GB2181587A - Motion sensor and alarm system - Google Patents

Abstract

The motion sensor comprises a vessel 31 containing a partially-conductive liquid, with at least one electrode A, B partially immersed in the liquid. Means are provided to respond to a change in current e.g. through the two electrodes A, B via the liquid, caused by motion or disturbance of the sensor. The sensor may be used in an alarm system, in which a preferably normally passive transmitter is triggered by the sensor to transmit a signal to a central monitor. <IMAGE>

Description

SPECIFICATION Alarm system This invention relates to an alarm system and also to a motion sensor which may be used in this or other forms of alarm systems.

Generally known intruder alarm systems require considerable amounts of cables to interconnect its various components, so that the installation time and installation costs are high.

In accordance with this invention as seen from one aspect, there is provided a passive alarm system comprising at least one sensing unit which includes a transmitter arranged to transmit a signal when the sensing unit is activated, and a central monitor including a receiverfor receiving any said transmitted signal and to cause the mo nitor to respond.

In this system therefore, the need for interconnecting cables is avoided and installation time and costs greatly reduced. Preferably also, the sensing units are battery-powered (but of low consumption) so that the sensing units can be located easily in anydesired positions without the need for consideration of a source of power. Typically the central monitorwill require a mains supply, but then the only cabling will comprise this supply cable and a cable to any outside alarm unit or further monitoring equipment.

The mode of transmission may be optical, for example, relying on infra-red or other light outside or within the visible spectrum: in particularthetransmitted signal, uniquely indicating activation of a sensor, may be a coded infra-red transmission. Instead however, the mode of transmission may comprise sonic, ultra sonic, radio or other means.

Motion or movement sensors are generally complex or otherwise prone to drawbacks. However, in accordance with the invention as seen from a second aspect, there is provided a motion sensor comprising avessel containing a partially-conductive liquid, at least one electrode partially immersed in the liquid, and means for responding to a change in current flow through the liquid and electrode, caused by disturbance ofthe liquid. For example, two electrodes may dip into the liquid (or one ofthese electrodes may be formed by the container-wall itself) and then disturbance of the sensor and hence of the liquid will cause a change in electrical resistance of the current path extending between the two electrodes.

Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: Figure iisa blockdiagram ofan intruderalarm system in accordance with this invention; Figure 2 is a diagram illustrating a pulse-coded signal transmitted by a sensing unit ofthe alarm system when this sensing unit is activated; Figure3isa diagram illustrating a pulse train developed in a central monitor of the system in response to reception of the transmitted signal; Figure4 is a block diagram of a central monitor of the intruder alarm system; Figure 5is a schematic diagram showing the principles of one embodiment of motion sensor in accordance with this invention;; Figure 6is a similar diagram showing the princi ples of a second embodiment of motion sensor; and Figure 7is a circuit diagram of a motion sensor based on the principles illustrated in Figure 6.

Referring to Figure 1 ofthe drawings, an intruder alarm system comprises a central monitor 10 poweredfrom the mains supply via a cable 11 and connected to an external alarm unit or other monitor equipment via a cable 12. Preferably, the central monitor is provided with battery back-up. The alarm system furthercomprises a plurality of sensing units which may be different or all the same, butforthe sake of illustration are shown in Figure 1 as indicating the following: a door mounted unit 13 which is fluid-sensor activated, a door mounted magneticswitch activated unit 14, a pressure-pad activated unit 15, a blinds or curtain mounted, fluid-activated unit 16 and a window mounted,fluid or magneticactivated unit 17.

Each of the sensing units includes a transmitter which transmits a unique signal when the unit is activated by the presence of an intruder. In order to relay a transmitted signal between rooms, a signal receiver may befitted on one side of a doorway and coupled to a transmitter on the other side.

In one form of transmitter, the current consumption when the sensor is not activated is extremely low, say 10 to 15 microamps, and when activated the current consumption rises to a mean value of approximately 5 milliamps. With these values battery life could exceed 12 months. The transmitter com- municates a signal to the central monitor by means of coded infra-red in the example shown. Thus, when a sensing unit is activated, a 10 second timer is triggered. The output of this timer releases a 5 KHz oscillatorthe outputofwhich drives one input of a logic AND gate. The oscillator output also drives an 8-Bit counter circuit. When the counter reaches a counted 32,theAND gate is enabled andforthe next 32 pulses the oscillator output is available at the output ofthe AND gate.The output ofthis circuit is an intermittent burst of 32 pulses for a duration of 10 seconds and each pulse is differentiated to give a pulse width of approximately 20 micro-seconds. The burst of pulses drive a pair of transistors in Darlington configuration which switch ON and OFF one or more Infra-Red L.E.D.'s (light emitting diodes) resulting in the emission of a pulse coded infra-red signal as shown in Figure 2.

