SHIELDED MAGNETIC REED SWITCH BACKGROUND OF THE INVENTION 1. Technical Field The present invention relates to a magnetic reed switch, and more particularly, but not by way of limitation, to a shielded magnetic reed switch which, when exposed to the magnetic flux of an approaching magnetic field source, does not actuate until the distance between the shielded magnetic reed switch and the approaching magnetic field source is less than or equal to a predetermined distance. In one embodiment, the shielded magnetic reed switch provides a sensor which actuates and thereby provides an input to an electrical circuit to indicate the distance between the shielded magnetic reed switch and the approaching magnetic field source.

2. Background Art Magnetic reed switches are well known in the art. A magnetic reed switch has associated with it a specific magnetic field strength (sometimes referred to herein as magnetic flux) required to actuate the reed switch. In the presence of a very strong magnetic field (e. g., a permanent magnet having at a magnetic field strength of at least 300 Gauss at a distance of 4 inches from the magnet's pole face), a typical magnetic reed switch will actuate at a distance of 12-18 inches from the source of the very strong magnetic field. Progress in the area of magnetic reed switches has largely involved development of ever more sensitive reed switches, i. e., reed switches requiring low magnetic field strength (low magnetic flux) for actuation.

Until now, no one has developed a magnetic reed switch which resists actuation in the presence of magnetic flux normally sufficient to actuate the magnetic reed switch.

Disclosure Of Invention The present invention provides a magnetic reed switch disposed within a ferrous metal housing, thereby requiring a stronger magnetic flux than would otherwise be required to actuate the unshielded magnetic reed switch. Based on the selection of the

reed switch and the architecture of the ferrous metal housing, the shielded magnetic reed switch of the present invention remains unactuated in the presence of the magnetic field strength normally resulting in actuation of the unshielded magnetic reed switch. The present invention further provides a sensor for detecting the presence of an approaching magnetic field source at a predetermined distance.

An object of the present invention is to provide a magnetic reed switch which, in the presence of a magnetic field source having sufficient magnetic flux to actuate the magnetic reed switch, remains unactuated until the magnetic field source is within a predetermined distance from the magnetic reed switch.

It is another object of the present invention to provided a series of shielded magnetic reed switches which, when used together, indicate the progress of an approaching magnetic field source.

It is another object of the present invention to provide a magnetic reed switch which indicates a predetermined distance between the magnetic reed switch and an approaching magnetic field source by actuating when the approaching magnetic field source is within the predetermined distance.

Brief Description of Drawings FIG. 1 is an enlarged cross-sectional view of a shielded magnetic reed switch of the present invention.

FIG. 2 is an enlarged cross-sectional view of a second embodiment of a shielded magnetic reed switch of the present invention.

FIG. 3 is an enlarged cross-sectional view of a third embodiment of a shielded magnetic reed switch of the present invention.

FIG. 4 is a representation of the operation of applicants'invention.

FIG. 5 is another representation of the operation of applicants'invention.

Best Mode for Carrying Out the Invention In the following description of the invention, like numerals and characters designate like elements throughout the figures of the drawings.

Referring now to FIGS. 1-3, shown therein are three embodiments of the shielded magnetic reed switch 10 of the present invention. In FIG. 1, a normally open magnetic reed switch 16, consisting of reeds 12,14 sealed in a glass envelope 17, is disposed within a ferrous metal housing 30. Electrical leads 18,20 are soldered to the portions of the reeds 12,14, and heat shrink 22,24 is applied as indicated. The magnetic reed switch 16 is then placed within the ferrous metal housing 30 and potting compound 36 holds the reed switch 16 and leads 18,20 in position within the ferrous metal housing 30.

For a permanent magnet M having a magnetic flux field of about 350 Gauss at a distance of 4 inches from the pole face P (see FIGS. 4 and 5), a ferrous metal housing 30 having a diameter of about 0.125 inch, a tubular wall 32 thickness of about 0.0125 inches, and a thickened end portion 34 of about 0.125 inches at its maximum thickness results in closure of the magnetic reed switch 16 when the permanent magnet M is within 3.5 to 5.0 inches of the magnetic reed switch 16. In the absence of the ferrous metal housing 30, the reeds 12,14 of the magnetic reed switch 16 close at a distance of about 12-18 inches between the pole face P of the permanent magnet M and the magnetic reed switch 16.

