WO1992015972A1

WO1992015972A1 - A detector

Info

Publication number: WO1992015972A1
Application number: PCT/GB1992/000362
Authority: WO
Inventors: Robert Paul Fletcher; Simon David Tredinnick
Original assignee: Sabed Limited
Priority date: 1991-02-28
Filing date: 1992-02-28
Publication date: 1992-09-17
Also published as: AU1331392A; EP0573518A1

Abstract

A detector for sensing proximity or contact of a detectable object, such as a user's hand or an article held by a user, suitable for use as a proxmity switch or as an alarm detector comprises a plastics material plate (12) to one face of which is secured a conductor (15) which may be a wire, a plate or the conductive element of a piezo-electric transducer, having terminals (15a, 15b) connected to a detector circuit comprising threshold detectors (41, 42; 86; 125, 126; 231, 235) sensitive to DC voltage changes and operative in response thereto to generate an output electrical signal which can be used to trigger an alarm (33, 34, 35; 97; 147, 148) or to commute a switching circuit (202). In dry conditions proximity of a detectable object such as the hand of the user causes a capacitive change which is detected by the DC voltage detectors; backup for moist conditions is provided by the piezo-electric detector generating a signal detectable by the DC voltage detectors.

Description

A DETECTOR

The present invention relates generally to a detector suitable for use in a whole range of circumstances and for many different purposes, including use in an alarm system, and particularly a special purpose alarm system which has potential application in a number of different fields.

The detector of the present invention also finds application in a number of other uses such as switching and as a proximity sensor.

Alarms are used generally to detect intrusion or the presence of an intruder. Intruder detection is frequently achieved by means of vibration sensors or accelerometers fitted to openings or to the closures for such openings (doors, windows etc.) or by optical means such as photo¬ sensors and light-beams spanning an opening. The presence of an intruder within a protected space may also be detected by ultrasonic means. Detectors sensitive to the presence of an intruder within a protected space also include microwave systems in which a microwave field is generated within the space being protected, which can be inductively coupled to sensors which detect a change in the field due to disturbances by the intruder's presence. Capacitive proximity sensors are also known. All such devices, however, involve some form of field generation and radiation in order to be able to operate remotely. This makes them unsuitable for some applications due to inadequate stability, especially the risk of being triggered by other electronic equipment nearby, which generates electromagnetic radiation at unpredictable frequencies and waveforms. Furthermore, known such devices are not at all suitable for the more rugged environment of, for example, the engine compartment or underside of the motor vehicle.

There is a need in general to provide a rugged detector capable of providing an output indication or signal in response to proximity and/or movement of a detectable object in the vicinity of the sensor.

According to one aspect the present invention provides a detector responsive to the proximity of and/or contact by a detectable object, comprising a sensor body of insulating material electrically connected to an electrically conductive element in turn connected to a detector circuit responsive to the static potential difference across the conductive element and operable, if the said potential difference crosses a triggering threshold, to trigger generation of an output signal. Preferably the insulating material is coated with a modified silicone conformal coating. It has been found that this, surprisingly, considerably enhances the sensitivity of the sensor. This is believed to be due to an enhanced ability of the insulating body to retain its charge.

One practical application of the present invention lies in the provision of a motor vehicle alarm having means for detecting the movement or presence of an object or of a human or a non-human animal or a part thereof in a protected region, which may be within the vehicle or between the ground and the underside of the motor vehicle.

It is believed that the detector of the present invention may be considered to function as a result of the build up of an electrostatic charge on the exposed face of the insulating body. This happens naturally due to the presence of ions in the surrounding air which tend to bind themselves to the surface as they pass due to the small ever present air currents in the atmosphere. It is probable that this is similar to the charging effect of thunder clouds.

In order to avoid spurious triggering of the detector by animals, especially when the detector is-used in an alarm system, there may be provided means for deterring non-human animals from entering the protected region, that is the region within which the sensor is operative. Such deterrent means may, for example, comprise means for generating and radiating an ultrasonic acoustic signal at a very high frequency to which the human ear is insensitive. A signal in the region of 20 - 40 KHz has been found to be virtually unnoticeable to people, but to be a source of discomfort to animals, especially domestic pets such as cats and dogs, resulting in a marked reluctance of such animals to enter or remain in the region of such ultrasonic signal.

Unlike previously known proximity sensors, the sensor of the present invention does not act to radiate any field and is therefore unaffected by electromagnetic radiation from nearby electronic apparatus, and is consequently very stable. It is thought that the proximity of a detectable object, such as a part of the human body, for example a limb or a hand, or even the body as a whole, causes leakage from the exposed surface of the sensor of negatively charged ions (electrons) naturally present due to environmental effects and the nature of the plastics material from which the sensor is made. This leakage of negative ions leaves the sensor momentarily slightly positively charged and this results in a small current flow through the conductive element in intimate contact with the insulator body. This brief current flow, which only takes place while the plastics material of the sensor reestablishes its previous equilibrium state by the movement through it of charged particles, is detected by a sensitive electronic trigger element or threshold detector, to generate an output signal which can be latched, amplified and processed as desired. In this respect it will be noted that in use the conductive element is held at one end at a reference potential, typically grounded. It is also of significance to note that the presence in the vicinity of the sensor of an ioniser acting to enrich the natural supply of negative ions in the immediate environment around the sensor, will increase its range of sensitivity without any other charges being required.

As mentioned above, the insulating material may be coated with a modified silicone conformal coating; the detection of the proximity of a detectable object may be made on the basis of the charge level as determined by the voltage sensed on the said conductor associated with, or connected intimately to, the insulator. The detectable object need not necessarily be animate; it has been found that the sensor of the present invention will respond to the proximity of inanimate objects as well. For example, when the sensor is armed it is possible to set it off by bringing up to it an inanimate object such as a piece of wood or metal .

As far as the alarm indicator itself is concerned, this may be controlled via a central control unit, which may be connected to receive input signals from other sensors in addition to the said sensor, and which may be sensitive to other phenomena to indicate, for example, attempts to gain access to the interior compartment of a vehicle such as the engine compartment, the passenger compartment or the boot. The said central control unit preferably energises at least one alarm indicator when triggered, which alarm indicator may be one or more of an acoustic indicator and/or an optical indicator and/or a remote communication device. The acoustic indicator may be a speech synthesiser which may, for example, be programmed to announce the nature of the alarm event and other information concerning the protected environment, such as the make, model, colour and/or registration number of the vehicle to which it is fitted. Alternatively, the central control unit may energise a remote communications unit for example via a cellular telephone link, to transmit a pre-recorded message identifying the vehicle, the alarm event, and perhaps also the location of the vehicle (which would have to have been entered onto the recording preliminarily, for example as the alarm is armed) . Various embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure l is a schematic view of a sensor panel formed as an embodiment of the present invention;

Figure 2 is a block schematic diagram illustrating the major components of an alarm system incorporating a sensor formed according to the principles of the present invention, for example the sensor of Figure 1;

Figure 3 is a wiring diagram illustrating the major components of a detector circuit forming part of the alarm system of Figure 2;

Figure 4 is a perspective view illustrating an alternative form of sensor;

Figure 5 is a schematic view of a part of the sensor of Figure 4, shown inverted for clarity;

Figure 6 is a circuit diagram showing a second embodiment of detector circuit;

Figure 7 is a schematic circuit diagram illustrating a further embodiment of the invention;

Figure 8 is a schematic diagram illustrating an embodiment formed as a proximity switch;

Figure 9 is a detail illustrating a component of the circuit of Figure 8; and

Figure 10 is an alternative switch circuit. Referring now to the drawings: Figure 1 illustrates, in sectional side view a sensor 11 formed as a flat panel suitable for fitting to the front of a casing to detect the proximity of a detectable object, such as a user's hand, in the region marked X enclosed within the broken outline. This region has been shown in broken outline in order to emphasise that its boundaries are not precisely defined and may even vary with time in dependence on atmospheric conditions.

The sensor 11 comprises a flat, rectangular plate 12 of plastics material, preferably a polycarbonate although other plastics materials such as thermoplastics also work satisfactorily (but not quite so well) , having a front face 13 and a rear face 14 to the latter of which is attached a thin conductive wire 15 which, as shown in the inset Figure la, follows a path in a loop spaced inwardly of the perimeter of the plate 72 so as to encompass a substantial part of the area of the rear face 14. The conductive wire 15 has two terminal ends 15a, 15b connected to a detector circuit as will be described in more detail, in relation to other Figures, below.

