A known door viewer comprises a system which extends between an inner surface and an outer surface of a door and includes a lens that allows a person applying an eye to the inner surface of the door to view a person adjacent the exterior surface of the door but prevents the exterior person viewing the interior person. Such a viewer works well when the interior person recognizes the exterior person but does not work well when the interior person does not recognize the exterior person but wishes to determine whether the exterior person is nevertheless someone who the interior person might wish to admit. In this circumstance, the interior person may try and converse with the exterior person but it can be difficult to conduct a conversation through the thickness of a door.

According to a first aspect of the invention, there is provided a door viewer comprising an optical system mountable in a door to allow a person at an inner side of the door to view a person adjacent an outer side of the door without permitting the outer person to view the inner person and a microphone for mounting on one side of the door and means for passing signals produced by the microphone to a receiver on the inner side of the door.

According to a second aspect of the invention there is provided a microphone system for a door viewer having an optical system mountable in a door to allow a person at an inner side of the door to view a person adjacent on outerside of the door without permitting the outer person to see the inner person and including an outer viewer part for mounting on the outside of a door and an inner viewer part for mounting on an inner side of a door, the system comprising an outer housing part and an inner housing part, at least one of said housing parts being mountable on an associated viewer part to mount the housing part on the door, one housing part including a microphone and the other housing part including a means for passing signals produced by the microphone to a receiver on the inner side of the door.

According to a third aspect of the invention, there is provided a method of mounting on a door a microphone system comprising an inner housing and an outer housing, one housing containing a microphone and the door including a door viewer comprising an inner viewer part and an outer viewer part interconnected through a hole in the door, the method comprising mounting at least one of the housings on an associated viewer part.

The following is a more detailed description of an embodiment of the invention, by way of example, reference being made to the accompanying drawings, in which:- Figure 1 is a front elevation of an outer unit of a microphone system for a door viewer,

Figure 2 is a similar view to Figure 1 but showing internal components of the outer unit of Figure 1, Figure 3 is a front elevation of an inner unit of the door viewer of Figures 1 and 2, Figure 4 is a similar view to Figure 3 but showing internal components of the inner unit of Figure 3, Figure 5 is a side elevation of the inner unit of Figure 3 and showing internal components of the inner unit and showing schematically both the outer unit and the inner unit mounted on a door Figure 6 is a partial cross-section of a door showing a door viewer extending through the door, Figure 7 is a similar view to Figure 6 but showing inner and outer viewer parts separated, Figure 8 is a similar view to Figures 6 and 7 but showing the inner and outer viewer parts connected to mount the inner and outer units on the door.

Referring first to Figures 1 and 2, the outer unit of the microphone system comprises a casing 10 formed, for example, from a moulded plastics material. The casing 10 has, in elevation, a generally rectangular lower portion with a grid 11 behind which a loud- speaker 12 is mounted. The upper portion 13 of the casing 10 has, in elevation, apart- 5 circular edge and is provided with a hole.

Referring next to Figures 3 to 5, the inner unit of the microphone system comprises a casing 15 and is of similar shape to the casing of the outer unit but with a longer lower portion. The lower portion is formed at its lower end with a battery storage compartment 16 with a removable lid 17 and containing four batteries 18 (see Figures 4 and 5) to provide a power source.

Above the compartment 16 is a grid 19 behind which a loud-speaker 20 is mounted.

This portion of the casing 15 also houses a switch 21 and a microphone 22 controlled by a control board 23 powered from the power source. An upper portion of the casing 15 is provided with a hole.

The inner and outer units are mounted on a door 27 by means of an optical viewer.

This is shown in Figures 6 to 8. Suitable optical viewers are known and will not be described in detail. An optical viewer includes an optical system that allows a person at the inner side of the door to view a person adjacent the outer side of the door with the outer person not being able to see the inner person. Such an optical viewer (see Figure 6) is formed by an inner viewer part 28 and an outer viewer part 29 that are interconnected by, for example, a screw thread. Each part comprises a barrel 28a, 29b with a flange 28b, 29bat one end. The connection is between the ends of the barrels remote from the flanges 28b, 29b with each flange 29b, 29b bearing against a respective side of the door 27.

In use, the inner and outer viewer parts 28,29 are separated (see Figure 7). The barrel 29a of the outer viewer part 29is inserted through the hole in the upper portion 13 of the casing 10 of the outer unit. The barrel 28a of the inner viewer part 28 is inserted through the hole in the upper portion of the casing 15 of the inner unit. The two barrels 28a, 29a are then inserted into respective opposite ends of an appropriately sized hole 31 in a door and interconnected. The flanges 28b, 29b on the barrels 28a, 29a press the associated housing parts against the associated door surface to mount the inner and outer units 10,15 on the door 27. Wires 32 connecting the microphone 22 to the loudspeaker 12 are passed through the hole in the door before the inner and outer viewer 28,29 parts are interconnected.

Thus, by pressing the switch 21 a person at the inner side of the door can speak to a person on the outer side of the door. A diagrammatic view of the arrangement is Figure 9. A microphone 32 may be provided on the outer casing with a second loudspeaker 33 in the inner casing to allow the outer person to speak to the inner person (see Figure 10). In this case a switch 34 may be provided on the inside of the door.

Of course, the shapes of the outer and inner casings are not material and they may be any suitable shape. They may be made of any suitable material. The power source need not be provided by batteries, it could come from a mains supply. The switch 27 is not essential; the system could be voice activated. Alternatively, the system could be activated by a detector 35 that detects the approach of a person to the outer side of the door. The detector could be an infra-red detector. This is shown in Figure 11.

The detector 35 could also provide a warning. The microphone system need not be provided separately from the optical viewer. The two parts could be provided together. The system could be provided with means such as a transmitter 36 for passing signals produced by the microphone to a remote receiver 37 incorporating, for example, a handset 38. This is shown in Figure 12. This might be in addition to or as an alternative to the loudspeaker 12. The system could incorporate an alarm 39 operable by a switch 40 from the inner side of the door 27. This is shown in Figure 13.