US 3562509 A
Description (OCR text may contain errors)
United States Patent l72| Inventor William G. Kalil, Jr.
[21 Appl. No. 743,067
 Filed July 8, 1968  Patented Feb. 9, 1971 [73 l Assignee Arrowhead Enterprises, Inc.
a corporation of Connecticut  ANTIDEPOSITION CIRCUIT 6 Claims, 3 Drawing Figs.
 U.S. Cl 240/4.2, 3.7/262; 350/61; 15/1.5  lnt.Cl G02b, F2lv 33/00  Field of Search 3.7/262;
 References Cited UNlTED STATES PATENTS 2,388,518 1 H1945 Baiocchi 350/61X FOREIGN PATENTS 698,874 10/1953 Great Britain 55/TCE OTHER REFERENCES Removing Impurities From Surfaces" by D. R. Young and .l. E. Thomas, Jr. Vol. 7, No. 3 August 1964 Primary Examiner-J. D. Miller Assistant ExaminerC. L. Yates AttorneyBuckler & Bramblett ABSTRACT: The deposition of dust on a mirror exposed to ambient air is prevented by mounting the mirror on a metallic plate which is positively charged. As more dust particles are positive than negative, such an arrangement is very effective in preventing the deposition of dust on the mirror surface.
PATENIEU FEB 9191:
ANTIDEPOSITION CIRCUIT BACKGROUND OF THE INVENTION This invention relates to the prevention of dust deposits on surfaces and, more particularly, on reflective surfaces such as those of mirrors used in photoelectric burglar alarms, announcer systems, and similar apparatus.
Photoelectric burglar alarms and customer announcer units are widely used in stores and commercial establishments. In systems of this type, a transmitter unit including a light source, such as an incandescent lamp, and a focusing mirror generates a beam of electromagnetic radiation which may be either in the visible region of the spectrum or in the invisible region (infrared, for example). The beam is transmitted, either directly or via mirrors, to a remotely located receiver unit. When the beam is broken, a suitable alarm or signal is given. Systems of this type, being relatively simple, should be capable of operating for extended periods of time without maintenance. However, difficulties have arisen in prior units due to the deposition of airborne dust on the mirrors. For example, the transmitter units employed in such systems normally include a heat-generating radiation source, such as an incandescent lamp. Suitable ventilation openings are provided in the transmitter case to permit ambient air to flow therethrough to cool the lamp and case by convection. However, this ambient air fiow results in entrained dust particles depositing on the mirror surface, thereby reducing its effectiveness. When excessive dust is present, or when the system has been installed for a comparatively long period of time, this deposition may be sufficient to render the system inoperative.
In order to overcome this problem, antistatic compounds have been applied to the mirror surface. While this has been relatively successful, it has certain drawbacks. These compounds are expensive and additional labor is required to apply them. Also, periodic cleaning of the mirror results in the coating being rubbed off. Furthennore, the antistatic compounds are not permanent.
Accordingly, it is a primary object of the present invention to provide simple and effective means for substantially reducing ambient dust deposition on a mirror surface. Other objects are to provide such means which is permanent, which reduces the cost of the system and its maintenance, and which dispenses with the need for antistatic compounds.
SUMMARY OF THE INVENTION In accordance with the present invention, an electrostatic field is established around an element to be protected. The polarity of the field in the vicinity of the element is the same as that of the particles to be repelled.
