1 PIR sensor with retractable controls FIELD OF THE INVENTION This invention relates to a PIR sensor, in particular a PIR senor in which the controls are normally hidden from view but may drop down for access 5 and subsequently retract. BACKGROUND TO THE INVENTION PIR (passive infrared) sensors are a popular choice for use in people detecting applications such as intrusion detection and security lighting control. PIR sensors are often made as stand-alone units that interface to 10 other equipment by means of relay contacts or the like. As such it is highly desirable for a PIR sensor to incorporate user controls such as detection sensitivity and output pulse duration to allow the sensor's operation to be set appropriately for a specific application or location. Such controls are used infrequently, typically only during the installation or maintenance of a 15 system. At these times It is highly desirable to have the controls readily accessible, however once set it is highly desirable to have the controls hidden away to prevent tampering or merely from aesthetic considerations. The PIR sensors in the marketplace have not addressed these opposing needs. They either have controls readily accessible at all times 20 making them prone to tampering and aesthetically unpleasing or have controls that are difficult to access thus making installation and maintenance inconvenient. The object of this invention is to provide a PIR sensor that alleviates the above problems, or at least provides the public with a useful alternative. 25 SUMMARY OF THE INVENTION Therefore in one form of the invention there is proposed a PIR (passive infrared) sensor comprising a first enclosure and a second enclosure with control actuators on an outer surface and telescopically engaged with the 2 first enclosure; wherein the second enclosure is free to move between a first position in which the control actuators are obscured by the first enclosure, and a second position in which the control actuators are not obscured by the first enclosure. 5 Preferably when pushed upon the second enclosure will move from the first position to a third position and then when subsequently no longer pushed upon will be propelled from the third position to the second position. Preferably when pushed upon the second enclosure will move from the second position to a fourth position and then when subsequently no longer 10 pushed upon will be propelled from the fourth position to the first position. In preference the second enclosure is propelled between the third and second positions and between the fourth and first positions by means of a spring mechanism. Preferably the second enclosure is cylindrical in shape and the control 15 actuators are constrained to move along an outer surface of the second enclosure. In preference the control actuators move along an arcuate path along said outer surface. It should be noted that any one of the aspects mentioned above may 20 include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a 25 part of this specification, illustrate a preferred embodiment of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: Figure 1 is a perspective view of a sensor with its controls hidden; 3 Figure 2 is a perspective view showing of the sensor with its controls exposed; Figure 3 is an exploded view of the sensor; Figure 4 is a view of the inner body of the sensor highlighting the track 5 of the retracting mechanism; Figure 5 is a perspective view of the inside of the outer body of the sensor showing the tracer of the retracting mechanism; and Figure 6 is a partially assembled view of the sensor showing the control actuators. 10 LIST OF COMPONENTS 20 Sensor 22 Outer body 24 Inner body 26 Mounting flange 15 28, 29 Mounting clips 30, 31 Control sliders 32, 33 Slider recesses 36 Slider aperture 38 Retaining slot 20 39 Retaining lug 40 Cable clamp 41 Cable entry 42 Cable aperture 43, 44 Tracers 25 45 Track 46 Clip retainer 47 Clip mounts 48 Spring 50 Inner base 4 52 Inner escutcheon 54 PCB and optics assembly 57 Main lens 58, 59 Control switchs 5 60, 61 Switch knobs 75 island 80 tracer pin 81 in stop 84, 91 intermediate stops 10 87 out stop 82, 85, 88, 92 track segments 83, 86, 90, 93 steps DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The following detailed description of the invention refers to the 15 accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration. 