At the central monitor, the received infra-red signal is detected by a sensitive infra-red receiver and the signal fed into a high gain amplifier. (The sensitivity of the receiver/amplifier is such that the receiverwill respond to a reflected infra-red signal, so the transmitterand receiver do not have to be in direct line of sight). The amplifier output feeds a pulse stretcher which increases the pulse width to approximately 100 micro-seconds and also has a threshold adjust ment set slightly above noise level. The output is shown in Figure 3, and comprises clean burstsof32 pulses, similarto the original transmitted pulses for a duration of 10 seconds.

Referring to Figure 4, the central monitor is controlled by a microprocessor 20 (for example, a Zilog Z80) having a ROM program store 21: during and after completion of a block of the program results are stored in a CMOS RAM 22 which has battery back-up to retain data in the event of the unit being powered down. The monitor has three inputs, namely a threeposition key-switch 23, a key-pad 24, and a detected coded infra-red signal input 25.

The lney-switch 23 has the following positions: 1. SET- So that a personal cancellation code of4 digits can be keyed in and stored in RAM 22; 2. INTERNAL-When an internal alarm on the premises is activated on intruder detection.

3. REMOTE - When an additional external alarm is activated or an output to other equipment is made available.

The key-pad 24 enables the system to be set and reset using a sequence of codes and the personal cancellation code of 4 digits selected when the keyswitch 23 is in SET position.

The monitor also has 3 outputs: an L.E.D. display 26 to display status ofthe system, an internal output to activate an alarm or an input to otherequipment on the premises, and a remote alarm output 28 to activate an alarm outside the premises or provide an input to remote equipment.

In operation, when the system is first powered up all three L.E.D.s on display 26 (green, yellow and red) are lit and an internal bleeper is activated, but neither ofthe internal and remote outputs are activated.

After 5 seconds, allowing for auto-resetting of the microprocessor during voltage stabilising in the cir cuitry ofthe monitor the LEDs are extinguished and the bleeping ceases. Atthis time the microprocessor program execution starts. The system has three modes of operation 1. With the key-switch 23 turned to the 'SET position the green L.E.D. is lit and when the * key on the key-pad is pressed the green L.E.D. flashes. The user's personal code of4digits can now be entered and foliowing the fourth digitthe green L.E.D. re mains continuously lit.When the + key is pressed the green L.E.D. flashes once more and the user can now enter 2 digits to setthetime in seconds between the system being set and the control monitor becoming active, and a further 2 digits to setthe time in seconds between the unit sensing intruder presence and the alarm output being activated. The green L.E.D.then remains continuously lit. Having set the two time periods, the user has sufficient time to leave the pre mises, after setting the system, before the system be comes active and, on entering the premises, has suf ficienttime to cancel the setting before the alarm output is activated.

2. With the key-switch in the 'INTERNAL' position no LE.D.'s are lit. The key is now removed. When the * key is pressed the green L.E.D. starts flashing and continues to flash for the period previously set bythe first two digits entered during the 'SET' mode. This may be of the order of 30 seconds. After this preselected period the green L.E.D. remains continuously lit and the system monitors the infra-red input 25. Infra red 'noise' may be detected from room lights, pas sing road traffic etc, butthesystem only responds to the intermittent bursts of 32 pulses.

Preferably, the system only responds if at least a predetermined number of bursts of pulses (say (10), each burst counterfalling between limits'(say 26 and 38), occurwithin a preprogrammed period, say 1/2 second, with no more than say 3 permissible noise pulses occuring between bursts. This affords some tolerance on the system in a 'noisy' environment or when the distances between sensors and monitor are approaching limits of operation orwhen the transmitter battery is approaching the end of its life.

When an intruder sensor is activated and the received code meets the above conditions, the yellow L.E.D. is additionally lit and the internal bleeper is activated. At this time the user's personal code may be entered to cancel the setting in which case all L.E.D.'s are extinguished and the bleeping ceases. If the setting is not cancelled then at the end of the second preset time entered during the 'SET' mode, the red L.E.D. is additionally lit andthe INTERNAL alarm output 27 is activated. The system may be reset using the personal code or inserting the key and turning the switch to 'SET' mode.

3. With the key-switch 23 in the 'REMOTE' position the operation is asfor'lNTERNAL' operation but additionally the remote alarm output 28 is activated.

The 'REMOTE' mode of operation is intended to be selected when the premises are completely vacated.

