GB2175722A - Security systems - Google Patents

Abstract

A security system for detecting whether electrical equipment normally plugged into the mains supply has been stolen, misplaced, or is operating incorrectly or for monitoring some conditions of the environment such as light or temperature, comprises within each plug coupling the electrical equipment to the main supply a means (18,22) for supplying a continuous or recurrent signal, preferably a unique continuous frequency signal, across the mains supply to a central monitor station connected to the mains supply, the monitor station monitoring all plug signals and having means for determining whether the signal from any particular plug has ceased and providing an indication thereof. <IMAGE>

Description

SPECIFICATION Security systems This invention relates to security systems, in particular systems for detecting whether electrical apparatus has been stolen, misplaced or is not working correctly or for detecting an environmental condition.

In the domestic environment and in industry more and more equipment, sometimes of considerable value, is electronic, portable and powered by the mains supply. This equipment is a natural target for burglary and further such equipment is easily misplaced.

There are of course available burglar alarm systems which are fitted to buildings to prevent or detect the entry of burglars.

The present invention however starts from the recognition that within a building portable electronic equipment is a prime target for burglary and therefore the present invention provides a system which is particularly directed to the protection of electronic equipment.

The present invention therefore provides a security system incorporated in a mains power electrical supply wherein each item of electrical equipment to be protected has a plug to be connected to a mains socket, each plug or the electrical equipment connected thereto being provided with means for supplying a continuous or recurrent signal to the mains supply, and the system including a monitor station for connection to the mains supply wherein the signal from each item of electrical equipment is monitored, the monitor station including means for providing an indication if an item of electrical equipment ceases to supply a signal to the monitor station, thereby indicating unplugging or abnormal operation of the equipment or an environmental condition.

Thus the present invention operates on the concept of having a mains supply which is continually turned on and a signal which is provided by each item of electrical equipment plugged into the mains supply being continuously monitored. As preferred each item of electrical equipment has a special three pin 13 amp plug which contains a means of transmitting a characteristic signal through the mains wiring regardless of whether the equipment is turned on or off. Whilst the signal may be of any nature, for example a pulse coded signal it is preferred from the point of view of reliability and simplicity and ease of distinguishing the signal from interference to provide each plug with an oscillator for providing a characteristic frequency signal. The monitor station contains means for distinguishing between the various frequencies emitted by the different plugs.

In the situation where the invention acts to indicate whether electrical equipment is functioning normally, e.g. a food freezer or computer equipment, means may be provided for detecting abnormal operation on the equipment, e.g. a relay coil reacting to power consumption by the equipment, which means operates to turn off the means supplying a continuous or recurrent signal, thereby indicating abnormal operation. Alternatively, the invention may indicate whether lights in a room are working by use of a light sensor or may indicate whether room temperature has fallen below or risen above some particular value by use of a temperature sensor attached to the plug, and therefore that the heating system should be switched on or off for that area.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a circuit diagram of an electrical plug incorporating signalling circuitry according to the invention; Figure 2 is a block diagram of a monitor station connected to a mains power supply according to the invention; and Figure 3 is a schematic view of a plurality of separate mains supplies systems each with their respective security system, the security systems being interconnected.

Referring now to the drawings, Fig. 1 shows signalling circuitry which is incorporated in a standard three pin 13 amp plug. The circuitry would for the most part be incorporated in an integrated circuit and there is therefore little problem in housing the circuitry within the plug.

A mains supply indicated at 2 provides live neutral and earth lines, L, N and E and the dotted line 4 indicates on its righthand side the circuitry within the plug. The connecting points EQ represent the points at which electrical leads are connected to the electrical equipment to be powered by the plug. A fuse 6 is provided. Disposed between the neutral and live lines is a high to low potential difference convertor 8 of conventional construction which provides through a diode 10 and a smoothing circuit provided by resistor 12 and capacitors 14 and 16 a steady 6 volt DC power supply. This power suply is employed to power a standard CMOS oscillator 18. The oscillator 18 provides an oscillatory signal of a selected frequency at its output 20 and this is coupled to the primary winding of a transformer 22.A capacitor 24 is connected across the primary winding to reduce the impedance load and the secondary winding of the transformer is connected via a decoupling capacitor 26 to the neutral line of the mains supply.

Thus a signal of a selected frequency which is continuous in nature is supplied across the neutral line from this plug.

In view of the small power requirements of this arrangement, the convertor 8 may be a resistor-zener diode network since there will be little power flow through the network in view of the load requirements.

It will be understood that each plug within the overall security system has a similar circuit there within but each plug has one frequency selected from a series of unique frequencies.

It will be understood that the electrical switch which is normally associated with the plug socket would be left switched on even while the equipment was not in use and overnight. The equipment connected to the plug would have to be turned off by its own power switch.

The electrical lead interconnecting the plug and the electrical equipment is not easily disconnectable and extends into the plug and into the electrical equipment and is there securely fixed. Thus the only alternative to unplugging the equipment to remove it is to cut through the power lead which is a dangerous operation.

Referring now to Fig. 2 which discloses a monitor station, this monitor station is disposed at some suitable point within a building, for example in the office of security personnel.

The monitor station comprises a small box with the electrical equipment therein and a display panel on the front face of the box. The display panel comprises an array of light emitting diodes, 50, 52 etc., one diode being provided for each plug of the system.

