Patent Publication Number: US-6714131-B2

Title: Anti-thief security sensor assembly using the opening of the projector cover for beam adjustment

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an anti-thief security sensor assembly and, more particularly, to the anti-thief security sensor assembly of a type wherein while an infrared beam emitted from an infrared beam projecting unit is constantly received by an infrared beam receiving unit, an alarm is generated when an unauthorized intruder traverses across the path of travel of the infrared beam from the infrared beam projecting unit towards the infrared beam receiving unit. 
     2. Description of the Prior Art 
     The anti-thief security sensor assembly is known in which a beam projector of an infrared beam projecting unit and a beam receiver of an infrared beam receiving unit are installed at opposite ends of a rectilinear guard area spaced an appropriate distance from each other with their optical axes aligned with each other. The infrared beam receiving unit is so operated that when the receiver senses the infrared beam the receiver can output an electric signal of a signal level proportional to the amount of the infrared beam received. The electric signal so outputted is, after having been amplified by an amplifier circuit, processed by a detecting circuit to remove a disturbance light component and then to convert it into a signal of a level proportional to the received beam signal, and the signal level from the detecting circuit is subsequently determined by a signal discriminating circuit as to whether or not the level of the signal is lower than a predetermined detection level. In the event that the level of the received beam signal attains a value lower than the predetermined detection level as a result of the infrared beam from the beam projecting unit towards the beam receiving unit having been intercepted by traverse of an unauthorized intruder, the signal discriminating circuit outputs a detection signal with which an alarm generator is driven to output an alarm signal warning that the unauthorized intruder has entered the guard area. 
     The anti-thief security sensor assembly is generally capable of monitoring the guard area ranging from a rectilinearly close distance to a rectilinearly long distance of a few hundred meters, and the longer the rectilinear distance, the more difficult it is to install the beam projector and the beam receiver with their optical axes aligned with each other as accurately as possible at respective locations a distance from each other. Accordingly, the conventional anti-thief security sensor assembly includes a sighting instrument so as to facilitate the alignment of the respective optical axes of the beam projecting and receiving units. To align the respective optical axes of the beam projecting and receiving units with each other at the time of installation or servicing of the anti-thief security sensor assembly, a servicing worker has to look through a viewing hole of the sighting instrument mounted on the beam receiving unit to adjust the angle of orientation of the beam receiver in both vertical and horizontal directions to roughly align the respective optical axes of the beam projecting and receiving units with each other with naked eyes. Once this has been done, while the signal level proportional to the amount of the infrared beam received by the beam receiver is read out with the use of a level meter such as, for example, a voltmeter electrically connected with and built in the detecting circuit of the beam receiving unit, the angle of orientation of the beam receiver in the vertical and horizontal directions are finely adjusted to render the reading of the signal level to match with a predetermined level of the infrared beam that ought to be received, thereby completing the job of aligning the respective optical axes of the beam projecting and receiving units. 
     In recent years, the anti-thief security sensor assembly has come to be known in which the beam receiving unit is provided with a signal output means for outputting a level display signal descriptive of the amount of the infrared beam received to the beam projecting unit and, on the other hand, the beam projecting unit is provided with a receiving means for receiving the level display signal and a display means for displaying the level display signal so received. See, for example, the Japanese Laid-open Patent Publication No. 4-71099. According to this prior art anti-thief security sensor assembly, the fine alignment of the respective optical axes of the beam projecting and receiving units is carried out by manipulating the sighting instrument of the beam projecting unit while the display of the level display signal received from the receiving means is monitored with naked eyes. While the optical adjustment on the side of the beam projecting unit had required intervention of at least two servicing workers, assigned respectively to sites of installation of the beam projecting and receiving units, who were required to communicate wireless with each other as to the level of the infrared beam received, this prior art anti-thief security sensor assembly disclosed in the above referenced publication requires only one servicing worker to accomplish a similar optical adjustment. 
