Abstract:
An apparatus selectively connectable to the prongs of an electrical plug that provides an indication to a person of the presence of power to the prongs.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     None. 
     BACKGROUND OF THE INVENTION 
     The subject matter of this application relates to a device that selectively attaches to the prongs of an electrical plug and provides a visual indication of a voltage across the prongs. 
     Some modern electrical extension cords often include indicator lights indicating the presence of power in the extension cord. For example, U.S. Pat. No. 4,671,597 to Grill discloses an extension cord having a female receptacle into which the prongs of an electrical plug may be inserted. The female receptacle includes a lamp connected to respective conductors wired to the hot and neutral prongs of a male plug at the opposite end of the extension cord, so that the lamp is illuminated when the extension cord is plugged into an outlet. These same conductors are positioned so that they contact the hot and neutral prongs of any plug inserted into the extension cord. The reason for the indicator lamp of Grill is that extension cords often extend around walls or other obstacles that prevent a person from knowing whether the extension cord is plugged into an outlet at the time it is being used. The indicator lamp provides this indication. 
     Also, many existing power strips have indicator lights indicating the presence of a voltage potential at any (or all) of the sockets provided by the power strip. For example, some power strips include a switch that alternatingly provides voltage to all the sockets in the power strip or disables voltage to all the sockets in the power strip; a light indicates which of these two states the switch is set to. Still other power cords include such switches and/or lights to each of the sockets in the power strip. Again, as above with respect to the extension cord of Grill, the indicator lights provide an easy indication of the presence of power to the power strip&#39;s sockets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings, in which: 
         FIG. 1  shows a wafer having slots into which the prongs an electrical plug may be selectively inserted, and a light indicating the presence of a voltage across the prongs. 
         FIG. 2  shows a first embodiment of circuitry by which voltage across the prongs of the plug of  FIG. 1  illuminates an LED light. 
         FIG. 3  shows a second embodiment of circuitry by which voltage across the prongs of the plug of  FIG. 1  illuminates a neon light. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, many existing extension cords and power strips include indicator lights that convey information about whether power is available in a socket that a plug may be, or is, inserted into. Thus, the absence of a light may indicate to a user that a plug of an extension cord needs to be inserted into a socket, or a switch of a power cord needs to be depressed. The present inventor noted, however, that such lights provide the additional functionality of informing a user that power is, or is not, being delivered to a device already plugged into a receptacle having such a light. Thus, when a device is plugged into a socket but is not working, a user may quickly determine whether the fault lies with the device, or with inadequate power being provided to the device. 
     However, a great number of appliances or devices are routinely plugged directly into a socket, as opposed to receiving power from a socket indirectly through an extension cord or a power strip. Given the impracticality of using an extension cord or a power strip for every device receiving power from a wall outlet, the present inventor therefore realized the need for an indicator light associated with a male plug of an appliance, rather than a female receptacle of an outlet, extension cord, or power strip. The present inventor also realized the benefit of a device that could be retrofitted on existing male electrical plugs. 
       FIG. 1  shows an exemplary wafer  10  having a plurality of apertures  12   a  and  12   b  sized and arranged to receive the prongs  14  of a plug  16 . In  FIG. 1 , apertures  12   a  are positioned and sized to receive the hot and neutral prongs of the plug  16 , i.e. the prongs through which current flows when the plug is inserted into an outlet. Conversely, aperture  12   b  is sized and positioned to receive the ground prong of the plug  16 . Though  FIG. 1  shows a 3-prong plug ubiquitously used in the United States to be inserted into a standard 120V receptacle having positive, neutral, and ground terminals, those of ordinary skill in the art will appreciate that other embodiments of the wafer  10  may have apertures sized and arranged to receive prongs of different types of plugs used in the United States or other countries, e.g. NEMA 14-50 plugs, a CEE 7/4 plug used in Germany, etc. 
     Preferably, the wafer  10  is sufficiently thin so that the prongs  14  of plug  16  may be inserted into the apertures  12  such that they extend through the wafer  10  a sufficient distance so that the plug  16  may be inserted stably into an electrical outlet (not shown) and receive power from that outlet. Preferably, the wafer  10  is less than ⅛ inch thick and more preferably is less than 1/16 inch thick. In this manner, the wafer  10  may fit snugly between a rubber base  17  of the plug  16  and an outlet into which the prongs of the plug  16  are inserted. 
     The wafer  10  also includes a light  18  that illuminates when a plug  16 , having prongs  14  inserted in the wafer, receives electrical power. That is to say, if the plug  16  is inserted through the wafer  10  and into an electrical outlet, the light  18  will illuminate if the electrical outlet delivers power to the plug  16 , as explained more fully below. Preferably, the light  18  is positioned so that it is easily visible to a person while the plug  16  is inserted in an outlet. For example, as shown in  FIG. 1 , the wafer  10  may be sized so that it has a height greater than that of an inserted plug, with the light positioned above the plug to be easily seen when illuminated. In other embodiments, the light  18  may be positioned to the side of a plug inserted into the wafer  10 , or any other appropriate, visible location. 