As shown in Fig. 2 a separate monitoring circuit is provided for each plug and each monitoring circuit 60 comprises a transformer 62 connected between the neutral and earth lines connected to an amplifier 64. The output of the amplifier 64 is filtered by a band pass filter 66 and holds on a monostable 68 which provides a steady signal to LED 50 in the event that a steady signal is received from the respective plug indicating the presence of the plug. The band filter 66 ensures that only the frequency selected for the particular plug be monitored is fed to the monostable 68. Similarly for each plug a further circuit is provided comprising a transformer, amplifier, band pass filter and monostable for holding on a light emitting diode.

In the event that a plug is removed from the mains supply circuit, the oscillatory signal from that plug is interrupted and the respective LED will go off thus indicating to security personnel that an undue occurrence has occurred.

The time constant of the monostable circuits may be made as long as 10 seconds to ensure that interference masking out the frequency signals for short periods of time does not cause accidental operation of the circuit.

An alarm circuit 70 is provided including a DTL OR gate circuit which ORs the signals from the LEDs together so that should any one LED not be illuminated an alarm output is provided to a speaker 72 to raise the alarm.

In addition a relay 74 is provided for any other desired alarm signaller.

A power supply unit 80 is provided connected to the mains supply circuit for providing power to the signal detection circuitry.

In a large building, it may be the case that the mains power supply enters from several sources, e.g. derived from different phases of a three phase supply. In this case signals from security plugs on one circuit could not be picked up on another. For this reason as shown in Fig. 3, a separate monitor station is associated with each separate ring circuit of the power supply, the ring circuits being indicated by circuit A, circuit B, circuit C and circuit D. Each monitor station is indicated at 90 and each plug of the security system is indicated at 92. As shown in Fig. 3 the monitor stations 90 may interconnect two separate ring circuits and the signals from the separate monitor stations may be fed through the ring circuits to a central checkpoint 94 on one particular ring circuit. This would be effected by each monitor station providing a signal of a unique frequency which is transmitted by the other monitor stations to the central checkpoint 94. It will be realised that the security system according to the invention may have many different applications; for example it may be used as a fuse monitor for locating faults in the plug or equipment failures of other natures.

Claims (15)

1. A security system incorporated in a mains power electrical supply wherein each item of electrical equipment to be protected has a plug to be interconnected with a mains socket, each plug or the electrical equipment connected thereto being provided with means for supplying a continuous or recurrent signal to the mains supply, and the system including a monitor station for connection to the mains supply wherein the signal from each item of electrical equipment is monitored, the monitor station including means for providing an indication if an item of electrical equipment ceases to supply a signal to the monitor station, thereby indicating unplugging or abnormal operation of the equipment or an environmental condition.

2. A supply system as claimed in claim 1, wherein each plug is a three pin plug having earth, line and neutral pins.

3. A security system as claimed in claim 1 or 2 wherein each plug contains means for transmitting a signal unique to that plug in a continuous or recurrent manner to the mains supply.

4. A security system as claimed in claim 3 including an oscillator for providing a continuous oscillation at a frequency unique to that plug to the mains supply.

5. A security system as claimed in claim 4 wherein the plug includes a voltage conversion means including an arrangement of resistances and zener diodes for stepping down and rectifying the mains supply to DC voltage level suitable for powering the oscillator.

6. A system as claimed in claim 4 or 5 wherein the oscillator is arranged to provide an oscillatory signal to a transformer which provides the oscillatory signal to the neutral and earth lines.

7. A system as claimed in claim 1 wherein the monitor station includes means for receiving the signal from each of the plugs and means for distinguishing between such signals and for providing an indication of the presence or absence of each signal.

8. A system as claimed in claim 7 wherein the monitor station includes a separate channel for each plug of the system, each channel being connected to the mains supply and including a transformer for receiving the signal from the mains supply, an amplifier connected to the transformer, a band pass filter having a frequency range selected for a particular plug, the band pass filter supplying a signal to a monostable circuit which holds on a signalling device should the signal from the plug be present in the system.

9. A system as claimed in claim 8 wherein the signal from each monostable circuit of each channel is fed to an alarm circuit including a plurality of OR gates for receiving the signals from monostable circuits and providing an output signal to alarm means.

10. A system as claimed in claim 7, 8 or 9 wherein a display panel is provided which indicates to an operator which plugs are in place and which plugs are inoperative.

11. A system as claimed in claim 1 wherein the main supply is provided by a plurality of separate ring circuits and for each ring circuit a separate monitor station is provided for all the plugs connected to that ring circuit, the monitor stations being connected to further ring circuits, and a central checkpoint being provided so that in the event of a monitor station detecting an irregularity in one particular ring circuit, the monitor station can transmit a signal indicating the irregularity through the other ring circuits to the central checkpoint.

12. An electrical plug for a security system as claimed in claim 1, the plug including means for supplying a continuous or recurrent signal to the mains supply for indicating that the plug is in position within the mains supply.

13. An electrical plug as claimed in claim 12, the plug including means for detecting faulty operation of the respective equipment or for detecting a condition of the environment surrounding the plug and means responsive thereto for causing said continuous or recurrent signal to cease.

14. A security system substantially as described with reference to the accompanying drawings.

15. An electrical plug for a security system substantially as described with reference to Fig. 1 of the accompanying drawings.