GB2305211A - Security locking circuit - Google Patents

Abstract

A security locking circuit is arranged to transmit 4 an interrogation radio signal and receive a return radio signal from a passive key circuit comprising a resonant tuned circuit. The security circuit issues a release signal for releasing a latch 50 on, for example, a pet door, if the characterisation of the return signal signal matches a pre-stored characterisation. The security circuit has a training mode in which a return signal characterisation is stored as one of the pre-stored characterisations.

Description

SECURITY LOCKING CIRCUIT AND METHOD OF MAKING KEY CIRCUIT FOR SAME This invention relates to a security locking circuit which finds particular, but not exclusive, use as a locking circuit for a latchable door such as a cat or pet door.

As described in GB-B-2,119,431, such a circuit is known which responds to a cat wearing a particular key to open the door to admit it into a household whilst not responding to the presence of other cats to prevent them doing so.

This prior art security locking circuit includes a transmitter coil and a means for pulse energising the coil.

When the animal moves the pet door a switch is closed which triggers the circuit to pulse-energise the coil and so emit a pulse of radio waves.

The field produced by the transmitter coil interacts with a tuned circuit in the adjacent key on the neck of the cat seeking entry causing a current to be generated in the tuned circuit consisting of a decaying a.c. current at a frequency determined by the resonant frequency of the tuned circuit.

This field produced by the tuned circuit is picked up by the transmitter coil and the a.c. components analysed by an oscillator/discriminator circuit which provides an unlatching signal, to unlatch the pet door, when the detected frequency is substantially comparable to the predetermined frequency as determined by the discriminator circuit. In this way, only a cat with a key whose tuned circuit operates at the correct frequency can gain entry through the particular pet door.

As a number of households in a given neighbourhood might wish to use such a pet door it is necessary to provide a number of distinct security circuits, each with a distinct predetermined frequency which it is arranged to discriminate, and a corresponding number of distinct designs of key circuits each of which produce one of these frequencies.

A householder with more than one pet needing to gain entry would have the appropriate number of key circuits tuned to the same frequency, one for each pet.

Usually the key circuits are attached to collars which are elasticated so they can come free of the animal if the animal's collar snags. It is therefore necessary for most users to purchase replacement key circuits from time to time.

It has been found that five distinct frequencies can provide adequate diversity in most cases. The particular frequencies selected were 20, 24, 28, 32 and 34 kHz, the security circuit and key circuits being turned to tlkllz.

This necessitates five variations of security circuit and key circuit to be manufactured and stocked. Further, the process of tuning the security circuit and key circuit is relatively time consuming and labour intensive.

It would be advantageous if more key frequencies could be selected within the available bandwidth to provide greater key diversity but this is not practicable with the prior art security circuit - key circuit system as this would result in yet greater stocking requirements and would increase the time and costs associated with tuning the components which would increase as the tuning tolerance tightened to less than +1kHz to accommodate the large number of distinct frequencies in the available band width.

The present invention provides a security locking circuit suitable for use with a latch for a door comprising: a transmitter arranged to transmit an interrogation signal; a receiver arranged to receive a reply signal emitted by a key circuit in response to transmission of the interrogation signal; means arranged to generate a characterisation of such a reply signal and compare the characterisation to one or more pre-stored characterisations and, on finding a satisfactory comparison between the characterisation and a pre-stored characterisation, to generate a release signal for releasing the latch; the circuit being arranged to be switchable to a training mode in which an interrogation signal is transmitted and a generated characterisation of a reply signal from a key circuit is stored as one of the prestored characterisations.

The security circuit functionality is most conveniently implemented by a microcontroller integrated circuit with stored program but other means of implementing the invention may be employed.

The interrogation signal is preferably an electromagnetic (e-m) radiation signal and the key circuit a passive resonant circuit device able to generate a reply signal without the need for a local power source.

Other forms of interrogation signal may be employed, for example ultrasonic signals. The e-m radiation is conveniently at radio frequencies but other portions of the e-m spectrum may be used, for example infra-red radiation.

Active key circuits may also be used if desired. However, the following discussion will assume the interrogation signal is a radio signal and the key circuit is a passive resonant circuit.

The transmission of the interrogation signal may be triggered by a switch operable by the animal or may be triggered at regular intervals by the controller regardless of whether an animal is near the circuit.

The present invention obviates the requirement for a pretuned discriminator circuit in the security locking circuit and a matching pre-tuned key tuned circuit. A key tuned circuit can be made with components which will place the resonant frequency somewhere in a desired bandwidth, as can be determined from component and manufacturing tolerances, but without the circuit being specifically tuned or its precise resonance frequency even being determined. The key can then be presented to the security circuit when operating in the training mode which circuit will then store its characterisation of that key in a memory store for future reference. The security circuit will then operate to provide a release or unlatch signal when that particular characterisation of key circuit is detected.

Thus, only one design of security circuit need be manufactured, and a very large key diversity can be provided by increasing the discriminatory power of the security circuit to +0.lkHz, for example, and manufacturing key circuits with random tuning to which a security circuit can be trained to respond, as required.

A stock of distinctly pre-tuned key circuits is not now required. If a key circuit is lost it can be replaced by a randomly selected key circuit, the security circuit being put into training mode once more so it will now be retrained to respond selectively to the new key circuit.

An embodiment of the present invention can provide access to a number of users of the same security circuit by providing storage of a number of characterisations of key tuned circuits, each one being obtained by operating the security circuit in training mode with the respective key circuits.

The elimination of the requirements for pre-tuning the security circuit and key circuit, particularly the latter, provides significant savings in manufacturing costs.

Typically, a tuned resonant circuit is made by push-fitting a ferrite rod inside the coil of a series coil-capacitor resonant circuit and finely adjusting the rod until the resonant frequency of the tuned circuit is equal to one of the preselected wavelengths, to within the accepted tolerance. The components were then fixed in position by potting the circuit into a holder.

A key tuned circuit specifically for use with the security circuit of the present invention, in the embodiment employing such a passive resonant circuit, can be made according to the method according to a second aspect of the present invention namely by forming an electrical resonant circuit including a capacitor in series with an inductance coil surrounding a ferrite core and potting the resonant circuit to fix the resonant frequency of the resonant circuit without performing any tuning of the resonant circuit. This method dispenses with the previously necessary, and time consuming, tuning stage of the manufacturing process.

The security circuit is preferably switched to training mode by a switch accessible from the side of the mounting for the circuit which is interior to the building to which access is required. This may, conveniently, be a recessed switch accessible through a housing to avoid accidental switching of the security circuit to its training mode. The switch is conveniently pressed to close the switch and biased to open when pressure is released.

The security circuit is preferably arranged such that after generation of the release signal a lock signal is generated in the event of either: a) the subsequent absence of the receipt of a reply signal; or b) the receipt of a reply signal having a characterisation which has a non-satisfactory comparison with a pre-stored characterisation in response to the transmission of an interrogation signal.

Preferably, the conditions for finding a satisfactory comparison with a stored characterisation to re-lock the latch are less stringent during this monitoring phase of operation to reduce the likelihood of a relocking-unlocking cycling of the door latch in the presence of an animal to be admitted.

The security circuit is, conveniently, battery operated but may be configured to be mains power supply operated, with, optionally, a battery back-up power supply, for example a trickle charged rechargeable battery.

If the security circuit is battery operated, it is preferable that the circuit is arranged to minimise power consumption. Accordingly, preferred battery powered embodiments will include power saving arrangements, for example intermittent operation of the transmitter and operation of the receiver only when required. This is conveniently achieved under the control of a microcontroller, itself operated in low-power, sleep mode, for a high proportion of the time.

A low battery warning circuit, again preferably operated intermittently, is conveniently provided to provide an indication of when the battery is running low of stored charge.

An apparatus, for example a pet door, with a latch lockable by a locking mechanism responsive to receipt of an release or latch signal may be used with the security circuit of the present invention. The locking means preferably includes a DC electric motor which can be impulse driven between a locking position and an unlocking position with only a relatively small current drain compared to that required by a solenoid operated locking mechanism, for example.

In a preferred embodiment of such an apparatus, the controller of the security circuit is also arranged to provide an a.c. drive current to the motor during the training mode so the motor emits distinctive audio tones to indicate the various stages of the training mode as an aid to the user configuring the security circuit. The frequencies employed are in the order of 500Hz so there is no appreciable rotation of the motor and the locked/unlocked state of the latch is unaffected. This mode of operation obviates the need for a separate sounder in the apparatus.

It will be understood that the use of a DC motor as an audio sounder is not restricted to use in conjunction with the security locking circuit of the present invention but is of general applicability to other apparatus having a DC motor in which audio-feedback to a user to indicate an operational mode, or other reason, is required.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a circuit diagram of a security circuit according to the present invention; Figures 2 and 3 are diagrammatic views of the latch and locking mechanism of a pet door controllable by the security circuit of Figure 1 with the latch in the locked and unlocked state, respectively; Figure 4 is a diagrammatic side view of the latch and locking mechanism as shown in Figure 2; Figure 5 is a diagrammatic side view of the latching and locking mechanism as shown in Figure 3; and Figure 6 is a diagrammatic side view of the latch and locking mechanism of Figure 4 with the latch about to latch the door.

Figure 1 is a security circuit 2 suitable for use with the cat flap latched door which will be described with reference to figure 3. The component list for this circuit is reproduced at table 1 at the end of this description.

The security circuit 2 comprises a transmitter circuit 4, a receiver/amplifier circuit 6, a microcontroller 8, a motor drive amplifier 10, a voltage regulator 12, a bubble switch 14 biased to an open position and a low battery detection circuit 16 and an aerial 18.

The voltage regulator 12 provides a stabilised +9V DC input from a mains adapter (not shown) applied to terminal 22 relative to a OV earthed terminal 24. The 9V DC input voltage is applied across a series voltage divider comprising resistor R13 and zener diode ZD2 to provide a stabilised voltage to the base of transistor T6 whose emitter is connected to the earth OV terminal 24 via resistor R12. The emitter voltage provides a stabilised voltage of about 5V over a range of currents drawn from the voltage regulator 12.

A terminal 26 coupled to supply rail 20 via diode D5 provides a connection for a battery to provide a battery back-up for the mains powered voltage regulator 12.

If battery-only use is to be adopted, a 9V battery can be directly attached to the supply rail at terminal 28 and earth at terminal 30.

Supply rail 20 is connected directly to the drive voltage input Vdd of the microcontroller 8, the Voo output of the microcontroller being directly connected to earth potential.

The supply rail 20 is also connected to a data line of the controller 8, in this embodiment the B6 input, via a red LED D6 and resister R11, in series. On polling the data line B6, the LED D6 emits light to indicate the security circuit is operative. This indicator light is optional.

The low battery detector 16 comprises a zener diode ZD1 and resistor R14 in series connection between a data bus line B5 of the controller 8 and earth potential. Data bus line B4 is connected to the junction of the zener diode ZD1 and resistor R14.

To test the status of the supply line 20, the line B5 is taken high and adopts a voltage slightly less than the voltage of the supply rail. The zener diode ZD1 provides a fixed voltage drop of about 3V so line B4 of the controller 8 sees a 2V input which is regarded by the controller 8 as a logic 1 input. This indicates to the controller 8 that the supply rail 20 is at an adequate voltage.

Should the voltage decrease to about 4V or less, due to battery failure for example, the voltage applied to line B4 of the controller 8 will drop to about 1V which is regarded by the controller 8 as a logic 0 input. This indicates to the controller 8 that the supply rail 20 is not at an adequate voltage and the battery has failed the battery test.

