GB2192026A - Intruder alarm assembly - Google Patents

Abstract

The alarm assembly built into a door handle detects rotation of the handle by an attitude sensor 10 specifically a mercury tilt switch which actuates an alarm. The circuit of Fig. 1 is constructed such that, once triggered, the alarm signal continues for a period; preferably there is induction time between triggering the alarm and the alarm functioning. The alarm signal may be a sound produced by an inductive coil and moving diaphragm housed in a resonant cavity 52 in the handle. A multivibrator modulates the frequency of audio oscillator to ensure maximum sound output. The circuit of Fig. 1 is built on a circuit board which slides between channels 54 of an electronics enclosure 40 placed inside an outer case (42, Fig. 4) of the handle, which also includes a back plate (44, Figs. 9, 10), an end cap (62 Fig. 6) removable for battery replacement and a return spring assembly. <IMAGE>

Description

SPECIFICATION Intruder alarm assembly The present invention relates to intruder alarms, and particularly to miniature alarms for providing a warning of the unauthorised opening of a door.

GB 2,152,257 (Wilkins) discloses an alarm of this type which is adapted to be built into a door handle assembly, most of the components actually being housed in a doorknob. US 4,196,422 (Teledyne Industries, Inc.) also discloses a door handle alarm. Such alarms are potentially advantageous, since they can be self-contained (and hence easy to install but difficult to detect). However, a miniature alarm tends to produce only a small alarm signal.

Furthermore, the alarms disclosed in the above specifications employ rather complex arrays of relatively rotatable electrical contacts, so that rotation of a door handle causes one contact to slide onto and over another.

Such contacts tend to be unreliable, depending on the resilience of small parts, and being affected by dirt and by wear which may reduce the resilient force or even cause a contact piece to break off.

An alarm of a preferred embodiment of the present invention can be small enough to be installed within a handle, but still produce a loud alarm signal. It can be mechanically very simple and reliable, dispensing with sliding contacts as just described.

The invention provides an alarm assembly for a door comprising a door handle and an alarm, the alarm comprising a gravity dependent attitude sensor, and means for producing an alarm signal coupled to the sensor so as to be actuable by a change of the sensor's attitude, at least the sensor being housed within the door handle. The attitude sensor may be a tilt switch, e.g. a mercury tilt switch. Detection of rotation may actuate an alarm sounder. This is preferably housed in a resonant cavity, e.g. a Helmholtz resonator, so that even a small sounder can produce a clearly audible output. For example, it is possible to use a sounder comprising a coil and a diaphragm, rather than a conventional bulky piezoelectric device.

The device will generally have an on/off switch. Preferably there is an induction time between actuating the switch and the alarm assembly becoming functional, so that an authorised person can switch on, and then open and/or close the door without triggering the alarm. Preferably there is an induction time between triggering the alarm and the alarm sounding, so that an authorised person can use the door and switch off the alarm before it sounds. Preferably it can be switched on again immediately. (The delay is also valuable for startling an intruder, who will then not be aware of how the alarm was triggered.) Such induction times may be provided by the slow charging of capacitors. It may also be arranged that the alarm, once triggered, sounds for a limited period. Generally, the circuitry will include an oscillator for driving a sounder.Preferably means are provided for scanning the frequency thereof to ensure that it encompasses the resonant frequency.

The assembly may include a housing, which may be shaped externally so as to serve as, and appear to be, a door handle. A doorknob portion may have one or more rearward openings to allow access to a switch, and/or to let the alarm signal be heard, and possibly for a visible signal of alarm status (whose reflection may be seen in the door or lockplate).

An embodiment of the invention will now be described in greater detail with reference to the accompanying drawings in which: Figure 1 is a circuit diagram; Figure 2 is a plan view of the housing for the electrical components; Figure 3 is a section on Ill-Ill in Fig. 2; Figure 4 is an axial section, and Fig. 5 is a rear view, of the outer housing; Figure 6 is an axial section of the end cap for the outer housing; Figures 7 and 8 are front and side views of a lid retaining plate; Figures 9 and 10 are a rear view and an axial section of the backplate; Figures 11 and 12 are a rear view and an axial section (on XV-XV) of a spring end plate; Figures 13 and 14 are a rear view and an axial section of a spring cam; and Figure 15 is a side view of a return spring in its relaxed state.