     According to the prior art, difficulty has been encountered that even when the level of the infrared beam being received by the beam receiving unit is desired to be ascertained, no transmission of the level display signal from the beam receiving unit to the beam projecting unit is possible. In other words, transmission of the level display signal from the beam receiving unit to the beam projecting unit is possible only when and after a receiver cover enclosing and protecting the beam receiving unit is physically opened and mere physical opening of a projector cover enclosing and protecting the beam projector does not allow the beam projecting unit to receive the level display signal transmitted from the beam receiving unit. Accordingly, unless those two covers for the beam receiving and projecting units are physically opened, no fine adjustment of the optical axis of the beam projector is possible. 
     In view of the above, it has hitherto been carried out for a servicing worker to first open the projector cover for the beam projecting unit, then to move to the site of installation of the beam receiving unit to open the receiver cover for the beam receiving unit so that the optical axis of the beam receiving unit can be adjusted in the manner described above, again to move to the site of installation of the beam projecting unit after the adjustment of the optical axis of the beam receiving unit to thereby effect the adjustment of the optical axis of the beam projecting unit, thereafter to return to the site of installation of the beam projecting unit to mount the once opened projector cover onto the beam projecting unit, and finally to again return to the site of installation of the beam receiving unit to mount the once opened receiver cover onto the beam receiving unit, thereby completing the alignment of the respective axes of the beam projecting and receiving units with each other. For this reason, the servicing worker has to make at least one round and half trip between the respective sites of installation of the beam projecting and receiving units. This is indeed a substantial burden on the servicing worker if the guard area extends a substantial distance, accompanied by a substantial length of time required to complete the optical axis alignment. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention is intended to provide an anti-thief security sensor assembly wherein only physical opening of a projector cover for a beam projecting unit is sufficient to achieve the adjustment of the optical axis of the beam projecting unit relative to that of a beam receiving unit. 
     In order to accomplish the foregoing object of the present invention, there is provided an anti-thief security sensor assembly including a beam projecting unit for projecting an infrared beam and a beam receiving unit for receiving the infrared beam projected by the beam projecting unit. The beam projecting unit includes a projector base having a sensor circuit mounted thereon and a projector cover detachably mounted on the base for enclosing and projecting the sensor circuit, an opening detecting switch for detecting a physical opening of the projector cover and a transmission request generating circuit operable in response to the detection of the physical opening of the projector cover by the opening detecting switch for outputting a received beam level transmission request signal to the beam receiving unit. The beam receiving unit is provided with a level output circuit operable in response to the received beam level transmission request signal to transmit to the beam projecting unit a level display signal indicative of the amount of the infrared beam received by the beam receiving unit. 
     With this anti-thief security sensor assembly according to the present invention, when the projector cover is physically opened at the time of installation or servicing of the anti-thief security sensor assembly, the opening detecting switch detects such physical opening of the projector cover and the transmission request generating circuit operates, based on an detecting operation of the opening detecting switch, to transmit the received beam level transmission request signal to the beam receiving unit. Accordingly, when the beam receiving unit receives the received beam level transmission request signal, the level output circuit transmits the level display signal indicative of the amount of the infrared beam received by the beam receiving unit to the beam projecting unit. Thus, regardless of whether a receiver cover enclosing and projecting circuit components of the beam receiving unit is physically opened, i.e., removed, the servicing worker can perform an accurate axis alignment while looking at the level display signal at the site of installation of the beam projecting unit and, therefore, the workability is high. 
     In a preferred embodiment of the present invention, the transmission request generating circuit is so designed as to superimpose the received beam level transmission request signal on the infrared beam projected from the beam projecting unit. Accordingly, with no need to provide a communicating means for conducting a communication from the beam projecting unit towards the beam receiving unit, the received beam level transmission request signal can be transmitted from the beam projecting unit towards the beam receiving unit. 
     In another preferred embodiment of the present invention, the beam receiving unit may include an amplifier for amplifying the infrared beam received by the beam receiving unit, and a received beam level suppressing circuit for controlling the amplifier to reduce the received beam level by a predetermined level corresponding to the amount of the infrared beam attenuated or reduced as it passes through the projector cover of the beam projecting unit. This is particularly advantageous in that since even though the projector cover is physically opened or removed, the amount of the infrared beam received by the beam receiving unit can be displayed by the level meter of the beam projecting unit at the same signal level as the signal level attained when the projector cover is mounted, the alignment of the respective optical axes of the beam projecting and receiving units can accurately be achieved. 
     It is to be noted that the term “physical opening” used in connection with the projector and receiver cover in the description made hereinabove and hereinafter is intended not only to mean that the cover is removed away from the associated base, but also to mean that the cover is hingedly opened relative to the associated base to which it is hinged and is thus used in the sense that when the cover is opened, internal component parts covered and projected by such cover are rendered open to the outside regardless of whether the cover remains hingedly affixed to the associated base or whether it be separated from the associated base. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In any event, the present invention will become more clearly understood from the following description of a preferred embodiment thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and: 
     FIG. 1 is a circuit block diagram showing an anti-thief security sensor assembly according to a preferred embodiment of the present invention; 
     FIGS. 2A and 2B are schematic diagrams showing respective manners of transmission of an infrared beam, a received beam level transmission request signal and a level display signal in the embodiment shown in FIG. 1; and 
     FIG. 3 is a schematic perspective view showing the anti-thief security sensor assembly shown in FIG. 1, with a portion of a beam receiving unit shown as cut out. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Hereinafter, an anti-thief security sensor assembly according to a preferred embodiment of the present invention will be described in detail. Referring first to FIG. 1 showing a circuit block diagram of the anti-thief security sensor assembly, the anti-thief security sensor assembly includes a beam projecting unit  1  and a beam receiving unit  2  fixedly mounted on corresponding wall surfaces or support poles, which are positioned at opposite ends of a rectilinear guard area spaced an appropriate distance from each other, with their optical axes aligned with each other. Each of the beam projecting and receiving units  1  and  2  is unitarized into a respective module. 
     The beam projecting unit  1  includes a beam projector  3  which includes a light emitting element such as, for example, an infrared light emitting diode, and a transmission side optics such as, for example, a beam projecting lens or a reflecting mirror for forming an infrared beam IR. A projector drive circuit  4  is used to drive the light emitting element of the beam projector  3  to cause the latter to emit an infrared beam IR at a predetermined frequency as shown in FIG.  2 A. The beam projector  3  and the projector drive circuit  4  forming respective components of a sensor circuit are utilized in a pair as will become clear from the subsequent description, but only one of the pair is shown in FIG.  1 . 
     A projector-side opening detecting switch  7  is a contact type or proximity type switch for detecting opening or closure of a projector cover  21 A, as will be described later, relative to a projector base  20 A. This projector-side opening detecting switch  7  concurrently serves as an existing damper switch utilized to detect a nuisance opening of the projector cover and then to output an alarm signal. A transmission request generating circuit  8  controls, when the opening detecting switch  7  is turned off as a result of detection of the opening of the projector cover  21 A, to output a received beam level transmission request signal B to the projector drive circuit  4 , then to superimpose it on the infrared beam IR generated from the light emitting element and finally to transmit the resultant superimposed signal IR+B to the beam receiver  11  as shown in FIG.  2 B. The beam projecting unit  1  shown in FIG. 1 also includes a signal receiving unit  9  for receiving a level display signal C transmitted from a beam receiving unit  2 , and a level meter  10  for displaying a signal level of the level display signal C received by the signal receiving unit  9 . It is to be noted that the infrared beam IR is in the form of, for example, a pulse modulated light and the received beam level transmission request signal B is in the form of a pulse signal different in frequency from that of the infrared beam IR. 