Referring now to FIG. 2, shown therein is another embodiment of the shielded magnetic reed switch 10 of the present invention. In FIG. 2, the tubular wall 32A is

about 0.0125 inches thick and the thickened end portion 34A of ferrous metal housing 30 is about 0.151 inches thick. The thickened end portion 34A is about 0.125 inches thick at its thickest point.

Referring now to FIG. 3, shown therein is another embodiment of the shielded magnetic reed switch 10 of the present invention. In FIG. 3, the tubular wall 32B is about 0.025 inches thick and the ferrous metal housing is about 0.151 inches thick.

The thickened end portion 34B is about 0.125 inches thick at its thickest point.

In FIGS. 1-3, the ferrous metal housing 30 is annealed to a full soft condition to maximize magnetic permeability.

In FIGS. 4 and 5, the operation of the shielded magnetic reed switch 10 (much enlarged) in the presence of a magnetic field associated with a permanent magnet M is illustrated. A distance D in FIGS. 4 and 5 defines the distance between the shielded magnetic reed switch 10 and the pole face P of permanent magnet M (about 3.5 to 5.

0 inches) at which closure of the magnetic reed switch 16 occurs.

Referring now to FIG. 4, the permanent magnet M is positioned out of range of the distance D at which closure of the magnetic reed switch 16 (see FIGS. 1-3) is desired. At distances exceeding D (i. e., distances greater than about 5 inches and within the 12-18 inches at which an unshielded magnetic reed switch 16 would normally close) the ferrous metal housing 30 (see FIGS. 1-3) absorbs the magnetic flux produced by the permanent magnet M and prevents the magnetic reed switch 16 from closing.

The magnetic reed switch 16 connected to a generic electrical circuit by electrical leads 18,20 remains open.

Referring now to FIG. 5, the permanent magnet M is positioned within the distance D at which the shielded magnetic reed switch 10 closes. The magnetic reed switch 16 connected to the generic electrical circuit by electrical leads 18,20 is now closed.

While the operation of the shielded magnetic reed switch 10 is illustrated in conjunction with a permanent magnet M, it will be understood by one skilled in the art that the shielded magnetic reed switch 10 of the present invention operates as illustrated with any magnetic field source (i. e., with either a permanent magnet or an electromagnet).

It will be understood by one skilled in the art that the ferrous metal housing 30 acts as a magnetic shield to prevent the reeds 12,14 from closing prematurely and, further, that the structure of the ferrous metal housing 30, including the thickness of the tubular wall 32,32A, 32B and the thickness of the thickened end portion 34,34A, 34B determine the distance D from the magnetic field source (e. g., the permanent magnet M) at which the shielded magnetic reed switch 10 closes. A relatively thinner tubular wall 32,32A, 32B will result in closure of the magnetic reed switch 16 at a relatively greater operating distance D. A relatively thinner thickened end portion 34,34A, 34B will also result in closure of the magnetic reed switch 16 at a relatively greater operating distance D. Selection of the magnetic reed switch 16 also affects the operating distance D. A magnetic reed switch 16 having relatively more flexible reeds results in closure of the magnetic reed switch 16 at a relatively greater operating distance D, whereas a magnetic reed switch 16 having relatively stiffer reeds effectively reduces the distance D at which the magnetic reed switch 16 closes.

It will be further understood by one skilled in the art that the present invention is for apparatus involving a magnetic reed switch disposed within a ferrous housing, so that the shielded magnetic reed switch actuates at a predetermined distance from an approaching magnetic field source. In the absence of the ferrous housing, the magnetic reed switch will actuate at a distance greater than the predetermined distance.

While applicants'invention is illustrated herein as being a normally open magnetic reed switch 16 disposed within a ferrous metal housing 30, it will be understood to one skilled in the art that reed switches can be either normally open or normally closed. A single-pole, single-throw (SPST), normally-open magnetic reed switch (also referred to by those skilled in the art as a Form"A"reed switch) is illustrated herein. Single-pole, single-throw (SPST), normally-closed magnetic reed switches (also referred to by those skilled in the art as Form"B"reed switches), single- pole, double-throw (SPDT), and break-before-make reed switches (also referred to by those skilled in the art as Form"C"reed switches) are known in the art and suitable for use in lieu of the magnetic reed switch 16 of FIGS. 1-3. Whether Form A, Form B, or Form C, each type of switch can be shielded in accordance with the present invention as taught herein.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The embodiments were chosen in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best use the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.