20 In a preferred embodiment the invention combines a PIR sensor and a light sensor in an enclosure suitable for fitting into an aperture in a ceiling or wall or in a device such as a luminaire. The sensor sits flush with the surface through which it is mounted with just a mounting flange and PIR lens being visible. By pushing on the PIR lens the internal body of the 25 sensor is extended to reveal user controls. A second push on the PIR lens retracts the internal body to conceal the controls again. Referring to the drawings now in greater detail where there is provided a sensor shown generally as 20. As seen in Figures 1 and 2 the main components of the sensor 20 are 30 an outer body 22 from which an inner body can extend to reveal a control 5 slider 30. The sensor 20 is mounted in a circular aperture of a ceiling or the like by means of mounting flange 26 in cooperation with mounting clips 29. Visible and infrared light enter the sensor via the main lens 57. Further components of the sensor 20 can be seen in the exploded view 5 of Figure 3. The outer body 22 incorporates a cable entry 41 allowing a cable to enter the sensor and pass via the cable aperture 42 of the inner base 50 to the PCB and optics assembly 54. The cable is secured to the outer body 22 by means of cable clamp 40. The outer body 22 includes clip retainers 46 10 and clips mount 47 to attach the mounting clips 28 and 29. The inner body 24 comprises an inner base 50, inner escutcheon 52, PCB and optics assembly 54 mounted within, and control sliders 30 and 31. The inner body 24 can extended from and retract into the outer body 22 by pushing upon the inner body. The movement is governed by a biasing 15 means, in this case spring 48 positioned between the inner body 24 and outer body 22 in conjunction with further components discussed below. The range of movement is limited by a retaining lug 39 of the inner base 50 cooperating with a retaining slot 38 of the outer body 50. The control sliders 30 and 31 serve to actuate the control switches 58 20 and 59. As seen in Figure 2, a control slider 30 is constrained to move in an arc along the slider aperture 36 between the inner base 50 and inner escutcheon 52. The control sliders 30 and 31 incorporate recesses 32 and 33, best seen in Figure 3, which engage knobs 60 and 61 of the control switches 58 and 59 as seen in Figure 6. The curvilinear movement of the 25 control sliders is translated in to rectilinear movement of the control switches. Other control means such as potentiometers could readily replace the control switches. Figures 4 and 5 illustrate the mechanism by which the inner body 24 can extend and retract with respect to the outer body 22. The outer body 22, 30 shown in Figure 5, includes tracer arm 43 attached to the outer body. The 6 distal end of the tracer arm 43 incorporates a tracer pin 80 that traces along a track 45 on the outer surface of the inner body 24 as seen in Figure 4. The inner body further incorporates a second identical track not visible in the Figures that co-operates with a second tracer arm 44. The sensor is 5 made of a plastic material which allows the tracer arm 43 to flex slightly allowing for movement of the tracer pin 80 with respect to the outer body 22. When the sensor is assembled the inner body 24 is placed within the outer body 22 and the tracer pin 80 is constrained by the track 45. The spring 48 mounted between the inner body and outer body will tend to push 10 out the inner body 24, however, this will be limited by the tracer pin 80 engaging the in stop 81 of the track 45 and the inner body 24 will remain retracted. If the inner body is then pushed further into the outer body the tracer pin 80 will begin to traverse a circuit around the island 75 of the track 45. Traversal of the tracer pin around the track is progressed along track 15 segments 82, 85, 88 and 92. Between the track segments there are a series of steps 83, 86, 90 and 93 which ensure that the tracer pin 80 traverses the track in a counter-clockwise direction only. When the inner body 24 is retracted and subsequently pushed inward the tracer pin 80 will leave the in stop position 81; move along track 20 segment 82; over step 83 and come to rest against intermediate stop 84. The tracer is prevented from returning to track segment 82 by the step 83. When the inner body is released the spring 48 can act and will push out the inner body whilst the tracer moves along track 85; passes over step 86 and comes to rest at out stop 87. The inner body is now fully extended and the 25 control sliders 30 and 31 are accessible. With the inner body 24 extended and subsequently pushed inward the inner body will be free to move with tracer pin 80 moving along track segment 88 and over step 90 before coming to rest against intermediate stop 91. Upon release the spring 48 will again tend to push out the inner 30 body. The tracer pin will move along track segment 92 over step 93 before 7 coming to rest against in stop 81. The inner body is now held in the fully retracted position and the control sliders are no longer accessible. The reader will now appreciate the operation of the sensor and how it provides a simple to use extend/retract mechanism to reveal and conceal 5 control actuators thus providing a sensor that is easy to install and maintain whilst providing the security and aesthetic benefits of hiding control actuators when they are not required. Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention 10 has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. 15 Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field. In the summary of the invention, except where the context requires otherwise due to express language or necessary implication, the word 20 "comprising" is used in the sense of "including", i.e. the features specified may be associated with further features in various embodiments of the invention.