The 'INTERNAL' mode is intended to be selected when the premises are occupied, for use in super markets, department stores, etc. when sensing units are attached to or located near valuable items.

The sensing unit transmitters may be activated by magnetic switches (reed switches, Hall Effect, magneto-resistors etc), pressure pads or any suitable device capable of triggering the transmitter. To detect entry at doors or windows, a motion or movement sensor can be employed which is low in powercon gumption, is battery operated and of small dimensions, and would be attached to curtains, blinds and items ofvalue.

The simplest form of such a movement sensor con sistsoftwoelectrical conductors or electrodes with a resistive liquid forming an electrical circuit between the conductors. The measured resistant the conductors is a function ofthe resistivity oF Pr.e liquid, the surface area of the conductor in contact with the liquid and the distance between the conductor. For example, as shown in Figure 5, two electro desA,B, dip into a container of liquid 31, and increased depth of immersion results in a reduced resistance measured between the electrodes and conversely, reduced depth of immersion results in an increase in measured resistance. An increase in the separation ofthe electrodes gives an increase in measured resistance and conversely a reduction in separation results in reduced measured resistance.

Disturbance ofthe liquid level will result in a change in measured resistance value between the electrodes.

Figure 6 shows an arrangement wit .

A,B,C, dipping into a liquid in a contain~ . If the cu tainer and electrodes are moved to the leftthen momentarily, the fluid at electrode Awill fall, the level at will rise and the level at B remain fairly constant: in this case the resistance measured between A and B will momentarily rise and that between B and Cwill momentarilyfall. Similarly, movementofthe container and electrodes to the right momentarily results in a lower measured resistance between A and B and a higher measured resistance between B and C. This effect is due to the inertia ofthe liquid.If the electrodes A, C, are connected via an electrical bridge circuit 32 to one input of an amplifier 33, and electrode B is connected to the other input of the amplifier, as shown in Figure 7, the horizontal movement of the container and electrodes results in a large change in amplifier output voltage, the change in output being dependent on the amplifier gain and the rate of movement. If this movement sensor is used for the above-described intruder alarm, the amplifier output is used to trigger the signal transmitter ofthe sensing unit.

The movementsensorwhen set to activate a suitable transmitter of infra-red, sonic, ultra-sonic, radio etc, could be attached to valuable items in supermarkets, department stores, art galleries, museums, exhibitions etc. Also it could be distributed amongst stocks in warehouses, or built into a suitable office desk item of equipment (e.g. a calculator) or placed on confidential documentsorthe like when a person is called away from his place of work, thus providing internal security.

Claims (11)

1. A motion sensor comprising a vessel containing a partially-conductive liquid, at least one electrode partially immersed in the liquid, and means for responding to a change in current flow through the liquid and electrode, caused by disturbance ofthe liquid.

2. A motion sensor as claimed in claim 1, com- prising two said electrodes partially immersed in the liquid, and in which said means is arranged to respond to a change in current flow through the two electrodes via the liquid.

3. A motion sensor as claimed in claim 1, comprising three said electrodes partially immersed in the liquid with first and second electrodes positioned to respective sides of the third, intermediate electrode, and in which said means responds to changes in currents flowing through the first and second electrodes respectively and the intermediate electrode via the liquid.

4. A motion sensor as claimed in claim 3, in which the first and second electrodes are connected in adjacent arms of a bridge circuit having its output con nected to one input of a differential amplifier, and in which the intermediate electrode is connected to the other input of the differential amplifier.

5. An alarm system comprising at least one motion sensor as claimed in any preceding claim.

6. An alarm system as claimed in claim 5,further comprising a transmitter which is normally passive and which is arranged to transmit a signal when triggered by the or a said motion sensor, and a central monitor which includes a receiver for receiving any said transmitted signal and causing the monitor to respond.

7. A passive alarm system comprising at least one sensing unit which includes a transmitter arran- gedtotransmita signal when the ore said sensing unit is activated, and a central monitor including a receiver for receiving any said transmitted signal and to causethe monitorto respond.

8. An alarm system as claimed in claim 6 or7, in which the signal which the transmitter is arranged to transmit is a coded signal.

9. An alarm system as claimed in claim 8, in which the coded signal which the transmitter is arranged to transmit comprises intermittent bursts of a predetermined number of pulses.

10. An alarm system as claimed in claim 9, in which the receiver is arranged to respond to a received signal only if the latter comprises at least a predetermined number of successive bursts of pulseswith each burst comprising atleasta predetermined number of pulses.

11. An alarm system substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings.