GB2181586A - An alarm circuit - Google Patents

Abstract

A burglar alarm circuit includes an oscillator 4 for supplying a signal to an electrically conductive film 1, 2, 3. A synchronous rectifier 8 is connected to the film and is connected by short and long time constant circuits (12, 13; 15, 16 to two inputs 11, 14 of a comparator 10 which activates an alarm 27 if there is a sudden increase in the resistance of the film. The film is applied to the glass of a window and a number of such films may be connected in series. The circuit can be supplemented with a second comparator 30 which operates the alarm in the event of a short circuit occurring either deliberately or by accident during installation. <IMAGE>

Description

SPECIFICATION An alarm circuit This invention relates to an alarm circuit, and in particularto a burglar alarm circuit which is employed in detecting a sudden change in the resistance of an electricallyconductivefilm such as an electrically con ductivefim on window.

The heat and light transmission, absorption and reflection characteristics of window glass can be controlled by the provision on the glass of a coating which comprises metal and metal oxide layers. Such coatings are electrically conductive and would usually be in the form of an electrically conductive film on the inner surface ofthewindowwhen the glass is glazed in position, oron one ofthe inner surfaces of the glass sheets which constitute a double glazing unit.

When such a window is broken there is a sudden change in the electrical resistance of the conductive film on the glass. In the case of partial breakage this is a sudden change in resistance value. When there is a crack extending right across the window or the com plete destruction ofthe window, there is an effective open-circuit between opposite sides of the window.

The film resistance of such metallic coatings on window glass is expressed is "ohms persquare" which is the resistance between opposing edges of a square of the film. This is independent of the size and shape of the window. Most windows tend to be ratherwiderthan they are tall and the resistance of the film can be monitored as the resistance in ohms between bus bars on opposite vertical edges of the window,which,fora given value ofthe film resist ance in ohms per square, is independent ofthe size but dependent on the shape ofthe window.

It is a main object ofthe present invention to provide an alarm ci rcuit which can be connected to an electrically conductive film on one or a whole series of windows, which circuit will automatically stabilise when power is applied, regardless of the number of window films which are connected in series orthe actual resistance of each window film, and will respond only to a sudden change in resistance. The circuit is insensitive to electrical noise and balances out automatically any slow resistance changes dueto temperature changes and ageing ofthewindow coatings.

According to the invention there is provided an alarm circuitfordetecting a sudden change in the resistance of an electrically conductive film, comprising an oscillatorforsupplying an oscillating signal to the film, a synchronous rectifier for connection to the film, which rectifier is synchronised to the oscillator and is operable to produce a signal level indicative of the integrity ofthefilm, and a comparator circuit having two inputs respectively connected to the output ofthe synchronous rectifier by long and short time constant circuits, which comparator is responsive to a sudden change in the signal level at the rectifier output to activate an alarm indicating circuit.

In a preferred embodiment of the alarm circuit one input ofthe comparator is connected buy a shorttime constant circuit to the rectifier output, and the other input is connected by a long time constant circuit to a tapping on a potentiometerwhich is connected to the rectifieroutputand is adjustableto control the sensitivity of the circuit.

In practicethere may also bey need to detectany short circuit in the installation which might occur inadvertently or deliberately, and the alarm circuit may also include a second comparator circu it for detecting short circuits, having two inputs respectively connected to the output of the synchronous rectifier by long and shorttime constant circuits, and an OR gate connecting the outputs of the comparators to the alarm indicating circuit.

In the preferred embodiment one input ofthe second comparator circuit is connected by a long time constant to the rectifier output, and the other input is connected by a short time constant circuit to a tapping on a potentiometerwhich is connected to the rectifier output and is adjustable to control the sensitivity of the circuit.

In a particularly effective alarm circuit according to the invention the time constants ofthe long and short time constant circuits differ by a factor of at least 100.

The invention also comprehends an alarm circuit according to the invention connected as a burglar alarm to an electrically conductive film on a window, and operable to activate the alarm indicating circuit upon breakage of the window.

The alarm circuit may be connected to seriesconnected, electrically conductive films on a plurality of windows.

An embodiment ofthe invention will now be described, by way of example, with reference to the accompanying drawing which is a circuit diagram of an alarm circuit for detecting a sudden change in resist anceofanyoneofa numberofseries-connectedel- ectrically conductive films on a plurality of windows.

Referring to the drawing a numberofseries- connected electrically conductive films 1,2,3 are indicated by resistances. These resistances indicate the resistance determined between bus bars at two oppositesidesofan electricallyconductivefilmcoating on a sheet of window glass which may be a single glass pane or part of a double glazing unit.

The output of a sine-wave oscillator4 is connected through a buffer amplifier 5 to one end of the seriesconnected films 1, 2, 3. The other end ofthefilms is connected to a current sensing resistor6 and the signal developed across the resistor 6, isfedthrough an amplifier7to one input of a synchronous rectifier 8.

The oscillator4 produces a sine-wave oscillation at a freq uency for exam ple of 20 KHz which is above the audio range butbelowthefrequencyrangeatwhich significant radiation would take place, such as would interfere with other electronic equipment. This signal is also fed through a squarer 9 to the synchronous rectifier 8 whose operation is thereby synchronised to the oscillator.

The buffer amplifier 5 provides sufficient power to generate a realistic signal level, for example 1 to 2 volts at the output ofthe synchronous rectifier 8 when thefilm resistance is as low as 10 ohms. The total film resistance 1,2,3 may be ofthe order of lotto 250 ohms depending on the number ofwindows which are connected together in series and the film resistance of each window.

The synchronous rectifier8 responds onlyto signals which are in phase with the output ofthe oscillator4,and operates in known mannerto produce a DC output level, for example 1 or 2volts, and this output level is indicative ofthe integrity of the film or films. Ifthere is a sudden increase in the film resistance orin the extreme case if a window is broken right across and an open circuit results, there is a sudden fall inthesignal level atthe rectifieroutput.

Acomparatorcircuit 10 has two inputs which are respectively connected to the output of the synchronous rectifier by long and shorttime constantcircuits. One input 11 ofthecomparatorisconnectedby a shorttime constant circuit 12,13 to the output ofthe rectifier 8. The other input 1 4 of the comparator loins connected by a long time constant circuit 15, 16 to a tapping 17 on a potentiometer 18which is connected to the output of the rectifier 8. The tapping is adjustable to control the sensitivity of the circuit.

In oneembodimentthe resistors 12 and 15 may both be 100 KQ while the capacitor 13 is 220 nFand the capacitor 16 is 22 yF so that the ti m e constants of the long and shorttime constant circuits differ by a factor of at least 100. The output of the comparator 10 is connected by a diode 19 of an OR gate 19,20 to an alarm indicating circuit which includes a relay 21 which is connected between the output ofthe gate 19,20 and a positive supply 23. In parallel with the relay 21 is a light-emitting diode 24 and the parallel connection ofthe relay with its diode is connected to earth through holding relay contacts 25 and a reset switch 26.In parallel with the relay 21, the contacts 25 and the switch 26, are an alarm device such as a buzzer 27 connected in series with an ON/OFF switch 28 and contacts 29 ofthe relay 21. The switch 28 and contacts 29 could be connected into a master alarm system ofwhich the window alarm forms a part.

In operation,when the alarm circuit is switched on, there is an initial period for stabilisation of the circuit due to the existence ofthe long and short time constants. During this initial period, the buzzer 27 or other alarm would normally become activated and would have to be reset by opening the reset switch 26. When the circuit has stabilized the switch 26 is closed again sothatthealarm indicatingcircuitis switched on. If required, avoidance of activation of the alarm on switching on can be effected by opening the switch 28temporarily and then closing itwhen the circuit has stabilized. This will be indicated by the light-emitting diode 24 which will light up during the stabilisation period and will go out when the circuit is ready for operation.So long as there is no change in the integrity of the electrically conductive film or films 1,2,3,the positive voltage atthe input 11 ofthe comparator 10 is only slightly greaterthanthevol- tageatthe input 14, the output ofthe comparator 10 stays positive, and the diode 19 is blocked.

In the event of a sudden increase in the window film resistance in series with the current sensing resistor 6, duetothe breakage of one or more windows, there is a sudden fall in the voltage level at the output ofthe synchronous rectifier 8. This fall is quickly transmitted through the short time constant 12, to the input 11 ofthe comparator.Because of the long time constant 15,16the input 14 ofthe comparator remains substantially at its stabilized level, and there is a drop in the output level of the comparator 10, so that the diode 19 becomes conductive and the alarm indicating circuit is activated by operation ofthe relay 21 which closes switches 25 and 29 so that the buzzer sounds, which gives an alarm ofinter- ference with the windows carrying the films 1,2,3.

The diode 24 is also energised.

Because of the nature of the two time constant circuits 12,13 and 15,16 any slowly varying signal output from the synchronous rectifier, for example due to slow changes in atmospherictemperature, automatically balances out at the inputs 11 and 14of the comparator 10 and does not result in an alarm.

Also the circuit automatically adjusts itselfto a threshold value of the output from the synchronous rectifier8depending onthe numberandmagnitudes of the window resistances so that the workman who is installing the alarm system when installing the windows does not have to make any adjustments to the alarm circuit to take into account the number of windows with electrically conductive films which are connected in series.

The circuit can be supplemented with a second comparatorwhich gives an alarm in the event of a short circuit occurring either deliberately or by accident during installation.

A seco nd compa rator 30, for detecting shortcir- cuits, has two inputs 31,32 respectively connected to the output of the synchronous rectifier 8 by a long timeconstantcircuit33,34and ashorttimeconstant circuit 35, 36 which is connected to atapping 37 on a potentiometer 38 which is connected between the output of the synchronous rectifier and earth. The position of the tapping 37 is adjustable to control the sensitivity ofthe circuit. The long and short time con stantcircuits33,34, and 35, 36 may havesimilar values to the time constant circuits which feed the first comparator 10 they may also differ by afactor of at least 100.

The comparator 30 is stabilized in the samewayas the comparator 10 and is insensitive to slow resistance changes, but if there is a short circuit there is a sudden rise in the voltage developed across the current sensing resistor 6 which results in an increase in the voltage level at the output ofthe synchronous rectifier 8. This is effective to produce a negative-going output signal at the output of the comparator 30 so that diode 20 conducts and the alarm indicating circuit is activated.

Because ofthewayinwhich the long and short time constant circuits are connected to the inputs of the comparators 10 and 30, when the comparator 10 producesafall in output level which unblocksthe diode 19, the comparator 30, which detects short cir cuits, maintains its output level and continues to blockthe diode 20.

In the event of a short circuit, the output of the comparator 10 rises blocking the diode 19 and the output ofthe comparator 30 falls unblocking the diode 20.

The whole circuit as illustrated thus provides an effective burglar alarm circuit which is operable upon breakage of a window orwindows, orthe inadvertent or deliberate introduction of a short circuit.

In practice such a burglar alarm system is intended for installation with a plurality of multiple glazing window units, with an electrically conductive film on one inner surface of each window unit and those films connected in series by means of bus bars within the window units at opposite sides of each film. It is also envisaged that the alarm circuit may be a hybrid chip mounted within one ofthe window units of an installation.

Claims (9)

1. An alarm circuit for detecting a sudden change in the resistance of an electrically conductive film, comprising an oscillatorforsupplying an oscillating signal tothefilm, a synchronous rectifierforconnection to the film, which rectifier is synchronised to the oscillator and is operable to produce a signal level indicative ofthe integrity ofthe film, and a comparator circuit having two inputs respectively connected to the output ofthe synchronous rectifier by long and short time constant circuits, which comparator is re sponsiveto a sudden change in the signal level atthe rectifier output to activate an alarm indicating circuit.

2. An alarm circuit as claimed in Claim 1 wherein one input of the comparator is connected by a short time constant circuit to the rectifier output, and the other input is connected by a long time constantcircuitto atapping on a potentiometer which isconnected to the rectifier output and is adjustable to control the sensitivity ofthe circuit.

3. An alarm circuit as claimed in Claim 1 orClaim 2, including a second comparatorcircuit,fordetect- ing short circuits, having two inputs respectively connected to the output of the synchronous rectifier by long and short time constant circuits, and an OR gate connected the outputs of the comparators to the alarm indicating circuit.

4. An alarm circuit as claimed in Claim 3, wherein one input ofthe second comparator circuit is connected buy a long time constant to the rectifier output, a nd the other input is connected by a shorttime constant circuit to a tapping on a potentiometer which is connected to the rectifier output and is adjustable to control the sensitivity of the circuit.

5. An alarm circuit as claimed in anyone of Claims 1 to 4, wherein the time constants ofthe long and shorttime constant circuits differ buy a factor of at least 100.

6. An alarm circuit as claimed in any one of Claims 1 to 5, connected as a burglar alarm to an electricallyconductivefilm on awindow,and operableto activate the alarm indicating circuit upon breakage of the window.

7. An alarm circuit as claimed in Claim 6, connected to series-connected, electrically conductive films on a plurality of windows.

8. An alarm circuit for detecting a sudden change in the resistance ofan electrically conductive film, substantially as herein described with reference to the accompanying drawing.

9. An alarm circuit connected as a burglar alarm to one or more windows having electrically conductive films, substantially as herein described with reference to the accompanying drawing.