In this embodiment, the controller conducts a battery test once every minute, approximately. If the battery test indicates failure, an audio signal is sent via lines B2, B3 of the controller to the motor drive amplifier 10 causing it to emit a short audio tone to indicate failure.

Terminals ol and o2 of the controller B are connected to earth potential via a respective capacitor C4 and C5 and connected to each other by a 4mHz ceramic resonator Xl which components provide the clock signal for the controller 8.

The transmitter 4 comprises a transistor T1 whose base is connected to the centre of a voltage divider comprising resistors R8 and R9 connected in series between the supply rail 20 and address line A2 of the controller 8. Drawing current at line A2 of the controller 8 causes the transistor T1 to conduct and so energise the transmitter coil 18 which is connected in series with the transistor T1 between the supply rail.20 and earth.

By pulse energising the transmitter coil 18 a pulsed e-m field is emitted which, in this particular embodiment interacts, in known manner, with any adjacent key's resonant tuned circuit which consequently emits radiation at a frequency determined by the tuned circuit as previously described in relation to GB-B-2,119,431.

This radiation emitted by the key tuned circuit in response to what may be called the interrogation e-m radiation signal from the transmitter induces corresponding signals in the circuit comprising the transmitter coil 18 the ends of which as connected in series by resistor R1 and parallel diodes D1 and D2 connected in opposition to provide an a.c. operable voltage divider.

The a.c. output from this voltage divider is coupled via a capacitor C1 to the input 2 of a receiver/amplifier integrated circuit IC1.

The receiver/amplifier 6 is powered by means of a voltage supply applied between terminals 8 and 4 of the integrated circuit IC1. The latter terminal 4 is connected to earth potential, the former terminal 8 is connected to line B7 of the controller 8 which is taken high to power-up the receiver/amplifier as required. The controller 8 can therefore power up the amplifier/receiver integrated circuit when, and only when, it is required to detect signals from a key turned circuit. A voltage divider formed by a pair of resistors R5 and R6 connected in series between earth potential and the terminal B7 of the controller 8 to provide suitably biased inputs for the noninverting inputs of IC1.

The amplifier biasing of the output of the amplifier IC1 is controlled as follows.

Before a transmitted pulse is generated the a.c.

voltage from line 7 of the receiver/amplifier coupled to line A0 of the controller IC2 via capacitor C3 will be small.

If the voltage at line AO is high enough to be seen as a logic 1 then line Al, connected to capacitor C3 by a resistor R7, is set low to remove charge from the capacitor C3 until the voltage at line A0 drops to a value seen as a logic zero.

Conversely if the voltage at line AO is low and seen as a logic 0, line Al is set high to add charge to the capacitor C3 to increase the voltage until it is seen as a logic 1.

That is, the controller operates line Al moving between a low and high O/P value to cause the voltage at line AO to oscillate between a 0 and 1 detected logic value.

If a transmission pulse is to be generated, the controller 8 ensures the last transition is to logic 0 after which no further transitions are caused by the biasing procedure whereupon the controller 8 causes the interrogation pulse to be transmitted.

This biasing procedure saves one stage of amplification which would otherwise be needed.

Appropriate biasing of the receiver/amplifier circuit allows the conversion of extremely small amplitude signals induced in the antenna by the key tuned circuit to be converted to logic level signals on which subsequent signal processing can be carried out as the transitions of the a.c. received signal are quantised into 0 and 1 logic level signals.

Referring now to Figures 2 and 3, a cat flap operable by the security circuit of Figure 1 has a door 40 pivotally mounted in housing 42 by stubs 44 received in slots in the housing 42. The door 40 is retained in position by a complementary housing (not shown) which fits on top of the housing 42 to provide a closed housing with an entrance way 46 closeable by the door 40.

The transmitter coil 18 is wrapped round upstanding flanges 48 with ends 50 exposed for connection with the security circuit of Figure 1.

A latch 50 engageable with the door 40 has associated with it a locking mechanism comprising a first arm 52 pivotally mounted by a screw 54 to the housing 42, a second arm 56 pivotally coupled to the latch 50 by means of a ball joint (not shown) captive in a slot in the latch 50, the first and second arms 54, 56 being joined together so as to allow relative rotational movement at joint 58.

A further arm 60 extends generally sideways from the joint 58 and can be moved between the positions shown in Figures 2 and 3 by a motor 62 which positions correspond to the locked and unlocked position, respectively (as will be explained in more detail with reference to Figures 4 to 6).

The movement is effected by a peg 64 eccentrically positioned on a disc 66 fixed to the motor spindle 68.

Further pegs 70 and 72 protruding from the disc meet a stop (not shown) mounted on the housing to limit the rotational movement of the motor 62 to approximately 180 , the peg 72 being in contact with the stop in the position of Figure 3, the peg 74 being in contact with the stop in the position of Figure 2.

Application of a suitably polarized d.c. pulse causes the motor 62 to flip between the locked and unlocked positions of Figures 2 and 3 as determined by the pegs 72 and 74 and the stop.

The motor 62 is secured to the housing 42 by a strap 78 which also captures a wire spring 80 which urges the latch upwards as will now be described in more detail with reference to Figures 4, 5 and 6.

Referring now to these Figures, the latch 50 is a snap fit on a cross bar 82 (see Figures 1 and 2) at the front end of a support 84 which is slotted to allow the latch 50 to pivot into the slot of the support 84 as shown in Figure 5.

The rear end of the support 84 is pivotally mounted on the housing 42 so as to be pivotable between the positions shown in Figures 4 and 6.

When the locking mechanism is in the position of Figures 2 and 4, i.e. in the locked position, the rear end of the latch 50 is held in the position shown by the aligned arms 54 and 57 to prevent the door 40 moving in the direction A of Figure 2 so preventing a cat from entering a household. The door 40 is, however, free to move in the opposite direction to allow exit of an animal from the household even when the locking mechanism is in this locked position.

The latch 50 has a generally triangular cross-section end portion 51 defined by ramp surfaces 51a and 51b which act as camming surfaces to urge the latch 50 downwards when the door 40 is pushed horizontally against either of them.

On detection of a cat with a key circuit that is permitted to enter the household, the security circuit operates the motor 62 to move the locking mechanism to the unlocked position as shown in Figure 3 and 5. Now pressure applied to the door in direction A by the animal will act on the ramped surface Sib to pivot the rear end of the latch 50 downwards with respect to the support 84 allowing the door to open and the animal to enter.

After the animal enters the household the security circuit may operate to activate the motor 62 to move the locking mechanism back to the locked position of Figures 2 and 4 before the door closes. In this case a spring (not shown) urges the door 40 against the latch 40 pivoting the support 84 downwards against the action of spring 80 as shown in Figure 6 as the door 40 pushes against the ramped surface 51a. This lowers the latch 50 sufficiently to allow the door to move to its closed position whereupon spring 80 pushes the support 84 upwards to the position of Figure 4 with the door now held closed by the latch 50.

The controller 8 has the following operational cycle in normal operation after initial power up on connection to the power supply.

The switch 14 is checked by reference to line A3 of the controller 8 to see if the switch 14 has been closed by the user.

If it has been, the controller 8 determines if it is held closed for about 5 seconds which, if so done, puts the controller into the training or programming mode which transition is indicated by the controller 8 causing the motor 62 to emit an audio tone by outputting a first audio tone on the line B3 of the controller 8.

This indicates to the user that a first collar should be presented to the security circuit and the switch 14 temporarily closed again whereupon the security circuit will switch on, bias the receiver amplifier 6, and transmit an interrogation pulse.

The received return signal from the key circuit as received at the line A0 of the controller is in the form of a series of 0 and 1 logic levels as the a.c. detected signal crosses the voltage level corresponding to the 0 to 1 logic level transition. This received signal is sampled 40 times at 2s intervals to form a 40 bit strings of Os and ls which is the characterisation of that key for this embodiment. This characterisation is stored by the controller in a memory store.

The controller then causes the motor 10 to emit an audio tone to indicate that a second key circuit is to be presented for characterisation.

The user will the temporarily close the switch 14 once more and the controller will repeat the characterisation steps but this time will store the characterisation as a second characterisation. This embodiment is arranged to store only two key circuit characterisations but more could be readily implemented for users with more than 2 animals requiring access.

If the switch 14 is not closed again for a preset time the controller 8 reverts again to normal operation.

If the switch 14 was initially closed for less than 5 seconds, this indicates that the latch should be unlocked despite absent the presence of a valid key circuit and the controller 8 issues the appropriate signal to the motor 10 to unlock the latch. This provides an override of the locking mechanism.

Absent any switch closure, the controller will check the battery if it is the time to do so (for example once every 256 cycles of the normal operation loop) and indicates failure of the test as described above if required.

The controller 8 then operates the security circuit to scan for a valid key circuit by switching the receiver/amplifier on and biasing it as previously described prior to transmitting an interrogation pulse.

Any return signal from a key circuit is characterised, as described above with reference to the training mode, to form a 40 bit characterisation string of 0's and l's. This is then compared to the stored characterisations to determine if a match is obtained.

In this embodiment, the characterisation is sufficiently close to be deemed a match if 32 or more of the bit 40 values of the key circuit characterisation match the bits in the corresponding positions of the characterisation string of one of the up to two stored characterisations. The controller 8 will then issue an output to the motor 10 to cause the latch to be unlocked.

If no match is found, no such signal is output by the controller and the latch remains locked.

The controller 8 then goes into sleep mode for about 1/3 of second before repeating the normal operation control loop.

Assuming the switch 14 is not closed in this sleep period, the controller will once again scan for a suitable key circuit in the vicinity but now will operate to issue an output to the motor 62 to lock the latch if no match is found but now according to a reduced criterion in which a match is defined as 27 or more bits of the key characterisation matching the correspondence bits in one of the stored characterisations.

The use of a 40 bit characterisation string and the two specified match criteria for unlocking and relocking the latch were determined by trial and error to provide satisfactory discrimination to unlock whilst preventing "hunting" while the animal enters the pet door. Other values may be found to appropriate with other embodiments of the present invention.

If the switch is closed for less than 5 seconds while the latch 50 is in the unlocked position due to the use previously overriding the security circuit, the controller 8 will now act to output a signal to the motor 62 to relock the latch 50. That is, short closures of the switch 14 act to toggle the locking mechanism between the locked and unlocked positions.

In the override setting, the controller 8 does not do any scanning for key circuits but merely monitors for the override to be toggled off once more whereupon it then continues the normal operational loop of battery checks and scanning and further sensor training if required.

On initial power-up of the security circuit, the controller is arranged to enter the training mode immediately so one or two key circuits can be characterised as described above.

A key circuit for use with this embodiment typically comprises a coil of about 100 to 120 turns about 10mum long of 9mm outer diameter on a ferrite rod 25mum long and 5mm external diameter in series with a capacitor of about 68 nF. These produce a resonant frequency of about 20-40 kHz with the particular value determined by the precise values and configuration of the circuit elements who variations thereby produce the desired randomisation of the resonant frequency for the required density of key frequencies without a tuning step. The circuit is then potted in the usual manner is a rectangular plastic housing about 13 x 13 x 35mm dimensions, the housing then being affixed to an elasticated collar or other means of attachment to the animal.