BRIEF DESCRIPTION OF THE DRAWING The manner in which the foregoing objects are achieved will be more apparent from the following description, the appended claims and the FIGS. of the attached drawing wherein:
FIG. 1 is a front view, partially broken away, of a photoelectric transmitter unit useable with this invention;
FIG. 2 is a cross section taken substantially along the line 2-2 of FIG. 1; and
FIG. 3 is a schematic diagram of an electrical circuit useable with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT With particular reference to the drawing of this application, there is illustrated in FIGS. 1 and 2 a transmitter unit for a photoelectric burglar system comprising a metallic case having an infrared transmitting window 12. The rear of the case is provided with suitable ventilating louvers l4 and internally threaded studs 16. An electrically conductive metallic mounting plate 18 is provided with openings 20 which are aligned with the studs 16. Mounted in each of the openings 20 is a grommet 22 of a suitable insulating material, such as rubber or plastic. Passing through the grommets 22 and threaded into studs 16 are mounting screws 24. Positioned upon mounting plate 18 by a suitable adhesive and in alignment with window 12 is a spherical mirror 26.
Also mounted upon mounting plate 18 by means of insulating grommets 28, bolts 30, and nuts 32 is an L-shaped bracket 34 to which is mounted a lamp socket 36. Lamp socket 36 supports an incandescent lamp 37 with its filament positioned substantially at the focal point of mirror 26. It will thus be seen that, by means of the insulating grommets 22, 28, the mounting plate 18 is electrically isolated from both case 10 and lamp socket 36.
The inventor has experimentally determined that the majority of ambient air dust particles have a positive electrical charge. Accordingly, a mirror in proximity to a positive electrical potential is less subject to dust deposition than a mirror in an electrically neutral condition or adjacent an area of negative potential. Accordingly, the apparatus of FIGS. 1 and 2 includes means for applying to the mounting plate 18 a positive electrical potential. This means includes a screw 38 and terminal 40 to which is secured an electrical lead 42. The manner in which the positive potential is achieved is illustrated in FIG. 3. A terminal block 44 serving the transmitter unit is connected to receive a 120 volt input from an AC power supply which is applied to the primary winding 46 of a transformer 48. Transformer 48 is of the stepdown type and has a secondary winding 50 with a 4 volt output which is applied by leads 52 to lamp 37. A current limiting resistor 54 and a diode. 56 are connected in series between one side of transformer primary winding 50 and lead 42, diode 56 being polarized to pass positive current to terminal 40. A conductor 58 connects the otherside of permanent winding 50 to a terminal 60 on case I0. A smoothing capacitor 62 is connected between terminals 40 and 60. It will thus be seen that a simple half wave rectifier is formed which maintains mounting plate 18 at a positive potential with respect to case 10. As this positive potential is immediately adjacent the mirror 26, it serves to effectively r'epel the positively charged dust particles from the area immediately surrounding the mirror and the mirror remains much cleaner than would otherwise be the case. The current limiting resistor 54 prevents overloading of the power supply in the event the plate 18 is accidentally shorted to the case 10.
It will be understood by those skilled in the art that circuit elements of various values may be employed in this invention. In at least one actual embodiment, the diode 56 was a one ampere diode and capacitor 62 had a rating of microfarads. Such a circuit produced approximately 5 volts across terminals 60 and 40.
It will also be understood by those skilled in the art that a number of variations and modifications may be made in this invention without departing from its spirit and scope. Accordingly, the foregoing description is to be construed as illustrative only, rather than limiting. This invention is limited only by the scope of the following claims.
1. Apparatus for preventing the deposition of electrically charged particles on a mirror to be protected which comprises: a mirror; an electrically conductive case enclosing said mirror; and an electrically conductive mounting plate supporting said mirror and electrically insulated from said case for applying a DC electrical potential between said case and a region adjacent said mirror.
2. Apparatus for preventing the deposition of electrically charged particles on a mirror to be protected which comprises; a mirror; an electrically conductive case enclosing said mirror, an electrically conductive mounting plate supporting said mirror and electrically insulated from said case; DC power supply means connected across said case and mounting plate; and on electrical lamp mounted within said case adjacent said mirror.
3. The apparatus of claim 2, wherein said lamp and said DC power supply means are energized from a common AC power source.
in series with said plate.
6. The apparatus of claim 5 wherein a smoothing capacitor is connected across said casing and mounting plate.