GB2146503A - Solid state switch - Google Patents

Abstract

A bi-lateral switch circuit comprises two similar solid state switches, the first comprising a C MOS oscillator circuit 7,8,9, R22 and R34 adapted to oscillate only upon receipt of a high level signal A. A pair of V MOS transistors TR11 and TR12 connected in series opposition have their gates coupled via isolating capacitors C11 and C12 and a rectifying circuit, e.g. a voltage tripler D11-D14 and C15-C17, to the oscillator output so that the V MOS transistors conduct only when the oscillator circuit is oscillating. The second switch operates on receipt of a high signal B, thus providing the equivalent of a pair of isolated relay contacts for controlling an alarm device when the inputs A and B are the normally high and normally low outputs, respectively, of an infra-red intruder detector. <IMAGE>

Description

1 GB 2 146 503 A 1

SPECIFICATION

Infra-red intrusion detector system This invention relates to an infra-red intrusion 70 detector system.

A passive infra-red intrusion detector system works on the principle of detecting the radiant energy emitted by a persons body as the person moves across the field or fields of view of the system. However, to discriminate between station ary objects that will also be emitting infra-red radiation, the system is designed to respond to a change in the level of radiation received so that only a moving object will be detected.

Some form of optical system is employed in order to collect and focus the received energy because the sensitive area of the infra-red detector element is only a few square millimeters. Thus, the amount of energy failing on such an area radiated from a human target at say 20 meters distant would be too low to be reliably detected with a good signal to noise ratio. An optical gain of about 1000 fold has been found to be satisfactory in practice. Either a mirror or a lens system may be used to achieve the necessary optical gain. A difficulty arises in that the wavelength (which is a function of the temperature) emitted by a human target is of the order of 10 micrometres and a conventional lens made from materials such as glass does not transmit this wavelength. Thus the use of a lens made from silicon or germanium while providing low transmis sion losses are expensive to manufacture. Thus, a mirror or a multiplicity of mirrors is generally employed.

An example of the prior art system is described in

Patent Specification No. IE 43420 (GB 1551541).

A weakness of the prior art system is that an intruder, prior to entering a room having an infra-red intruder detector system can see in which direction the detector is aligned and can therefore take appropriate action to avoid the field of view of the detector. A simple but expensive solution to this problem is to provide several such detectors in one room so as to give substantially total infra-red 110 protection to the room.

Another possible solution to the problem is to house the detector system in a housing and by providing suitable ball and socket arrangements which project from the housing for moving the detector system within the housing, attempt to conceal the alignment of the detector system. In addition to the relatively high cost of such a solution, another difficulty arises in so far as a section of the housing would contain a window which would either 120 be open or covered by a material suitable for transmitting infra-red radiation to permit radiation to be collected by the optical system of the detector system while at the same time the material should have sufficient opaqueness so as to prevent the intruder from seeing in which direction the detection system is aligned. As mentioned above, the use of a material such as glass to cover the opening would effectively stop infra-red radiation from being re- ceived by the detector system and the use of 130 expensive material also mentioned above, would seriously reduce the sensitivity of the detector system.

It is an object of the present invention to overcome these problems.

The invention therefore provides an infra-red intrusion detector system which comprises an infrared detector assembly adapted for mounting in a housing, the assembly having an inf ra-red sensitive detector element for producing an electrical signal corresponding to a change in the level of infra-red radiation falling on the detector element, the housing having an optical system comprising a plurality of fresnel lenses mounted therein for gathering infra-red radiation and for focussing the radiation on the detector element, and wherein means is provided for moving the detector element independently of the optical system, the arrangement being such that movement of the detector element alters the field of view of the detector element so as to receive radiation via a selected number of fresnel lenses.

It will be appreciated that the fresnel lenses also act as a window in the housing in which the detector assembly and associated electronic components are housed as well as the optical system for the detector element.

The optical system or window must have sufficient strength and therefore thickness for reasonable handling without creating obstruction or a reduction in the ultimate performance of the detector system. Polythene has an infra-red transmittance loss of the order of 50% for a thickness of 0.5 mm and at that thickness is acceptable. The invention, therefore, further provides a detector system in which the optical system comprises a sheet of plastics material, preferably polythene, having a thickness of about 0.5 m m onto which has been hot pressed or moulded an array of fresnel lenses. Preferably, the optical system is of the wrap-around form spanning a field of view of approximately 180'. For ease of manufacture, the optical system is hot pressed in the flat and mounted in the housing in wrap-around form. The optical system preferably comprises from 3 to 30 fresnel lenses. For a horizontal field of view of approximately 180', twelve fresnel lenses are preferably employed, each lens providing a detection zone having an effective field of view of the order of 5' with a 50 buffer zone on either side of each zone. A significant advantage in using a fresnel lens arrange- ment is that the detector element may be housed in the back of the housing whereas with a mirror arrangement, the detector element has to be located at the front of the housing. Thus, false alarms which are triggered by ambient change in temperature are reduced significantly.

The detector element itself has a useful angle of view of approximately 120'. Thus, at any one position of the detector element, only about eight of the twelve lenses are employed and the remaining lenses are redundant.

When the assembly is mounted in a convenient location in the room, it is usually necessary to position it for maximum detection of an intruder. By simply moving the detector element, the desired field of view of the detector can be obtained without

2 GB 2 146 503 A 2 moving the detector assembly per se. This not only provides a reduction in the number of components necessary for mounting the assembly (e.g. ballsocket joints to enable the assembly to swivel) but also provides the additional advantage that an intruder will not be aware of the field of view of the detector as the lenses will prevent him from seeing the orientation of the detector.

As an alternative to a twelve fresnel lens system, a nineteen fresnel lens system may be employed. The zone of detection of a nineteen fresnel lens system increases the useful number of detection zones with thirteen of the lenses in use at any one time, the remaining six lenses being redundant.

As the present invention provides a moveable detector element only, there is a reduction in the size of the enclosure required to house the complete assembly and also there is an increase in the uncertainty in knowing which area of a room is protected. The means for moving the detector element preferably comprises a spindle and a knob for moving the spindle, the arrangement being such that the knob projects outside the optical system which knob when rotated moves the detector ele- ment which is mounted on the spindle.

One problem associated with an optical system which can provide a large panoramic window as described above is radio frequency penetration problems which could give rise to false alarms. The detector employed is preferably a pyro-electric crystal or film mounted in a detector housing and having a very high impedence. The detector housing preferably has a filter to allow wavelengths of between seven and fifteen micrometres to pass. It is known to use a built-in field effect transistor (F.E.T.) source follower in the detector housing. While radio frequency interference will not have undue effect on the components inside the detector housing, the relatively weak signal coming from the drain of the

F.E.T. to the amplification circuit external to the assembly is prone to radio frequency interference. To overcome this problem, the invention also provides a detector housing having a pyro-electric crystal or film and an F.E.T. mounted in the housing which further comprises a direct coupled transistor circuit mounted in the detector housing, the base current of which direct coupled transistor is fed from the drain of the F.E.T. By employing such a system, a thousandfold amplification in the signal coming from the detector housing can be achieved thereby swamping all but very high level radio frequency interference.

A simple form of pyro-electric crystal consists of a flake or film of pyro-electric material such as lithium tantalate on which is deposited a rear and front electrode thus forming a capacitor. If an image of, say, a human target is focussed on the front electrode the resulting temperature rise will cause a change in the polarisation of the material which in turn results in a change in the charge of the capacitor which may be detected as an electrical signal. The detector is equally sensitive to the image moving in the X or Y axis. In order to overcome the problem of false alarm generation in cases where the detector itself is subjected to a temperature change (due to normal room temperature fluctuation), it is now common practice to use either two detectors mounted in close proximity or dual elements on the same crystal or film. With this arrangement, the electrical signals are generated with an opposite polarity for a given temperature change and so cancel out. If the target moves horizontally across the field of view, the image of the target will first of all fall on one of the elements before the other creating an inbalance in the temperature of the two detectors so generting an electrical signal.

The disadvantage of the above system is that if the image moves vertically towards the two detectors, a detectable signal may not be generated as the two detectors could simultaneously detect a temperature change.

To overcome this problem, four detectors instead of two are interconnected in such a way that an inbalance occurs when the image moves towards or across the field of view of the detector.

In the case of a dual or quadruple detector arrangement, the individual elements may be connected either all in series, all in parallel, or a combination of series-parallel, so long as signal cancellation is arranged to take place when the entire assembly is subject to a uniform change of temperature. A parallel arrangement enables a single load resistorto be employed atthe same time ensuring that there are no 'floating electrodes'.

'Floating electrodes'are to be avoided as an electrode with no discharge path may build up an excessive charge which is undesirable and amongst other things can resit in excessive electrical noise.