Over the wire 15 is an insulating layer 16 and a conductive screening layer 17 to which is secured a printed circuit board carrying the electronic components of the detector circuit which produces an output electrical signal when a detectable object enters the region X or touches the exposed or front surface 13 of the plate 12.

The sensor may be used for a number of purposes, but will be described hereinbelow with special reference to its use as an alarm sensor or as a switch sensor for switching an electric current on or off.

As can be seen. Figure 2 shows a circuit for a motor vehicle alarm incorporating a sensor such as that described in relation to Figure 1. It is envisaged that in this embodiment the sensor 11 would be shaped differently to enable it to be fitted to the underside of a vehicle to detect any attempt to attach anything to the vehicle. The conductive wire 15 is connected to the input of a spurious noise rejector or filter 19 which filters out any unwanted frequencies and/or any voltage spikes, in particular the 50 Hz signal which may be picked up from nearby mains cables or the like. The filtered signal is fed to an amplifier 20 which amplifies the filtered signal and passes the amplified signal to an analogue-to-digital converter 21. A part of the amplified signal from the amplifier 20 is also fed back to the filter 19 along feedback line 22. This helps to cancel out unwanted disturbances and increases the efficiency of the filter. The analogue-to-digital convertor 21 converts the irregular signal to a positive voltage swing which is then fed to a current amplifier 23. The current amplifier 23 acts as a buffer to interface with a central control unit 24 which receives the signal from the current amplifier 23 as one of its inputs.

It is important to appreciate that the central control unit which forms the heart of the alarm system, is not a microprocessor based control system and, therefore, has no clock frequency. It is considered that a control system having a clock would be inappropriate for a vehicle or other alarm since it would be possible to corrupt it by interrupting the microprocessor clock frequency thereby causing the CPU to crash. The central control unit 24 is a circuit employing mainly gates and buffers. In addition to the input from the current amplifier 23, the central control unit 24 has an input from a series of secondary sensors, such as sensors detecting the closed state of the bonnet, the boot, or the vehicle doors, which provide an indication of any attempt to enter the vehicle. The central control unit also has an input from the car ignition on line 26 which will disconnect the alarm system when the vehicle engine is running (and therefore by implication there is a driver present in the vehicle who could detect any attempted intrusion) and further inputs from an arm/disarm keypad 27 which is used by the operator to set the alarm to its armed condition and to disarm the alarm once the alert state is no longer required. A final input on line 28 from the voltage supply via a voltage regulator and filter 29 is also applied to the central control unit 24. The voltage regulator and filter 29 is supplied with voltage from the motor vehicle 12 volt battery supply on line 30 and, since it is important that the alarm be maintained alert even if the battery supply should fail, a supplementary power supply backup battery 31 is also provided, this being normally recharged when the vehicle battery is recharged, and brought into operation only if the voltage supply from the main battery should fall below a critical threshold value. The output from the voltage regulator and filter 29 is also supplied to the ultrasonic generator 17 the signal from which is fed to a series of ultrasonic transducers on lines 32.

The output from the central control unit, generated when an^" input signal from the sensor wire 15 arrives via the current amplifier 23, or when an input signal on line 25 arrives from any of the secondary sensors, is supplied to one or all of the alarm indicator devices with which the system is provided. In Figure 2 there is shown a voice synthesis unit 33 which acts to announce the occurrence of an alarm event at a high volume. The voice synthesiser unit 33 is also connected to a transmitter unit 34 of a paging transmitter or telephone interface of a mobile telephone system by which the vehicle owner or an appropriate supervising authority can be alerted remotely to the occurrence of an alarm event at the protected vehicle.

Finally, the central control unit 24 provides an auxiliary output to an auxiliary output interface 35 by which subsidiary functions such as inhibition of the central locking unit of the vehicle may be achieved.

The central control unit 24 also directly controls two indicator lamps 36, 37 the first of which indicates that the system is in its armed condition and the second of which indicates that a tamper sensor has generated a signal indicating that an attempt to tamper with the vehicle security system has been made. Such tamper sensor may include means for detecting short circuit or open circuit of sensor wire 15 or any attempt to isolate the system from the battery supply in the vehicle.

Figure 3 illustrates the detector circuit in more detail. Across the ends 15a, 15b of the sensor wire 15 are connected two spark gap units 38, 39 which are grounded to an earth line 40 for protection of the subsequent electronic circuit against voltage spikes which may be induced on wire 15. Each end 15a, 15b of the wire 15 is connected to a respective NAND gate 41, 42 connected as a buffer, the inputs of which are connected to the ground line 40 via reverse biased diodes 43, 44. The outputs from the NAND gates 41, 42 are supplied via current limiting resistors 45, 46 to respective pairs of further NAND gates 47, 48, 49 and 50 which are strapped together by feedback resistors 51, 52. The outputs from the NAND gates 48, 50 are supplied to a further NAND gate 53 acting as a summing gate, the output from which is AC coupled by resistor 54 and capacitor 55 to a differential amplifier 56, the output of which is fed back via capacitor 57 to the inverting input of the amplifier 56 itself, and, via resistor 58 to the junction between two capacitors 59, 60 of low capacitance (in the low picoFarad range) which reduces the high frequency response of the circuit.

The output from amplifier 56 which is now a clean, analogue signal, is AC coupled via resistor 61 and capacitor 62 to an analogue-to-digital convertor 21. The output from this circuit on line 63 goes high when the input signal exceeds a threshold value, thereby biasing a latch transistor 64 into the conductive state allowing passage of the signal arriving on the clock input to the convertor 21, which is fed on line 65 to the collector of the transistor 64 thereby maintaining this transistor in the conductive state. The output signal is also fed on line 66 to the trigger of the central control unit to trigger the alarm.

Figures 4 and 5 illustrate an alternative form of sensor, generally indicated 70. In this case, the sensor includes an elongate element of insulating material, in the illustrated embodiment in the form of a generally U-shaped channel element 71, having a base 72 and two parallel side walls 73, 74. Fixed to the edges of the walls 73, 74 is a conductor 69 having conductor terminals 75, 76 which, as schematically illustrated in Figure 5, are connected to a detector circuit mounted on a printed circuit board generally indicated 77. Over the conductor 69 is fitted a flat cover strip 78, also of the same insulating material as the channel 70, and the ends of the channel 70 are closed by end caps 79, 80. It is to be emphasised that the illustration in Figures 4 and 5 is a schematic representation of the sensor; in practice the conductor 69 is entirely enclosed within the insulating casing formed by the channel 70, the cover strip 78 and the end caps 79, 80 so that the sensor defines an entirely enclosed cavity from which two leads, an output signal lead 81 leading to an alarm device and a power supply lead 82. As will be seen in Figure 4, a plurality of polystyrene beads 110 may be located in the cavity defined by the channel 70 and cover 78. It has been found that these, too, act to increase the ability of the sensor to retain its charge.

The elongate sensor 70 operates on the same principle as that described in relation to Figure 1, although in this case the whole of the channel casing serves the same purpose as the plate 12. The sensor 70 may be used as a portable sensor, for example as a sensor bar which can be placed across the front seats of a vehicle or in any other location where it is desired to detect proximity or contact.

The circuit on the printed circuit board 77 is illustrated in Figure 6. One of the two conductor terminals 76 is shown grounded at 84, and the other conductor terminal 75 is shown connected to the input of a Schmidt trigger 86.

The terminal 75 is connected via a high value resistor 85 to the Schmidt trigger 86, which is connected via a further resistor 87 to a Schmidt trigger pair 88, 89 with a feedback resistor 90. The output from the Schmidt trigger 89 is fed via a buffer trigger 91 to the input of a NAND gate 92 connected via an RC circuit 93 comprising resistor 94 and capacitor 95 to the inputs of a second NAND gate 96 the output of which constitutes the trigger output from the sensor circuit applied to an output terminal 97. The power supply, applied to a positive terminal 83, is supplied via an LC circuit comprising inductor 98 and capacitor 99 to a voltage regulator 100. A further capacitor 101 is connected in parallel with capacitor 99 and acts as a reservoir capacitor. Across the output of the voltage regulator 100 is a capacitor 102 which serves as a filter. The output from the voltage regulator 100 is applied to a relaxation oscillator constituted by a unijunction transistor 103, resistor 104 and capacitor 105. The output from the relaxation oscillator is applied to the input of a NAND gate 106 the output from which is fed as a supply to the Schmidt trigger 86. Periodically, switching off the Schmidt trigger and effectively allowing clamping diodes (not shown) ^• ithin the integrated circuit package of which the triggers 86, 88, 89 and 91 form a part, allows the circuit to form a leakage path for discharging the charge accumulated on the insulator of the sensor to prevent this from building up to a level which will fire the Schmidt trigger.