TABLE 1 Resistors (all 0.125 watt carbona film to MIL-R-11/DIN 449051 / CECC 40100 except as indicated) R1 10K R2 680K R3 10K R4 470K R5 47K R6 10K R7 330K R8 1K R9 1K R10 22K R11 560R R12 2K2 R13 1K R14 2K2 Capacitors C1 390pF Ceramic Medium k # 10% C2 1nF "" "" "" C3 100nF Ceramic C4 100pF Ceramic Medium K # 10% C5 100pF "" "" " C6 22 F electrostatic C7 100nF Ceramic high K Diodes D1-D5 IN914/4548 D6 RED LED(5MM) D7 IN914/4148 ZD1 2.7V 400mW ZENER ZD2 6V7 400mW ZENER Integrated Circuits IC1 LM358 (Source SGS Thompson) IC2 PIC 16C54XT/P (Arizona Microchip) Transistors T1 BC327-16 Source Thosiba T3 BC337-16 "" T2 BC337-16 "" T3 BC337-16 "" T3 BC337-16 "" T3 BC337-16 "" Misc.

X1 4MHz ceramic resonator (MuRata CSA 4.00MG)

Claims (18)

1. A security locking circuit suitable for use with a latch for a door comprising: a transmitter arranged to transmit an interrogation signal; a receiver arranged to receive a reply signal emitted by a key circuit in response to transmission of the interrogation signal; means arranged to generate a characterisation of such a reply signal and compare the characterisation to one or more pre-stored characterisations and, on finding a satisfactory comparison between the characterisation and a pre-stored characterisation, to generate a release signal for releasing the latch; the circuit being arranged to be switchable to a training mode in which an interrogation signal is transmitted and a generated characterisation of a reply signal from a key circuit is stored as one of the prestored characterisations.

2. A security circuit as claimed in claim 1 and including a switch which when closed switches the circuit to the training mode.

3. A security circuit as claimed in either one of claims 1 and 2 in which the transmitter includes a transmitter coil and a pulse generator arranged to apply an electrical pulse to the transmitter coil.

4. A security circuit as claimed in claim 3 in which the transmitter coil serves on an antenna for the receiver.

5. A security apparatus as claimed in any preceding claim in which the circuit is arranged such that after generation of the release signal a lock signal is generated in the event of either: a) the subsequent absence of the receipt of a reply signal; or b) the receipt of a reply signal having a characterisation which has a non-satisfactory comparison with a pre-stored characterisation in response to the transmission of an interrogation signal.

6. A security apparatus as claimed in claim 5 in which a satisfactory comparison is determined by more stringent criterion before issuance of the release signal than after issuance of the release signal.

7. A security circuit in which the characterisation comprises a series of logic 0 or 1 bits corresponding to respective samples of the received signal which has been quantized into two levels.

8. A security circuit as claimed in claim 7 in which the series comprises 40 or more bits.

9. A security circuit as claimed in claim 8 in which the release signal is generated if 32 or more bits of the characterisation of a reply signal match the corresponding bits of a pre-stored characterisation.

10. A security circuit as claimed in claim 9 in which the lock signal is generated if 26 or fewer bits of the characterisation of a reply signal match the corresponding bits of a pre-stored characterisation.

11. An apparatus including a security circuit as claimed in any preceding claim and a closure securable by a latch, and locking means coupled to the latch responsive to receipt of a release or lock signal to operate the locking means to allow or prevent opening of the closure, respectively.

12. An apparatus as claimed in claim 11 in which the controller is arranged to provide audio-frequency electrical signals to the motor to generate audio frequency tones to assist a user of the apparatus during the training mode of the security circuit.

13. An apparatus as claimed in claim 11 and 12 in which the apparatus is a pet door including a frame securable in the door of a household.

14. A method of making an external key circuit for use with a security circuit as claimed in any one of claims 1 to 6 comprising forming an electrical resonant circuit including a capacitor in series with an inductance coil surrounding a ferrite core and potting the resonant circuit to fix the resonant frequency of the resonant circuit without performing any tuning of the resonant circuit.

15. A method of making a pet key collar comprising fixing the external key circuit as claimed in claim 13 to a pet collar.

16. A security locking circuit substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

17. A method of making a pet key collar substantially as hereinbefore described.

18. A pet door substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.