The illustrated embodiment employs electrical components connected as shown in the circuit diagram (Fig. 1).

The system is powered from a battery 1, e.g. a single 9v primary cell, preferably of the alkaline or zinc chloride type. Diode 2 is placed in circuit so as to protect the integrated circuit and associated components in the event of the battery being connected with its polarity reversed with the switch 3, being in the 'on' position. The switch 3 is arranged such that when in the 'off' position, the V+ line to the rest of the circuit is connected to Ov. This is done so that all the capacitors that constitute the various time-delays are all fully discharged on power-up, thus ensuring that all timings will remain constant, even if the device is turned off and then on again quickly.

The circuit is built around a single integrated circuit 4, which consists of six identical Schmitt input inverters, and should ideally be of the type MC14584BCP, manufactured by Motorola, aithough any device of the same type will suffice as long as due attention is paid to the trigger levels and their effect on the timing periods. As all the gates are identical in the device, the six gates are referred to as the item number 4 plus a letter a to f to denote which gate is used. The gate allocations may be altered to suit. All diodes are of the generic type 1N914 or 1N4148, although any small silicon signal diode with a forward current handling of 75mA will suffice. The transistors are general purpose PNP (ZTX500) and NPN (BC183) switching types with a hfe of greater than 100.

On switching the slide switch 3 to the 'on' position, the input to gate 4a is held low whilst capacitor 6 is charged from resistor 5.

This voltage will rise exponentially at the input of 4a until it reaches the trigger threshold of the device. Whilst this voltage is rising, the output of gate 4a is at a '1' or V+ potential.

This causes current to flow through the light emitting diode 7, causing it to radiate for the time that the input takes to reach the threshold. When this is passed, the gate changes state and the output goes to a '0' or 0v potential, extinguishing the LED 7. The resistor 8 is to limit the current through the LED and protect the output of the gate. The LED is a special CMOS compatible part, requiring only 2mA to operate. The Zener diode 9 is included in the circuit so that the voltage applied to the LED must be greater than approximately 7.2v. If this voltage is less than this, the LED will not illuminate, thus warning that the battery is nearing the end of its useful life and should be changed. The circuit still remains fully operational until the battery has discharged to an effective voltage of approximately 6.5v.

Activation of the alarm is achieved by the operation of tilt switch 10. In normal use, the tilt switch is arranged such that it is mounted in the vertical plane with the contacts open circuit. Rotation of the doorknob will cause the mercury ball contained within the switch to short circuit the contacts. During the power-up delay both contacts of the switch are at the same potential, i.e. a '1' because the output of gate 4a is high and the input of 4b is pulled up via the resistor 11. Therefore if the circuit is operated during this period the alarm will not trigger. This delay is to allow the user to switch on and be able to go through the door.

When the circuit is armed, the input to gate 4b is a '1' because the tilt switch is on open and resistor 11 pulls the input up. On rotation of the knob, the input to gate 4b will be pulled down to Ov when the tilt switch contacts are shorted. This in turn causes the output of gate 4b to go to a '1' which instantaneously charges capacitor 14 and changes the state of gate 4c from a '1' to a '0'. The diode 12 is in the circuit to prevent the capacitor 14 discharging via the output transistors of gate 4b. Thus they can only discharge through resistor 13. The values of components 13 and 14 are quite large and determine the time that the alarm is allowed to sound. Whilst the input to gate 4c is slowly decreasing towards the threshold, the output of 4c, having gone to '0', allows capacitor 17 to discharge through resistor 16.Diode 15 is in circuit to allow instantaneous charging of 17 at the end of the cycle and during powerup. The input of this network is applied to gate 4f which will not change state until its input voltage has decayed to the threshold, so transistor 22 is turned off and the audio circuit is disabled. When the input to gate 4f reaches the threshold, the output changes state and switches transistor 22 on, allowing the audio circuit to operate. This delay between the trigger and activating the sounder is to allow authorised users to enter the door and have time to switch the system off before the alarm sounds.

The output of gate 4c is also connected to the input of gate 4d. When the output of gate 4d goes to a '1', diode 18 is reverse biased and allows the astable multivibrator constructed around gate 4e with resistor 20 and capacitor 19 to run. The output of gate 4e is connected via resistor 23 to the base of the audio oscillator constructed with transistors 26 and 27 along with resistors 24 and 29 and capacitors 25 and 30. The effect of the multivibrator is to modulate the frequency of the oscillator at approximately 2Hz such that the audible tone is swept through the resonant frequency of the audio transducer 31, thus ensuring maximum sound output in spite of variations in frequency due to component tolerances. The diode 28 is included in the circuit to protect the transistors 26 and 27 from the 'fly-back' effect of the inductive load presented by the transducer.

The oscillator continues to run until the capacitor 14 has discharged via resistor 13 through the threshold of gate 4c, when the output of gate 4c returns to a '1'. When this happens, the input of gate 4f goes instantaneously to a '1' because capacitor 17 is charged via diode 15. The output of gate 4f goes to '0' turning off transistor 22 and stopping the audio oscillator. At the same time, gate 4d changes state and diode 18 conducts, stopping the astable multivibrator 4e running. The system remains active in that if the tilt switch is triggered, the whole sequence begins again. The cycle can be stopped at any time by moving switch 3 to 'off'.

The door knob alarm assembly consists of three main parts in addition to the electrical parts. They are: (A) internal electronics enclosure (40); (B) outer housing comprising outer case (42) and backplate (44); (C) spring assembly (46).

The whole device is assembled such that the electronics circuit (Fig. 1) battery 1 and audio transducer 31 are mounted into the electronics enclosure 40 which is in turn placed inside the outer case 42. The spring assembly 46 fits into the backplate 41, and two screws are introduced through the rear of the spring assembly and driven into the plastic base of the electronics enclosure to hold the various components together.

The electronics enclosure 40 (Figs. 2 and 3) comprises a plastics cup having an interrupted cylindrical wall 48, and a pair of paraxial walls 50 in an intermediate region. Diametrically opposite the interruption, the cup has wall portions 51 defining a cavity 52, here cylindrical.

Adjacent the wall interruption are ribs 53 defining opposed channels 54. The circuit (Fig.

1) is built on a circuit board which slides down between the channels 54. The battery 1 sits in the compartment formed by the two walls 50, electrical connection to the circuit being provided by the flying leads of the battery clip. The audio transducer or sounder is located in the cavity 52 which by virture of its dimensions forms a Helmholz resonant cavity.

Many alarm systems use a piezoelectric sounder, but because of the restrictions placed on the design by the need to keep the finished item not much larger than a conventional door knob, a piezo device with a suitable sound output level was found to be physically too large. The device thus employed in this embodiment is a transducer comprising an inductive coil and moving diaphragm, which has the advantage of being small in size (12mm diameter), but has the disadvantage of not being very loud. It was found that housing the device in a tube in the manner shown had the effect of significantly improving the volume of sound emitted. Electrical connection to the circuit is provided by flying leads, and the device is held in place by an indent in the wall of the tube.When the printed circuit board, battery and transducer are all in place, the lid retaining plate 56 (Figs. 7,8), is positioned into slots 58 in portions of the walls 50 which project beyond the cup. The plate 56 has a threaded boss 60 and is mounted with this facing away from the battery. This component serves both to secure the battery in position and to present a stud for the end cap 62 (Fig. 6) to be screwed onto. The end cap 62 needs to be easily removed for battery replacement, but should be distiguised enough so that only au thorised users have access to the battery compartment.

The outer case 42 (Figs. 4,5) can be manufactured from aluminium, brass, plastic or any material with a suitable finish. The outer components, i.e. the outer case 42, the backplate 44 (Figs. 9,10) and the end cap 62 (Fig. 6), when assembled, form the actual door knob.

The shape and dimensions of this example have been chosen only for convenience, and the design could be quite easily altered to accommodate changes in style or presentation.

When the printed circuit board is in position in the plastic enclosure 40, and this is in turn mounted inside the outer case 42, the 'on/off' switch 3 has to be accessible, the light emitting diode 7 needs to be visible and provision has to be made for the sound to be heard from outside the enciosure. The outer case rear view (Fig. 5), shows how these provisions are met. The slot 70 allows the toggle on the slide switch to protrude such that it may be operated. The hole 72 is to allow the light emitting diode to protrude also, although the light emitted is not viewed directly, but is seen as a reflection on the upper surface of the backplate. The hole 74 is to allow the sound waves emitted from the sounder to have free access to the atmosphere. It was found that the best results were obtained by leaving this aperture as an open hole.Various exit configuration were tried, but in all cases some reduction in sound volume was experienced. The sound is bounced off the door and backplate into the room, which also has the effect of disguising exactly where the alarm is being generated.

The spring assembly 46 consists of three parts: the spring end plate (46a), suitably of plastic (Figs. 11,12); the spring cam 46b (Figs. 13,14); and the return spring (46c) (Fig.

15). The return spring 46c is mounted on the spring cam 46b such that the extensions on the spring, when placed under compression, line up and are held in place by the lug 80 shown in Fig. 13. The spring end plate 46a is then clipped on top of the spring to hold it in position, and the whole sub-assembly is introduced into the rear of the backplate 66. The ends of the spring 46c protrude from the plastic and engage onto the spigot 82 formed inside the rim of the backplate. Two screws are introduced through the rear of the assembly to engage in the plastic of the electronics box 40. This now means that when the backplate 66 is secured to a door, the outer case, electronics box (and tilt switch), will rotate with the operation of the knob, but against the spring which will always return the knob to the 'rest' position, thus ensuring that the alarm is allowed to switch itself off after activation.

Claims (11)

1. An alarm assembly for a door comprising a door handle and an alarm, tha alarm comprising a gravity dependent attitude sensor, and means for producing an alarm signal coupled to the sensor so as to be actuable by a change of the sensor's attitude; at least the sensor being housed within the door handle.

2. An alarm assembly according to claim 1 wherein said signal production means comprises a sound producer in or in communication with a resonant cavity in the door handle.

3. An alarm assembly according to claim 2 wherein said sound producer comprises a coil and a moving diaphragm.

4. An alarm assembly according to claim 2 or 3 wherein the sound producer comprises an oscillator; a means for modulating the frequency of the output of the oscillator; and a transducer for receiving the modulated signal and producing an audible signal whose frequency scans across the resonant frequency of the cavity.

5. An alarm assembly according to any of the preceding claims, having an on/off switch, and delay means for providing an induction time between actuating the switch and the alarm becoming functional.

6. An alarm assembly according to any of the preceding claims, having delay means for providing an induction time between the triggering of the alarm by the sensor and the alarm signal being emitted.

7. An alarm assembly according to any of the preceding claims, said alarm having its power supply means located inside the handle.

8. An alarm assembly according to any of the preceding claims having a latching circuit arranged so that, once the signal production means is actuated by a change in the sensor's attitude, it is not deactuated by restoration of the sensor's original attitude.

9. An alarm assembly according to any of the preceding claims housed in a door handle and having an on/off switch; the door handle having a rearward opening allowing access to the switch.

10. An alarm assembly according to any preceding claim further including a visual indicator of the status of the alarm said indicator being mounted at the rear of the handle so that, in use, its condition can be assesed by observing its reflection from a lockplate or other reflective surface portion of an associated door.

11. An alarm assembly substantially as herein described with reference to and as illustrated in the accompanying drawings.