     In a preferred embodiment, the wafer  10  is made of a flexible material, such as a soft plastic, so that the wafer may bend when needed. For example, 120 volt electrical outlets commonly used in the United States are arranged in stacked pairs within a receptacle. Where, as shown in  FIG. 1 , the wafer  10  has a height greater than that of the plug, and is inserted between a plug and a socket, it may be desirable to bend the wafer  10  out of the way so that a second plug may be inserted in the other socket in the receptacle. 
     The exemplary wafer  10  is preferably fabricated using a very inexpensive procedure that requires neither precise tracing of an electrical path on a substrate between the apertures  12   a  and the light  18 , nor bulky electrical contact pads positioned around the periphery of the apertures  12   a . Preferably, the wafer  10  is made of two plies  20  and  22  of flexible plastic connected to each other by, e.g. fasteners  24  and/or  26 . Between the plies  20  and  22  is inserted conductive fabric material  28  positioned to electrically connect each aperture  12   a  to a respective electrical contact  38  (shown in  FIGS. 2 and 3 ), which are in turn electrically connected to the light  18 . Conductive fabric material  28 , such as 3M™ Fabric Tape CN-3190, may preferably comprise a flexible Nickel/Copper-plated backing material with a conductive acrylic adhesive layer. Such conductive fabric material is ordinarily used for electromagnetic shielding of circuitry, i.e. to block spurious electromagnetic fields that might otherwise interfere with sensitive electronic components shielded by the fabric. The present inventor realized that the high conductivity of such fabric requisite for shielding applications, provides an inexpensive alternative to either tracing an electrical path from an aperture  12   a  to the light  18 , and/or providing bulky and precisely positioned contact pads around the periphery of the apertures  12   a.    
     Specifically, by simply placing conductive fabric  28  between the plies  20  and  22  of the wafer  10  at respective locations that extend between each aperture  12   a  and an associated contact  38  of the light  18 , an electrical connection will be made that will complete a circuit from the hot prong of a plug  14  inserted in the wafer  10 , to the light  18 , and back to the neutral prong of the plug  14 , as can easily be seen in  FIG. 1 . In some embodiments, the conductive fabric  28  may form a barrier in the apertures  12   a  that can be punctured by prongs  14  when the wafer  10  is first used, thus ensuring an electrical connection between the plug  14  and the fabric  28 . In other embodiments, the fabric may be pre-cut in a manner that ensures such an electrical connection. In some embodiments, fasteners  26  such as screws, rivets, or pins, help ensure an electrical connection between the fabric  28  and the contacts  38 . 
     It should be understood that the strips of conductive fabric material  28  are preferably electrically isolated from each other. This prevents a short in an electrical circuit that begins from the hot prong of a plug  16 , through a first strip of conductive fabric to a positive terminal of a lamp circuit  30 , and ends at the neutral prong of the plug  16  connected to the negative terminal of the lamp circuit  30  by a second strip of conductive material. 
       FIG. 2  shows one embodiment of a lamp circuit  30  that may be used in the wafer  10 . The lamp circuit  30  may include two contacts  38  that are provided with a voltage potential across them when a plug, having the wafer  10  over its prongs, is inserted into a socket having power. Any such voltage potential across the contacts  38  will therefore propagate electric current through an LED  32 , which then illuminates. A current-limiting resistor  34  preferably includes a resistance sufficient to limit the current to the LED  32  to a range that will not damage the LED  32 . Those of ordinary skill in the art will appreciate that the resistance value of the resistor  34  will depend both on the voltage of the outlet the wafer  10  is intended to be used with, as well as the power rating of the LED  32 . For example, if the voltage supplied by an outlet is 120 volts and the power rating of the LED 0.5 watts, the resistance of the resistor  34  may be 30 kOhms. 
     The lamp circuit  30  may also preferably include a diode  36  that limits current to flow in only the direction from the positive to the negative side of the LED  32 . Diode  36  prevents current from flowing in the reverse direction through the LED  32 , which might otherwise damage the LED  32 . The frequency of AC current is high enough that fluctuations in the light emitted by the LED  32  will not be noticeable. Those of ordinary skill in the art will appreciate that other mechanisms may be used to prevent reverse current through the LED  32 , such as a rectifier 
       FIG. 3  shows an alternate lamp circuit  30  that includes a neon light  37  instead of an LED  32 . Because the neon light  37  may operate directly on AC current, a diode  36  (or rectifier, etc.) is not needed. Those of ordinary skill in the art will appreciate that other light sources (filament bulbs, etc.) may also be used. 
     It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.