Charge accumulates on the sensor 70 by the friction with the air as this passes over the sensor. As explained above a separate negative ion generator may be provided in the vicinity of the sensor to increase its range of sensitivity by causing a more rapid build up of ions on the sensor surface. It has been found that a sensor formed as an embodiment of the invention, for example as illustrated in Figures 1, or 4 and 5, is capable of maintaining an electrostatic charge for very extended time periods if suitably treated. The treatment proposed comprises the application of a known conformal coating sold under the reference SCC3 and made by Electrolube Limited. This material is a clear lacquer normally used for protecting electronic components from environmental agents. It comprises a modified silicone coating, containing Xylene. It has been found that this material surprisingly enhances the charge-retaining quality of the dielectric material.

Turning now to Figure 1 , the alternative sensor circuit illustrated is adapted to be connected to a sensor unit of the same general type as that described in relation to Figure 1 or Figures 4 and 5, across terminals 121 and 122.

As in the preceding embodiments, the sensor is located in the area in which movement or contact is to be detected and is insulated from its environment apart from the connection to the two terminals 121, 122. In the same way as before, therefore, an electrostatic charge builds up on the insulating component of the sensor due to environmental agencies. The terminal 122 is connected to a ground line 120, whilst the terminal 121 is connected to a high value resistor 123 which determines a very high input impedance of the circuit. A variable capacitor 124 is connected between the resistor 123 and ground for varying the sensitivity of the circuit as will be described below. The capacitor 123 is connected to the inputs of a twin-input Schmidt trigger 125 the output of which is connected to a further, identical, twin-input Schmidt trigger 126. The two Schmidt triggers form part of an integrated circuit package including two further Schmidt triggers 137, 138 the function of which will be described in more detail below and also including protective diodes (not show) in the biasing circuit which normally protects the triggers from voltage spikes which might occur in the power supply. These diodes are periodically forward biased by reducing the biasing voltage to zero by means of a part of the circuit which will be described in more detail below. The effect of this periodic forward biasing is to allow the diodes to form a leakage path for the charge which builds up on the sensor 70 so that despite fluctuating environmental conditions the maximum charge on the sensor 70 remains below a certain threshold. The discharge frequency, which is selectable by choice of components, is chosen in dependence on the sensor circumstances so that the voltage on the sensor is maintained just below the trigger threshold of the Schmidt trigger 125. This discharge frequency is typically in the region of between 1 and 7 seconds. In quiescent conditions, therefore, the voltage on the input to the Schmidt trigger 125 is effectively zero.

Although the sensor may detect and respond to 50 Hz signals in the vicinity the filter constituted by the high value resistor 123 and the variable resistor 124, supplemented by an additional capacitor 119 in parallel therewith, acts as an AC attenuator or low pass filter to substantially eliminate the 50 Hz signal. Resistor Rl at the same time acts as a current filter or voltage attenuator since the input of the Schmidt trigger 125 is sensitive to static charges.

