claimed in Claim 1 in which each sensor switch includes first and second series-connected resistors and a normally-closed sensor contact connected in parallel with the first resistor, the display device being connected in series with a diode across the first and second series-connected resistors, a second switch being provided to short-circuit the display device when the sensor switch is other than in its normallyclosed condition.

10. A system as claimed in any one of Claims 1 to 9 in which the display device includes a seriesconnected diode.

11. A system as claimed in any one of Claims 1 to 10 in which the display device comprises a light-emitting diode.

12. A system as claimed in any one of the preceding claims in which the test means is operable to apply an intermittent potential difference to the sensor circuit when in the test mode.

1 3. A burglar alarm system substantially as herein described with reference to the accompanying drawings.