To avoid the 'floating electrodes' problem with a series or series-parallel arrangement without the necessity of two or more load resistors, the pyroelectric crystal may be doped to make it slightly conductive. The electrical time constant should be at least as long as the thermal time constant and preferably longer i.e. several seconds.

Multi-element detectors may be constructed either by inter-connecting individual discrete detectors or by an arrangement of inter-connected electrodes deposited on a common pyro-electric flake or film, or a combination of both techniques.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a perspective view of an infra-red intruder detector system according to the invention; Figure 2 is an exploded view of the system of Figure 1 but not showing the cover; Figure 3 is an elevation of the system of Figure 1 viewed in the direction of the arrow 1 of Figure 11; Figure 4 is a plan view of the system of Figure 1; Figure 5 is a plan view of one lens used in the system of Figure 1; Figure 6 is a schematic view showing the effective field of view (approximately 5) of each lens of the detector system of Figure 1; Figure 7 is a schematic view of the zones of detection of the system of Figure 1 which can be obtained by movement of the detection element vertically.

Figure 8 is a circuit diagram illustrating the 3 GB 2 146 503 A 3 electronic signal processing circuitry used in the system of Figure 1; Figure 9 is a circuit diagram of a switching mechanism for operating the alarm in response to a signal from the electronic signal processing circuit of 70 Figure 8; Figure 10 shows a pyro-electric crystal on an enlarged scale; together with a schematic diagram Figure 10a showing the inter-connection between the individual electrodes of the crystal; Figures 10b-10h are schematics showing a num ber of other inter-connection examples between the individual electrodes of the crystal of the pyro electrical crystal of Figure 10; and Figure 11 is a diagrammatic representation of zones of detection of a detector system according to the invention.

Referring firstto Figures 1-5, the infra-red intrusion detector system 1 comprises a housing having base 2 and a terminal cover 3 secured together by suitable screws (not shown). Mounted on the base 2 is an optical system 13 under which is a printed circuit board 4 which contains electronic circuitry 5 for use with an infra-red detector element 6. The optical system 13 forms a window which is substantially D-shaped in cross- section as shown in Figure 3.

The printed circuit board 4 has a cut out 7 therein in which is mounted a detector element 6. The detector element 6 is mounted on a spindle 6a which projects under the cover 3 and terminates in a knob 6b. The spindle 6a and the detector element 6 are mounted so that rotation of the knob 6b enables the detector element 6 to move through 180'and that by turning or pulling or pushing of the knob 6b, the detector element may be moved in the direction of the arrows 9.

To enable the knob 6b to be moved in the required direction it is only necessary to remove the cover 3; the optical system 13 remaining on the base 2.

The detector element 6 is connected to the printed circuit board 4 by wires 11 which do not interfere with the movement of the detector element 6. The spindle 6a is held against the printed circuit board 4 by means of clips 10 which permit rotational or longitudinal movement of the spindle 6a but prevent undesirable movement thereof.

To manufacture the optical system 13, twelve fresnel lenses, one of which is shown in Figure 5, are hot pressed or moulded onto a polythene sheet having a thickness of approximately 0.5 mm. Alternatively a continuous roll of such polythene may have hot pressed onto it a plurality of fresnel lenses and the roll cutto the appropriate length so as to provide the optical system 13. The cut length is then mounted on the frame 12. The dimension of each lens is approximately 8mm by 50mm.

When the cover 3 is in position, the lenses of the optical system 13 are arranged so that infrared radiation passing through any selected one of the lenses focusses on the detector element 6. The cover 125 3 also has a Light Emitting Diode (LED) 15 visible through the cover 3 which serves as an alarm indicator for'walk testing'. Alternatively, the LED 15 may be mounted beside the detector element 6 so that when the detector element 6 is moved by the knob 6b, the LED 15 also moves.

The detector element 6 comprises four pyroelectric or similar rate of change detector components. The detector element 6 may be provided in a conventional manner with a coated germanium or silicon window (not shown) on its surface facing the optical system 13, the germanium window serving to exclude unwanted infra-red radiation and provide RF shielding. The individual components may be connected either all in series, all in parallel or a combination of series-parallel so long as cancellation of an alarm condition is arranged to take place when the entire detector element 6 is subject to ambient changes in temperature. In the present example, a series parallel arrangement is used.

Referring now to Figure 8, the detector element 6 comprises four pyroelectric detectors (one of which is shown), a Field Effect Transistor (F. E.T.) direct coupled from the drain thereof to the base of a transistor TR1 and a resistor R1 and all housed in the same housing. Thus it will be appreciated that the F.E.T. which confers a voltage gain of the order of a factor of ten, when coupled with the transistor TR1, which itself gives a voltage gain of the order of a factor of 100, provides a voltage gain of the order of 1000 at the collector of the transistor TR1. The amplified signal is fed to the coupling network R4 and C5, C6 and R6. RV5 is a preset gain control and R3 is a feedback resistor to stabilise the working point of the F.E.T. and TR1 combination. R2 is a self biasing resistor for the F.E.T, and C4 prevents signal degeneration and contributes to the low frequency roll off of approximately 0.2 Hz. The band pass requirements of the circuit are determined by these values and C7 and is typically 0.2 Hz to 5 Hz.

By employing the F.E.T. as an applifier, very low noise operation is possible. It is, however, necessary to operate the circuit at very low currents in orderto enable C4 to have a practical value. In the circuit, the F.E.T. is stabilized at approximately 1 microamp and TR1 at approximately 2 microamps, the entire arrangement requiring only a j volt supply. Inverter 1 has a feed back applied between output and input to stabilize the working point of the output to approximately half the supply voltage. This also automatically sets the window detector threshold of inverters 2, 3 4 and 5. The input supply voltage at X is between 3.2V and 18V and the regulated supply is 3 V at Y.

R1 0, 11, 12 & 13 form a potentiometer ensuring that pin 13 of inverter 2 is approximately 75mV above the switching threshold and pin 3 of inverter 4 is 75 mV below its switching threshold. It can therefore be seen that if the signal output of the detector element 6 exceeds a preset value of voltage (in this case + or - 75 mV) an alarm will be given.

Inverters 3 and 5 ensure a clean switching threshold and TR2 combines the outputs of pins 10 and 6 respectively.

Further buffering is provided by inverter 6. The output from invertor 6 goes high when the alarm threshold is reached, turning on, via resistor network R31 and R32, transistor TR7 whose collector load is R33. Thus the collector of TR7 goes low during alarm. Thus, during an alarm condition, the output at 4 GB 2 146 503 A 4 A is high and output at B is low.

All the inverters are of the CMOS type and are on a single hex inverter chip.

TR3, TR4 and TR5 form a voltage regulator providing the 3 volt stabilized rail at Y from the input 70 at X of between 3.2 volts to 18 volts. Input TR6 is a start up device to ensure the circuit will 'boot strap' itself into operation.

The whole circuit will operate down to a voltage of just over 3 volts with a total consumption of approximately 10 microamps. This in turn enables the alarm to operate from batteries for extended periods of time.

It is traditional to employ a relay at the output interface in such circuits. However, the relay coil drive requirement is relatively high and in the case of the above circuit would be approximately one thousand times higher than the consumption of the entire electronic circuit.

In order to overcome this problem a solid state 85 bi-lateral switch has been developed for this alarm which works in the following way:

Referring to Figure 9, CMOS inverters 7, 8 and 9 form a first oscillator, the output of which is coupled via Cl 1 and Cl 2 to a voltage tripler consisting of Dl 1, D1 2, D1 3, D1 4, Cl 5, Cl 6 and Cl 7. This arrangement applies between 6V and 9V D.C to the gates of VMOS transistors TR1 1 and TR1 2.

Similarly, CMOS inverters 10, 11 and 12 form a second oscillator, the output of which is coupled via Cl 3 and C14 to a voltage tripler consisting of D1 5, D1 6, D17, Dl 8, Cl 8, Cl 9 and C20. This arrangement applies between 6V and 9V D.C. to the gates of VMOS transistors TR14 and TR14. The values of Cl 1, C12 C13 and C14 are very small typically 100p17 and may be a high voltage type.

The CMOS inverter oscillators need draw only a few microamps from the 3 volt supply to turn on the bi-lateral switch. Cl 1 and Cl 2 or Cl 3 and C14 couple the A.C. output of each oscillator and at the same time provide isolation (arrow W) of the bi-lateral switch from the rest of the circuit. R24, R25, R28 & C21 comprise a first network and R26, R27, R29 and C22 comprise a second network to prevent interfer ence which may originate from the external circuit or wiring connected to the terminals of the bi-lateral switch causing false operation.

The first and second oscillators are controlled via diodes D9 and D1 0 respectively which are fed by the outputs A and B respectively from Figure 8. R34 and R35 are present to prevent excessive loading of the outputs A and B respectively. If the input to either diode D9, D10 is taken high then that oscillator with which it is associated is free to oscillate. Conversely if the inputto either diode D9, D10 is lowthe oscillator is inhibited from running. The A output from the alarm sensing circuit is normally low, therefore the first oscillator is inhibited resulting in transistors TR1 1 and TR1 2 receiving no drive and are therefore not conducting so that they form the normally open output. Conversely the second oscil lator is free to run as the output B from the alarm sensing circuit is normally high. Thus, TR1 3 and TR14 are turned on by the rectified output of the oscillator so that they constitute the normally closed 130 output. When an alarm condition occurs the above order is reversed i.e. output A goes high and output B goes low. Thus there is provided the equivalent to a pair of isolated relay contacts. The VMOS devices are connected back to back so that A.C. or D.C. of either polarity may be switched.

The'on' resistance may be as low as 10 ohms while the'off' resistance is typically 10 million ohms so that the arrangement replaces all functions of a relay and requires only a few microamps of drive power to operate.

It will be appreciated that a voltage tripler is only necessary when the circuit is operated from a low voltage. A simple rectifier circuit would suffice with higher supply voltages.

In cases where isolation of the bi-lateral interface output from the preceding circuitry is not necessary in the case, for example, of a battery operated unit, the oscillator part of the arrangement may be omitted and the VMOS devices controlled directly from the alarm output.

In use, the infra-red intrusion detector 1 is mounted in an appropriate location in a room and by means of the knob 6b, the detector element 6 is aligned in the appropriate direction. This aligning of the detector 1 may be carried out with the terminal cover 3 removed and the knob 6b may have alignment marks etched on it so the alignment of the detector element 6 can be determined. As shown in Figure 6, the detector element 6 is aligned so that infra-red radiation entering the housing passes through the lenses labelled a-f which provide cover for approximately a 90'field of view, the remaining lenses being, in this case, redundant. In the case where the LED 15 is mounted beside the detector element 6, the light from the LED 15 will be projected through the same lenses which are in use by the detector element 6. Thus a person conducting the 'walk test'will have a visual indication as to which zones are in use.

In Figure 7, the infra-red intrusion detector 1 is mounted 2 meters above the ground. With a quadruple detector arrangement which provides equal sensitivity to changes in the X and Y axes, the detector element 6 will respond to the movement in both the X and Y axes and movement towards the detector will trigger the alarm.

The detection zone can be continuously varied as shown in Figure 7. By moving the detector element 6 in a vertical direction relative to the lenses then the zones (a), (b) or (c) may be protected. Movement into or out of these zones will trigger the alarm. To provide a plurality of detection zones in the vertical plane for instance zones (a), (b) and (c) of Figure 7 together then additional arrays of fresnel lenses or detector elements may be fixed to the apparatus.

Referring now to Figure 10, there is shown on a very enlarged scale a representation of a quadruple element pyro-electric crystal 100. The pyroelectric crystal 100 comprises a flake or film having four electrodes M, N., 0 and P with positive polarity on the rear face and negative polarity on the front face thereof. Two output leads R and S are shown. The electrodes may be interconnected in a variety of different configurations. For example, in Figure 10 GB 2 146 503 A 5 and the schematic Figure 1 Oa, a particular configuration is shown wherein the negative terminals of electrodes M and 0 are interconnected by a lead D; the negative terminals of electrodes N and P are interconnected by a lead E; the positive terminals of electrodes M and N are interconnected by a lead F; and the positive terminals of electrodes 0 and P are interconnected by a lead G. In Figure 10b, a configuration is shown which gives two pairs of series assist electrodes in parallel opposition. In Figure 1 Oc, a configuration is shown which gives two pairs of series opposed electrodes in parallel assist. In Figure 'I Odthere is shown an all in parallel configuration whereas in Figure 10e, an all in series configuration is shown. Although what is shown in each of Figure 10 and related schematics Figures 1 Oa-10e is a single pyro-electric crystal having four electrodes, it will be appreciated that four separate pyro-electric crystals may be linked in the manner shown in the schema- tics or alternatively two dual element pyro-electric crystals may be linked together as shown in the schematics. It will, however, be appreciated that using a single pyro-electric crystal having four electrodes, a considerable saving in space and expense may be achieved.

The use of a four (or more) element detector arrangement as described above enables a target to be detected in both the X and Y axes, at the same time cancelling out unwanted signals that may be generated if the detector were subject to a temperature change.

Even so, in the system described there is no provision to ascertain in which direction the movement of the target occurs, only that a movement has occurred in the X and Y axis or a combination of the two (diagonal).

If instead of combining the outputs of the individual elements (or detectors) by series, series parallel, or parallel arrangements to provide con- figurations as described above with respect to Figures 1 Oa-1 Oe, the individual signals produced by each element or detector are monitored (by bringing a connection out from each element or detector), a great deal more information about the nature of the target movement may be obtained. Examples of such configurations are shown in Figures 10f-10h. It would, for example, be possible to eliminate such false alarm hazards caused by unwanted signals generated by such objects as a heater being switch- ed on or off, or by a swinging sign in a shop that was being protected. In the case of the heater it may happen thatthe image of the heater may be focussed on one of the elements only, causing an out of balance signal to be generated. The image of the swinging sign may be failing alternatively on one 120 element and then another, also causing an alarm signal to be generated.

However, by suitable signal processing that is made possible by monitoring each individual ele- ment, repetitive signals could be rejected (the swing- 125 ing sign) or single events ignored (the heater). The ability to obtain direction information of the target also makes the device very versatile in industrial applications outside of the field of intruder detectors.

In Figure 1 Of, there is shown an individual output 130 indicationconfiguration. In Figure 1 Og, there is shown a series paired output configuration. In Figure 10h, there is shown a parallel paired output configuration. The polarity shown in Figures 10g and 10h indicate the polarisation of the individual electrodes so that it is only necessary to provide one signal wire per pair of detectors as the polarity of the signal will reverse when the target moves in the opposite direction. Each of the signal wires would require buffering by an arrangement an example of which is shown at 6 in Figure 8.

Referring now to Figure 11 there is shown the detection zones of a nineteen element f resnel lens system and the detector element 6 is shown. The angle of view of the detector 6 is about 120' measured between the zone of lens 4 and the zone of lens 16. Lenses 1-3 and 17-19 are redundant. The maximum width of the zones of each tens varies between 0.5 metres for lenses 4 and 16 to 1.0 metres for lens 10. The range of detection varies between 3 metresfor lenses 4 and 16to 16 metresfor lens 10. It would be appreciated that by rotating the detector element 6 for example, clockwise as viewed in Figure 11, lenses 17-19 may be viewed by the detector element 6 and lenses 1-7 will be redundant. In this case, lens 13 will have the maximum range of detection.

It will be appreciated that a 360o window may be provided and more than one detector element 6 be mounted in the instrusion detector assembly so asto provide increased field of cover. Such an intrusion detector system could be mounted from the ceiling in the centre of a room and either the intrusion detector system could be rotatable or preferably one detector element maybe mounted therein which may be rotated by an electric motor to provide a 360' field of view.

Claims (4)

1. A solid state bi-lateral switch for use with a system as claimed in any of claims 1-14 of parent application No. 8317194, which switch comprises a first oscillator and a second oscillator, and wherein the first oscillator, in use, oscillates by virtue of the receipt of an alarm signal from the system and vice versa and the second oscillator, in use, is deactivated by the receipt of said alarm signal and vice versa and wherein each oscillator is operatively associated with a pair of VMOS transistors via respective isolating capacitors and voltage multipliers, the arrangement being such that said VMOS transistors are activated when the operatively associated oscillator is oscillating.

2. A switch substantially as hereinbefore described with reference to Figure 9 of the drawings.

New claims or amendments to claims filed on 27 9 84 Superseded claims 1 and 2 New or amended claims:- CLAIMS 1. A solid state switch comprising an oscillator 6 GB 2 146 503 A 6 circuit adapted to oscillate only upon receipt of an input signal having a given level, a pair of VIVIOS transistors connected in series opposition, and means including isolating capacitors and a rectifier circuit in that order connecting the output of the oscillator in common to the gates of the transistors, the arrangement being such that the VIVIOS transistors conduct only when the oscillator circuit is. oscillating.

2. A circuit as claimed in claim 1, wherein the connecting means further includes a voltage multiplier.

3. A circuit as claimed in claim 1 or 2, wherein the oscillator circuit comprises CIVIOS inverters.

4. A circuit as claimed in claim 1, substantially as described herein with reference to the accompanying drawing.

Printed in the UK for HMSO, D8818935.2i85,7102. Published byThe Patent Office, 25 Southampton Buildings. London, WC2A lAY, from which copies maybe obtained.