     On the other hand, the beam receiving unit  2  includes a beam receiver  11  made up of a receiver optics such as, for example, a beam receiving lens or a beam receiving mirror, and a light receiving element such as, for example, a phototransistor. This beam receiver  11  is operable to receive the infrared beam IR projected from the beam projecting unit  1  and then to output an electric signal of a signal level proportional to the amount of the infrared beam IR received thereby. This electric signal is, after having been amplified by an amplifier circuit  12 , fed to a detecting circuit  13  where the electric signal is, after a disturbance light component contained in the electric signal has been removed, converted into a signal of a level proportional to the level of the received beam signal solely in the form of a pulse modulated wave, which signal level is subsequently determined by a signal discriminating circuit  14  as to whether or not the signal level is lower than a predetermined detection level. In the event that the level of the received beam signal attains a value lower than the predetermined intrusion detection level as a result of the infrared beam IR from the beam projecting unit  1  having been intercepted by traverse of an unauthorized intruder, the signal discriminating circuit  14  outputs a detection signal with which an alarm generating circuit  17  is driven to output an alarm signal warning that the unauthorized intruder has entered the guard area. This alarm signal may be utilized in numerous way and may be communicated to a security center (not shown) and/or utilized to trigger an alarm and/or a lighting instrument on. 
     A level meter  18  such as, for example, a voltmeter is electrically connected with the detecting circuit  13  so that the signal level proportional to the amount of the infrared beam received by the beam receiver  11  can be displayed by the level meter  18 . The amplifier  12  referred to above is gain-controlled by an AGC (automatic gain control) circuit  19  in dependence on the level of the signal received from the beam receiver  11  so that an output from the amplifier circuit  12  can be controlled to a value lower than a predetermined signal level at all times. It is to be noted that the beam receiver  11 , the amplifier circuit  12 , the detecting circuit  13  and the alarm generator circuit  17  forming respective components of the sensor circuit are also utilized in a pair as will become clear from the subsequent description, but only one of the pair is shown in FIG.  1 . 
     The beam receiving unit  2  also includes a receiver-side opening detecting switch  50 , a received beam level suppressing circuit  51 , a level output circuit (an level output unit)  52  and a signal extracting circuit  53 . The signal extracting circuit  53  outputs a detection signal when the received beam level transmission request signal B is extracted from the signal received by the beam receiver  11 . The receiver-side opening detecting switch  50  may be a contact type or proximity type switch for detecting the opening and closure of a receiver cover  21 , as will be described later, relative to a receiver base  20  and concurrently serves as an existing damper switch utilized to detect a nuisance opening of the receiver cover  21 . The level output circuit  52  is operable, when the detection signal is inputted thereto from the signal extracting circuit  53 , to output either by wireless or through a signal feed line a signal level from the detecting circuit  13  as a level display signal C indicative of the amount of the infrared beam received by the receiving unit  2  to a signal receiver  9  included in the beam projecting unit  9 . 
     The received beam level suppressing circuit  51  controls, when the receiver-side opening detecting switch  50  is turned off in response to detection of the opening of the receiver cover  21  or when the detection signal is inputted from the signal extracting circuit  53 , to lower the gain of the amplifier circuit  12  through the AGC circuit  19  and then to amplify the signal level of the received beam signal from the beam receiver  11  by lowering such signal level by a predetermined level corresponding to the amount of the infrared beam attenuated (for example, 30%) as it pass through the cover of one of the beam projecting unit  1  and the beam receiver  11 . The received beam level suppressing circuit  51  also controls, when the receiver-side opening detecting switch  50  is turned off and the detection signal is inputted from the signal extracting circuit  53 , to amplify the signal level of the received beam signal from the beam receiver  11  by lowering such signal level by a quantity con-esponding to the amount of the received infrared beam attenuated (for example, 60%) as it passes through the projector and receiver covers. 
     FIG. 3 illustrates the beam receiving unit  2  with a portion thereof cut out. It is to be noted that since the beam projecting unit  1  is of a structure substantially similar to that of the beam receiving unit  2  and, accordingly, reference numerals allocated to similar component parts of the beam projecting unit  1  are also shown in FIG. 3 although the following description is directed to that of the beam receiving unit  2 . 
     The beam receiving unit  2  includes a casing  22  comprised of a mounting base  20  fixed to a wall surface or a support pole and the receiver cover  21  detachably supported by the mounting base  20 . The receiver-side opening detecting switch  50  shown in and discussed with reference to FIG. 1 for detecting a physical opening of the receiver cover  21  is mounted fixedly on the mounting base  20 . The receiver cover  21  is made of a material such as, for example, a synthetic resin of a kind capable of relatively favorably passing an infrared rays of light, but attenuating it by about 30% and is treated to represent a black color or a similar dark color sufficient to inhibit passage of visible rays of light therethrough. 
     The mounting base  20  has upper and lower spaced support members  28  and  29  fixed thereto so as to protrude perpendicular to the mounting base  20 . A generally box-like receiver chassis  27  is swingably supported in between the upper and lower support members  28  and  29  by means of coaxially aligned vertical stud shafts  30  and  31 . A generally rectangular lens holder  37  having coaxially aligned horizontal stud shafts  33  and  34  protruding laterally outwardly therefrom is accommodated within the receiver chassis  27  and is tiltably supported by opposite side walls of the receiver chassis  27  with the horizontal stud shafts  33  and  34  journalled thereto. Upper and lower beam receiving lenses  23  positioned one above the other are retained by a lens casing  24  which is in turn carried by the lens holder  37 . A light receiving element (not shown) forming the beam receiver  11  in cooperation with the beam receiving lenses  23  is mounted on a printed circuit board (not shown) and is accommodated within the lens holder  37  and positioned at a location rearwardly of the beam receiving lenses  23 . 
     Accordingly, it will readily be seen that the beam receiver  11  has an adjustable angle of swing in a horizontal direction, shown by the arrow h, as the receiver chassis  27  can be adjustably swung about a common axis defined by the vertical stud shafts  30  and  31  and also has an adjustable angle of tilt in a vertical direction, shown by the arrow v, as the lens holder  37  can be adjustably tilted about a common axis defined by the horizontal stud shafts  33  and  34 , wherefore the optical axis can be aligned relative to the beam projector  11 . As will be described in detail later, the optical axis alignment is carried out by the aid of a sighting instrument. Also, the sensor circuit of the circuit configuration shown in FIG. 1 is mounted on the printed circuit board referred to above, which is in turn mounted inside the lens holder  37 . Thus, the sensor circuit is covered and protected by the receiver cover  21  through the lens holder  37 . 
     One of the stud shafts, that is, the lower stud shaft  31  rotatably extends through the lower support member  29  and terminates with an adjustment knob  32  secured thereto for rotation together with the lower stud shaft  31  and, hence the receiver chassis  27  for adjustment of the angle of swing in the horizontal direction about the common axis defined by the upper and lower stud shafts  30  and  31 . The receiver chassis  27  has a mounting flange  38  extending upwardly from a generally intermediate portion of a lower front edge of a lower wall of the receiver chassis  27 , which flange  38  has an adjustment screw  39  turnably coupled thereto. This adjustment screw  39  is loosely through the mounting flange  38  and is then threaded into a projection  40  protruding downwardly from a lower end of the lens holder  37  so as to occupy a position behind the mounting flange  38 . Accordingly, it will readily be seen that turn of the adjustment knob  32  results in adjustment of the angle of swing of the beam receiver  11  through the beam receiver chassis  27  and turn of the adjustment screw  39  results in adjustment of the angle of tilt of the beam receiver  11  through the lens holder  37 . 
     It is to be noted that FIG. 3 illustrates a configuration wherein two structures each substantially similar to the beam receiving unit  2  shown in FIG. 1 are arranged one above the other and, therefore, the alarm generator circuit  17  can output the alarm signal when alarm signals are outputted from those paired beam receiving units  2 . 
     A sighting instrument for aiding the optical axis adjustment is provided at a position substantially intermediate of the lens holder  37  with respect to the lengthwise direction thereof and generally between the beam receiving lenses  23 . This sighting instrument  41  includes a pair of right and left viewing holes  43  and  44 , a sighting hole  47  and  48  employed in association with each of the viewing holes  43  and  44 , and a reflecting mirror employed in association with each of the viewing holes  43  and  44 . This sighting instrument  41  can be operated in such a manner that while a servicing worker looks into one of the viewing holes  43  or  44 , one or both of the adjustment knob  32  and the adjustment screw  39  have to be turned to adjust the angle of swing and/or the angle of tilt of the beam receiver  11  until an image of the beam projector  3  cast on the associated reflecting mirror aligns with an associated one of the sighting holes  47  or  48 . In this way, the optical axes of the beam projecting and receiving units  1  and  2  can be aligned with each other. It is to be noted that the beam projector  3  shown in FIG. 1 has a physical structure similar to that of the beam receiver  11  discussed above. 
     To align the respective optical axes of the beam projecting and receiving units  1  and  2  with each other at the time of installation or servicing of the anti-thief security sensor assembly of the structure described above, when the projector cover  21 A of the beam projecting unit  1  shown in FIG. 1 is physically opened, the physical opening of the projector cover  21 A is detected by the projector-side opening detecting switch  7  and the transmission request generating circuit  8  operates in response to the detection by the opening detecting switch  7  to control the projector drive circuit  4 . In this way, the projector drive circuit  4  controls the beam projector  3  so that the beam projector  3  can output the infrared beam IR superimposed with the received beam level transmission request signal B. On the other hand, in the beam receiving unit  2 , even though the receiver cover  21  is physically not opened, the signal extracting circuit  53  extracts the received beam level transmission request signal B from the signal received by the beam receiver  11  and outputs the detection signal to the level output circuit  52 . Then, in response to receipt of the detection signal the level output circuit  52  transmit the signal level of the detecting circuit  13 , as a level display signal C indicative of the amount of the infrared beam received by the beam receiving unit  2 , to the signal receiving unit  9  of the beam projecting unit  1 . The signal receiving unit  9  of the beam projecting unit  1  subsequently causes the level meter  10  to display the level display signal C received by the signal receiving unit  9 . 
     In view of the foregoing, when a single servicing worker is assigned to perform the optical axis alignment of the anti-thief security sensor assembly of the present invention, the servicing worker first has to physically open the projector cover  21 A of the beam projecting unit  1  and then to turn the projector adjustment knob  32  and the projector adjustment screw  39 , both in the beam projecting unit  1 , one at a time while looking into one of the viewing holes  43  or  44  in the beam projecting unit  1  to thereby roughly align the optical axis of the beam projector  3  with the beam receiving unit  2  with naked eyes. Once this has been done, the servicing worker has to move to the site of installation of the beam receiver  11  where the servicing worker has to physically open the receiver cover  21  of the beam receiving unit  2  and then to turn the receiver adjustment knob  32  and the receiver adjustment screw  39 , both in the beam receiving unit  2 , one at a time while looking into one of the receiver viewing holes  43  or  44  in the beam receiving unit  2  to thereby roughly adjust the optical axis of the beam receiver  3 . 
     Thereafter, by looking at the display made on the level meter  18  of the beam receiving unit  2 , the servicing worker has to finely turn the adjustment knob  32  and the adjustment screw  39 , both in the beam receiving unit  2 , one at a time in a manner similar to that described above until the display on the level meter  18  indicates a maximum reading, thereby completing a fine adjustment of the optical axis of the beam receiver  11  to align exactly with that of the beam projector  3 . 
     During the optical axis adjustment of the beam receiving unit  2 , the opening detecting switch  50  detects the physical opening of the receiver cover  21  to thereby provide the received beam level suppressing circuit  51  with the detection signal. Accordingly, in response to the detection signal, the received beam level suppressing circuit  51  controls the gain of the amplifier circuit  12  through the AGC circuit  19  so that the signal level of the signal received by the beam receiver  11  can be lowered by 30% that corresponds to the amount of the infrared beam attenuated as the latter pass through the receiver cover  21 . For this reason, even though the receiver cover  21  is physically opened, the level meter  18  displaying the output from the detecting circuit  13  in the beam receiving unit  2  displays the received signal at the same level as that exhibited when the receiver cover  21  is mounted. Accordingly, with the anti-thief security sensor assembly of the present invention, it is possible to accurately adjust the optical axis of the beam receiver  11  with only the receiver cover  21  of the beam receiving unit  2  opened physically. After the optical axis adjustment of the beam receiver  11  has been finished in this way, the receiver cover  21  is mounted to the original position. 