The output from the Schmidt trigger 125 is a "high" (i.e. 5 volt) signal in quiescent conditions when the input is low. The output from the Schmidt trigger 125 is fed to a following Schmidt trigger 126, which is therefore subjected to a high input signal and thus generates a low (effectively zero volt) signal at its output. The output of the Schmidt trigger 126 is connected to a twin T filter generally indicated 147 and comprising two capacitors 148, 149 in series. The junction between the capacitors 148, 149 is connected to a resistor 150 and two resistors 151, 152 in series with one another and in parallel with the capacitors 148, 149, with the junction between them connected to a capacitor 153 connected, together with the resistor 150, to the output of an inverter 127 which introduces a 180 degree phase shift into the signal. The twin T network 147 acts as a band pass filter centred on 50 Hz to effectively remove any 50 Hz component from the signal. The output from the inverter 127 is thus, in quiescent conditions, at the "high" level and upon triggering of the sensor, for example by rapid discharge of its accumulated charge, when the Schmidt trigger 125 is in its normal biased state produces a clean square negative-going pulse. This is applied to an RC network comprising resistor 128 and capacitor 129. In quiescent conditions capacitor 129 is charged and this, therefore, discharges through resistor 128 upon occurrence of the negative-going pulse resulting from detection from an event by the sensor 70. This acts to slow down the response of the circuit to ensure that triggering response is only given to events which are certain. Capacitor 129 and resistor 128 are connected to the input of an inverter 130 the output of which is supplied via a diode 155 to a capacitor 131 which is connected to the ground line 120. Because the output of the inverter 130 is low in quiescent conditions, the capacitor 131 also acts to slow the transmission of the "event" signal due to the time taken for the capacitor 131 to charge. The cathode of the diode 155 is also connected to the input of a further inverter 132 the output of which is fed via an inverter 147 and a resistor 157 to a light emitting diode (LED) 148 which is thus illuminated when the output from the inverter 132 goes low and consequently the output from the inverter 147 goes high to indicate the occurrence of an "event".

The output from inverter 132 is supplied via a resistor 158 to a switching transistor circuit comprising a PNP transistor 145 and an NPN transistor 146. The latter is connected via a relay coil 147 between a positive supply voltage on line 159 and a ground line 156. The base of transistor 146 is connected to resistor 158. Transistor 146 is normally biased to its conducting state by the "high" signal normally appearing at the output of the inverter 132 in the quiescent state of the circuit. Current therefore flows through relay coil 147 to keep the contacts 148 normally open; contacts 148 close when current ceases to flow through coil 147 to provide an output trigger'signal on terminals 160 and 161.

Returning now to the Schmidt triggers 125 and 126, the output from trigger 126 is also connected to two capacitors 133, 135 the former of which is connected to resistor 134 and to the input of the Schmidt trigger 137 referred to above which forms part of the common package with triggers 125 and 126. The output of trigger 137 is connected to the inputs of a further trigger 138 the output of which is connected via a resistor 161 to a light emitting diode 139 the cathode of which is connected to the ground line 120. The RC network comprising the capacitor 133 and resistor 134 acts as a potential divider and low pass filter allowing any 50 Hz signals to feed through to the triggers 137, 138 which thus effectively act to square the signal supplied to the light emitting diode 139. Illumination of this diode will assist, when the sensor is being located, to provide an indication of the proximity of a 50 Hz source which may disrupt the operation of the sensor.

A similar RC circuit comprising capacitor 135 and resistor 136 is connected in parallel to the circuit 133, 134 and acts, likewise, as a low pass .filter feeding the 50 Hz component of the output from Schmidt trigger 126 to the input of an inverter 141 connected via a switch 140 and a diode 142, along line 143, to resistor 144 connected to the base of PNP transistor 145. The emitter of transistor 145 is connected in the base circuit of transistor 146 and is normally at the high level. Transistor 145 is normally non¬ conducting but becomes conductive when the voltage at its base exceeds 0.6 volts, that is when a 50 Hz signal is detected across the terminals 121, 122. This is transmitted to transistor 145 only if switch 140 is closed, and therefore this circuit can be used, after installation, to detect the occurrence of a 50 Hz signal which, for example, may be introduced deliberately to disturb the operation of the circuit by someone intending to "jam" its operation. When the transistor 145 becomes conductive the voltage at its emitter falls thereby turning off the normally conductive transistor 146 causing the alarm relay contacts 148 to close thereby initiating an alarm output.

The repeated cyclical discharge of the sensor is effected by means of a free running oscillator comprising a unijunction transistor 149 connected between two biasing resistors 163, 164 between line 159 and the ground line 156. A further resistor 165 is connected between line 159 and the gate electrode of the transistor 149 and a capacitor 166 is connected between this gate and the ground line 156. Resistor 165 is chosen to allow the oscillation period to be such as to maintain the sensor just below the triggering threshold of the Schmidt trigger 125 and the output from the oscillator is taken via an inverter 150 to output line 167 which, as described above, is connected by means not shown to the biasing circuit of the Schmidt triggers whereby periodically to reverse bias the protective diodes once each cycle of the oscillator which, as mentioned above, may be between 1 and 7 seconds.

Turning now to figures 8 and 9 there is shown the application of the same basic principle to a switching circuit adapted to effect switching of a mains voltage signal line such as in a lighting or power circuit of a domestic, commercial, or industrial establishment. In this device the sensor acts to detect the proximity of or contact by a user's hand to allow a circuit such as a light circuit or power circuit to be switched on or off without requiring the movement of contacts. Such switching circuit has the considerable advantage of generating no spark and may therefore be of particular interest in hazardous environments such as those in petrol stations, and other establishments where there is a risk of the generation of combustible or explosive gases which may enter the atmosphere.

The circuit illustrated in Figure 8 comprises a modular sensor generally indicated 201, and illustrated in more detail in Figure 8, the output signals from which are supplied to a counter 202 used as a bistable circuit to produce an output on line 203 to the base of a transistor 204 via resistor 205. Transistor 204 is connected to an opto-isolator 206 which couples the signal generated by switching of the transistor 204 to a DIAC 208 acting as a bi-directional diode conductive above 30-35 volts in both directions, the output from which is applied to the gate electrode of a TRIAC 209 in series with the load line 210.

The circuit includes a single chip power supply 211 which is connected between a positive supply line 212 and a neutral line 213, and acts to produce a rectified and smoothed 5 volt DC output. Power supply 211 also acts as a voltage regulator. This is a commercially available chip and, apart from generating a regulated 5 volt output for the circuit described in relation to Figure 8, its operation is known to those skilled in the art and will not be further described here.

Detection of an "event" by the detector circuit 201, operation of which will be described in more detail in relation to Figure 9, causes the transistor 204 to be switched on and latched by the counter 202, and this is transferred via the opto-isolator 206 to the switching TRIAC 209 to control the load line 210.

Referring now to Figure 9, the modular unit 201 is shown in more detail. The main component of this circuit comprises a VMOS field»-effect transistor 220. The input signal from a sensor such as the sensor 11 of Figure 1 the insulating element of which in this case may be a rectangular plate such as the pattress of an electrical fitting of the same dimensions as a light switch (without, of course, the normal movable switching contacts, so that the pattress is simply a flat blank plate like a terminal box blanking plate) is applied to terminal 221 connected to the source junction of the field effect transistor 220 via a high value resistor 222.

Sensitivity control is achieved via terminal 223 which is connected to a circuit which clamps the input of the field effect transistor to ground potential (how?) .

The output of the field effect transistor 220, biased via a

Schmidt trigger 224 and resistor 225 is supplied to a further Schmidt trigger 226 the output of which is fed to a pump circuit comprising diode 227 and capacitor 228 which latter charges, via the diode 227 upon detection of a rise from the normal quiescent "zero" state at the output of

Schmidt trigger 226. This, delayed, voltage rise is applied to a further trigger 229 (also acting as an inverter) and to a final trigger 230, also acting as an inverter. Output signals are taken from the outputs of inverters 229 and 230 so that positive-going or negative-going signals upon detection of an event can be used in the switching circuit. This capability enables the modular unit 201 to be connected into different circuits from that illustrated in Figure 8 for other functions.

Figure 10 illustrates a switch circuit which can be used in place of the two circuits illustrated in Figures 8 and 9, for which reason the same reference numerals will be used to identify the same components. The primary difference between the circuit of Figure 9 and that of Figure 10 lies in the form of the primary trigger elements. In the embodiment of Figure 9 the sensor is connected to an input terminal 22, leading to a field-effect transistor 220. In

Figure 10 the input terminal 221 leads via a resistor 222 to the inventing input of an operational amplifier 231. The operational amplifier 231 has a negative feedback circuit comprising resister 232 and capacitor 233 acting both as a low pass filter and to set the gain of the operational amplifier. In particular, the low pass filter reduces the gain of the amplifier in response to an AC signal effectively attenuating the signals above about 30 Hz thereby isolating the remainder of the circuit from 50 Hz signals. The use of operational amplifiers in place of the field-effect transistor of Figure 9 or the Schmidt triggers of Figures 3, 6 and 7 is to adapt the circuit for a modified form of sensor in which the function of the conductor is performed by a piezo-electric element. The piezo-electric element has the additional function that, in circumstances where the electrostatic proximity effect is reduced, for example, by means of moisture, the user may nevertheless operate the switch by touching it lightly or tapping it with the tip of a finger thereby generating a voltage signal across the piezo-electric element which thereby causes an output signal from the operational amplifier 231. This output signal, whether generated by the piezo-electric element acting by the piezo-electric effect or acting simply as a conductor (in this connection the piezo-electric crystal of the transducer is intimately bonded to a conductive disc or plate) is fed via a resistor 234 to the non-inverting input of a second operational amplifier 235 which is biased to act as an open loop comparator. The inverting input of this amplifier 235 is supplied via a voltage divider comprising two series connected resistors 236, 237 and its output is fed, via a dipde 238 to the input of a counter circuit 202 in all respects identical to the counter 202 in Figure 8.

The output from counter 202, which effectively acts as a bistable or flip-flop circuit is supplied on Line 203 via resistor 205 to transistor 204. The transistor 204 is connected to an opto-isolator 206 which -isolates the detector circuit described above from the power switching circuit comprising DIAC 208 and TRIAC 209. The output signal from the counter 202 is also fed to a bi-coloured light emitting diode, illustrated as two diodes 210, 211 in Figure 10, to demonstrate that, in quiescent conditions when power is supplied to the circuit via the power supply 211 the output from counter 202 illuminates the bi-coloured diode to emit a green light whilst, when triggered to switch the TRIAC 209 on, thereby energising the load, the light emitting diode is energised to emit red light.

In an embodiment of the invention including the circuit illustrated in Figure 10, the sensor, which may be formed as flat plastics pattress for the front of a conventional domestic light switch, is provided with a piezo-electric element in the form of a piezo-σrystal disc secured to a metal conductor: the whole element is attached to the rear face of the plastics pattress securely by partly melting the plastics and displacing this to form overlapping connecting spots which hold the disc intimately in contact with the rear face of the pattress plate.

Claims

1. A detector responsive to the proximity of and/or contact by a detectable object, comprising a sensor body of insulating material electrically connected to an electrically conductive element in turn connected to a detector circuit responsive to the static potential difference across the conductive element and operable, if the said potential difference crosses a triggering threshold, to trigger generation of an output signal.

2. A detector as claimed in Claim 1, in which the sensor body has a wall portion defined by two opposite faces a first of which is in use exposed to the environment in which the detectable object is located and a second of which is contacted by the said electrically conductive element.

3. A detector as claimed in Claim 1 or Claim 2, in which the said sensor body of insulating material comprises or forms part of a substantially enclosed housing within which the detector circuit is located.

4. A detector as claimed in any of Claims 1 to 3, in which at least the exposed surfaces of the said insulating sensor body are coated with a modified silicone conformal coating.

5. A detector as claimed in any preceding Claim in which the said sensor body is a substantially flat plate of insulating material to one face of which the said conductive element is fixed.

6. A detector as claimed in Claim 1, in which the said conductive element is a piezo-electric crystal and the said detector circuit is responsive to the D.C. voltage generated across it in response to a bending stress applied to the said insulating sensor body by contact with a user's hand or finger.

7. A detector circuit as claimed in Claim 1, in which the detector circuit includes a low pass filter and a D.C. voltage threshold detector constituted by at least one Schmidt trigger or at least one operational amplifier.

8. An electric switching circuit comprising a detector as claimed in Claim 1 and solid state switching means operable to provide a conductive path and to interrupt 'the said conductive path alternately in response to successive detections by said detector circuit of proximity or contact by a detectable object.

9. An electric switching circuit as claimed in Claim 8, further including an opto-isolator between the said detector circuit and the said solid state switching means whereby to isolate the detector from switching transients during operation thereof.

10. An electric switching circuit as Claimed in Claim 9, in which the said solid state switching means includes a TRIAC the gate of which is connected via a DIAC to the output of the said opto isolator.

11. An alarm unit comprising a detector as claimed in Claim 1, in which the said isolating sensor body constitutes the housing for the said detector circuit and an alarm indicator operable to produce an audible and/or visible alarm output signal when triggered by the detection of proximity of or contact by a detectable object.