GB2200748A - Door alarm system - Google Patents

Abstract

The system comprises a source 5 and a detector 6 for a modulated light beam, both situated within the door housing, and a fiber-optic cable 7 within the door completing the optical path from source to detector when the door is closed. The latch handle of the door maybe arranged to interrupt the light path as soon as it is operated. The system is applicable to microwave ovens, ie to turn off the magnetron automatically when the door is opened or the handle is operated to open the door. <IMAGE>

Description

DOOR ALARM SYSTEM The invention relates to door alarm systems. For many purposes it is necessary to provide a signal indicating whether or not a door is in the closed position. Such a need arises in particular, in microwave ovens.

If a microwave oven is operated without the door securely closed, it is possible for harmful radiation to escape and pose a danger to health. It is, therefore, desirable to provide a secure system which will monitor the closure of the door and ensure that the supply of power to the magnetron of the oven is switched off before the door is opened. Apart from microwave ovens, there are of course many other applications for a door alarm, for example, in security systems.

An object of the present invention is to provide a door alarm which is simple and reliable, and in which the risk of obtaining a false indication that the door is closed, either by reason of accidental failure of the system, oversight by a user, or by deliberate intervention, is minimised.

The invention is defined in the claims appended hereto. The underlying concept of the present invention depends on the use of a modulated light signal passing along a path completed by a fiber-optic when the door is closed. In use, the path is interrupted by the opening of the door, and this is caused to give an alarm signal which may be audible and/or visual. The path may further be interrupted by a shutter attached to the door handle, so that, when the handle is operated, the alarm is given before the door actually begins to open. If desired, the shutter may be attached to another part of the door, the frame surrounding the door, the door hinge or an extension thereof.

The use of a modulated light beam avoids a false signal being given by reason of stray light reaching the detector of the system, and for this purpose, modulation should be at a frequency other than the power supply frequency or its harmonics since stray light modulated at these frequencies may be present in normal electric room lighting.

Alternatively, the light may be modulated in accordance with a coded signal which is recognised when the beam is demodulated.

The system will be further described by way of example with reference to the accompanying drawings, in which: - Figure 1 is a highly diagrammatic view showing the application of the invention to a microwave oven; Figure 2 is a detail section of a latch handle and shutter for use in connection with the invention; Figure 3 is block diagram of a system according to the invention; and Figure 4 is a circuit diagram of an interlock system.

Referring first to Figure 1, a microwave oven comprises a body 1 in which the right-hand portion 2 houses the magnetron, its driving circuits, and the associated circuits and controls. The greater part of the oven body comprises a cooking enclosure closed by a door 3 having an electrically screened window 4 through which the progress of cooking can be observed.

For safe operation it is desirable to ensure that power cannot be supplied to the magnetron unless the door is in its fully closed position. If the door is opened, even to a limited extent, while power is being supplied radiation can escape. It is, therefore, desirable that the door interlocking arrangement should not be liable to mechanical failure, or to be put out of action by misuse.

In accordance with the present invention a light source 5 producing a modulated beam of light is positioned in a housing or frame surrounding the door and a light detector 6 is also positioned in the housing or frame surrounding the door as indicated in Figure 1 at position displaced from the source 5. The detector 6 is connected to a demodulating circuit and a comparator which is designed to respond to the modulation applied to the light source 5 and to give an output signal when the modulated signal is recognised.

The source 5 and the detector 6 are each installed behind a small hole in the wall of the housing or frame surrounding the door. In addition, there is a corresponding hole in the door itself opposite each of the said holes and a fiber-optic cable 7 extends between the two holes' in the door as indicated in Figure 1 to conduct light from the source 5 to the detector 6 when the door is in the fully closed position. The distances between corresponding holes in the housing or frame surrounding the door and in the door itself is such as to maintain adequate light transmission.

In normal operation, with the door fully closed, a modulated beam of light is generated at the source 5, passes via the aforementioned holes into the door, and through the fiber-optic cable 7, where it is detected by the detector 6. The detected signal is demodulated, and compared with- the modulation applied to the source. If it matches, a control signal allowing the magnetron to be energised is generated. Opening of the door interrupts the light, and immediately switches off the magnetron.

The door is kept closed by means of a latch, which is not shown in Figure 1.

In a modification the latch mechanism itself may be arranged additionally to interrupt the modulated light beam, so that power is cut off as soon as the handle is operated, and before the door itself begins to move.

The modification is shown in Figure 2, which is a section through the door and part of the latch in the plane of the fiber of the cable.

The door latch is operated by a handle 8 integral with a member 9 pivoted in a recess 10 in the thickness of the door 3. The fiber-optic cable 7 terminates flush with the walls of the recess 10, and the optical path is continued by a hole 11 through the member 9.

As soon as the handle 8 begins to move about its pivot, and before the latch begins to operate to unlatch the door, the hole 11 moves out of alignment with the ends of the fiber-optic cable 7, the light beam is interrupted, and power is cut off from the magnetron of the oven.

Instead of a mechanical latch, the door may be kept closed by a magnetic catch and opened by a hinged plastic cantilever which is pressed to disengage the door from its frame. This lever may carry the shutter for interrupting the light beam.

The light beam may be modulated by a simple sinusoidal oscillation, or by a coded signal. If a simple oscillation is employed, it is desirable that it should not be at normal mains frequency or its harmonics, especially at twice the mains frequency because light fluctuating at such frequencies is normally present in room lighting, particularly where fluorescent lamps are employed.

For certain applications it may be convenient to reverse the above construction, placing the light source and detctor, with part or all of their associated circuits, within the door, and the fiber-optic cable in the door housing.

Figure 3 is a functional block diagram of the invention in its more general aspect. A modulator 100 supplied a modulated signal to a light source 101.

Light from the source 101 enters a fiber-optic cable 102 within the door 103, and is conveyed to a detector 104.

The output from the detector is demodulated by a demodulator 105, and a comparator 106 is designed to compare the demodulated signal from the demodulator with the modulation imposed on the source by the modulator 100, and to deliver an output signal when they correspond.

As long as the door 103 is closed, the light path is completed through the fiber-optic cable 102 and the comparator 106 delivers its output signal. As soon as the door starts to open, the cable 102 is displaced and the light beam interrupted, so that no output from the demodulator 105 reaches the comparator 106, and the output signal ceases. Constant illumination, or incorrectly modulated light at the detector will not cause an output signal and the signal will also cease if the power supply to any of the system components fails, thereby preventing a false signal being obtained by accidental or malicious interference with the system.

Figure 4 is a circuit diagram of an interlock system suitable for use with the pres-ent invention in which components corresponding to those of Figure 3 are allotted the same reference numerals.

As indicated in the above description of Figure 3, an output signal from the optical sensors is transmitted from the comparator 106 into a logic array L which which compares it with the modulated signal transmitted. In addition, interfaced status signals from power switching relay contacts are fed into the logic array L. The circuitry is designed so that only when the total system integuity is validated, will an output from the logic a.rray L be passed to the power relays in accordance with the following table.

INPUTS OUTPUTS I1 12 13 14 01 02 03 1 1 1 1 1 1 1 0 1 1 1 001 1 0 1 ,1 0 0 0 1 1 0 1 0 0 0 1 1 1 0 0 0 0 In the above table and Figure 4, the following legend is used, namely: A - Transmitted Modulated Signal D - Received Modulated Signal 11 - (A=B) I2 - Contact status from primary relay (PRI) 13 - Contact status from secondary relay (SEC) 14 - Contact status from monitor relay (MON) - - Coil drive of primary relay M/O 2 - Coil drive of secondary relay N/O 03 - Coil drive of monitor relay N/C Although particular reference has been made to monitoring the alarm system in the region of that edge of the door remote from the hinged edge, the system may, if desired, be mounted in the region of the hinged edge of the door.

Claims (8)

C L A I M S:

1. A door alarm system for use with a door member mounted within a door mounting member, the system comprising a source of modulated light carried by one of the members, a light detector carried by the other member, a demodulator coupled to the light detector and a comparator coupled to the modulated source and the demodulator, and arranged to provide a signal indicative of whether or not the modulated light is being received by the detector, and an optical fiber so disposed in the door as to provide an optical path from the light source to the detector, which path is interrupted by the opening of the door.

2. A door alarm system according to Claim 1 in which the door member is hinged to the door mounting member and the optical fiber is interrupted by a shutter coupled to a latch handle for the door so arranged that on operating the latch handle the light path is interrupted by the sh-utter before the door starts to open.

3. A door alarm system according to Claim 1 in which the light source and detector are each mounted in the door mounting member, and the optical fiber is disposed in the door.

4. A door according to Claim 1 in which the source of modulated light and the light detector are each mounted in the door member and the optical fiber is carried by the door mounting member.

5. A door alarm system according to any preceding claim in which light is modulated, and the demodulation is arranged to detect, a sinusoidal oscillation at a frequency other than the power supply frequency and its harmonics.

6. A door alarm signal according to any of Claims 1,2 or 3 in which light is modulated by, and the demodulator is arranged to detect a coded signal.

7. A door alarm system according to any preceding claim in which the door is the door of a microwave oven and the signal from the demodulator controls the engergising of the magnetron of the oven to prevent operation except while the door is fully closed.

8. A microwave oven including a door alarm system according to anyone of Claims 1 to 7.