     Thereafter, the servicing worker has to return to the site of installation of the beam projecting unit  1  where by looking at the display made on the level meter  10  of the beam projecting unit  1 , the servicing worker has to finely turn the adjustment knob  32  and the adjustment screw  39 , both in the beam projecting unit  1 , one at a time to perform a fine adjustment of the optical axis of the beam projector  3 . When the display on the level meter  10  shows a reading higher than the predetermined level, it means that the optical axis of the beam projector  3  is exactly aligned with that of the beam receiver  11  and, accordingly, the projector cover  21 A of the beam projecting unit  1  is then mounted to the original position, thereby completing the optical axis alignment between the beam projecting and receiving units  1  and  2 . Since at this time the receiver cover  21  of the beam receiving unit  2  has already been mounted to the original position, the servicing worker need not again move to the site of installation of the beam receiving unit  2 . Thus, it will readily be seen that only one round trip between the site of installation of the beam projecting unit  1  and the site of installation of the beam receiving unit  2  is needed for the servicing worker to accomplish the required optical axis alignment. 
     Although during the optical axis adjustment of the beam projecting unit  1 , only the projector cover  21 A of the beam projecting unit  1  is physically opened, the received beam level suppressing circuit  51  controls through the AGC circuit  19  to reduce the signal level of the signal received by the beam receiver  11  by 30%. Accordingly, even though the projector cover  21 A is physically opened, the amount of the infrared beam received by the beam receiving unit  1  can be displayed by the level meter  10  of the beam projecting unit  1  at the same signal level as the signal level attained when the projector cover  21 A is mounted and, therefore, the alignment of the respective optical axes of the beam projecting and receiving units  1  and  2  can easily be achieved. Thus, with the anti-thief security sensor assembly according to the present invention, even though the receiver cover  21  of the beam receiving unit  2  is not opened physically, only physical opening of the projector cover  21 A of the beam projecting unit  1  is sufficient to achieve the alignment of the optical axis of the beam projector  3 . 
     Also, when both of the projector and receiver covers  21 A and  21  of the beam projecting and receiving units  1  and  2  are physically opened, the received beam level suppressing circuit  51  operates, in response to receipt of the detection signal from the opening detecting switch  50  and the detection signal from the signal extracting circuit  53 , to control the gain of the amplifier circuit  12  through the AGC circuit  19  so that the signal level of the beam receiver  11  can be reduced by 60% that corresponds to the amount of the infrared beam attenuated as it pass through both of the projector and receiver covers  21 A and  21 . Accordingly, even in this case, the received beam signal can be displayed on the level meter  18  or the level meter  10  at the same level as that exhibited when the projector and receiver covers  21 A and  21  are mounted on the respective bases. Thus, regardless of whether one or both of the projector and receiver covers  21 A and  21  are opened physically, the accurate alignment of the respective optical axes of the beam projector  3  and receiver  11  with each other can be achieved. 
     It is to be noted that in the foregoing description of the preferred embodiment of the present invention, the received beam level transmission request signal B has been described as superimposed on the infrared beam IR before it is emitted from the beam projector  3  so that the received beam level transmission request signal B can be transmitted with no extra need to employ any communicating means between the beam projecting and receiving units  1  and  2 . However, for the transmitting means for transmitting the received beam level transmission request signal B, a wireless wave transmitter or an optical signal output device may be provided in the beam projecting unit  1  together with a wave receiver or an optical signal receiver provided in the beam receiving unit  2 . Also, in place of or in combination with the level meter  18 , only the beam receiving unit  2  may be provided with, as a level output device, a light emitting device using a plurality of light emitting elements such as light emitting diodes (LED) which light-emit different colors according to the varying amount of the received beam, so that a beam of a certain color depending on the amount of the received beam can be emitted therefrom towards the beam projecting unit  1  as the level display signal C. In such case, the servicing worker looking at the level output device can recognize the signal level in reference to the color of the signal emitted therefrom. 
     Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings which are used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention. Accordingly, such changes and modifications are, unless they depart from the scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein.