BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to air filtration systems, and, more particularly, to an air filtration system that contains an alarm for notifying the user that the air filter in the system has become clogged and dirty.  
         [0002]     Heat and ventilation systems, commonly known as HVAC systems, are equipped with air permeable filters that are placed in a special opening connected to a return air duct. Conventional air filters comprise a grid of plastic or metal, within which a porous, air-permeable membrane is secured. With time, the filter becomes clogged with dust or dirt, which causes reduction in the airflow on the downstream side of the filter. Sometimes the heat and air system is malfunctioning due to ice formation on the intake which can be caused by the differences in the humidity level between the inside of the house and outside of the building. The dirt, icing, and other such airflow reducing factors cause the fan unit to overload and overheat, which can ultimately lead to the destruction of the motor. The replacement of the motor can be very expensive and as a result, the building owners are encouraged to regularly replace or clean the air filters and to check the intake of the air conditioning system for ice and dirt accumulation.  
         [0003]     However, visual inspection is tedious and time consuming, especially if the technician has to check multiple filters in an apartment building or an office building. The disadvantage of the visual inspection is that the technician or the homeowner may not properly detect the degree of clogging in the filter, thereby making it difficult to determine whether the airflow that passes through the filter is of the right amount or is greatly reduced due to clogging.  
         [0004]     The present invention contemplates elimination of drawbacks associated with detection of a clogged filter and provision of a simple and inexpensive device that can be incorporated into the existing air filtration systems or supplied as a kit as part of the HVAC system.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the present invention to provide a warning device for a homeowner or a service technician that the air filter needs replacement or cleaning.  
         [0006]     It is a further object of the present invention to provide a visual means for indicating the degree of clogging of the air filter.  
         [0007]     Still another object of the present invention is to provide an audible warning device for signaling when the airflow across the filter falls below a pre-determined threshold value.  
         [0008]     These and other objects of the present invention are achieved through a provision of a warning system for use with furnace or air conditioning systems. The warning device comprises a sensor mounted on the downstream side of the filter, which reacts to the reduction in the air flow across the filter. The sensor is operationally connected to a generator of electrical power, or motor means for generating electrical current when the sensor is forced to move by the air current.  
         [0009]     The electrical power generator is operationally connected to one or more light emitting members mounted on the upstream side of the air filter in a location, where the light emitting member(s) can be seen by a user. In a system, which employs a plurality of light emitting members, a relay is mounted between the motor and the light emitting members for directing a pre-determined electrical potential to different light emitting members. The light emitting members may be provided with different color lenses, which are illuminated depending on the degree of air flow acting on the sensor. For instance, when the airflow has a pre-determined value indicative of a clean air filter, the green light emitting member may be illuminated. When the air flow is reduced to a non-acceptable level and the sensor stops moving, a red light emitting member may be illuminated.  
         [0010]     The system further contemplates he use of an audible signal corresponding to the unacceptable air flow across the filter, as well as a means for setting the value of air flow considered acceptable during heating or cooling seasons. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Reference will now be made to the drawings, wherein like parts are designated by like numerals and wherein  FIG. 1  is a schematic view of the warning system for dirty air filters in accordance with the present invention.  
         [0012]      FIG. 2  is a schematic view of an alternative system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     Turning now to the drawing in more detail, numeral  10  designates a return air filter mounted in an opening leading to an air passage  12 . The filter  10  has an upstream side  11  and a downstream side  13 . The return airflow, schematically illustrated by arrows  14 , flows from the interior of the room through the air filter  10  and is delivered into the air flow passage  12  on the opposite, downstream side  13  of the filter  10 . With time the air permeable material forming the air filter becomes clogged with dust and other dirt particles sucked in from the upstream side  11 . As a result, the amount of air delivered to the downstream side  13  is substantially reduced. To facilitate detection of the degree of contamination of the air filter  10 , the present invention provides for a system of warnings incorporated into the HVAC system.  
         [0014]     The warning device of the present invention comprises a sensor  20 , which can be for instance a fan  20  having rotating blades  24 . The sensor  20  is operationally connected to an electric current generator  22 , which can be a low voltage motor means. The low voltage motor  22  generates electricity as the blades  24  of the fan  20  rotate in response to the air flow passing through the passage  12  into the air duct  18 .  
         [0015]     The sensor  20  and the generator  22  are conveniently mounted on a bracket  26  which is secured to the wall of the air duct  18  in the direct path of the air flow. The generator  22  is electrically connected to a control panel by a plurality of electrical wires. Some of the wires, shown in phantom lines designated by numeral  32 , are connected to one or more of visual warning means, such as for instance light emitting members  34 . The light emitting members  34  may be light emitting diodes (LEDs) mounted on the control panel  30 .  
         [0016]     Another electrical wire  36  lead to an audible warning indicator  38  mounted on the control panel  30 . The control panel  30  may be mounted in close proximity to a thermostat (not shown) to allow visual and audible indication of the amount of airflow passing across the filter  10 .  
         [0017]     The LED lights  34  are preferably provided with different color lenses to allow a more precise determination of the degree of clogging of the filter  10 . For instance, one of the lights  34  may have a green color lens, which would be indicative of an acceptable level of airflow across the filter  10 . The green colored LED is designed to be illuminated when the fan  20  operates at its full capacity and generates the largest amount of electricity, for instance 1 millivolt.  
         [0018]     Another LED light  34  may have a yellow lens; this LED is designed to be illuminated when the generator  22  generates less electricity than is sufficient to illuminate the green LED. Another LED light  34  may have an orange lens which will connect to the generator  22  when the electrical current is insufficient for illuminating the green or yellow lights. Finally, a red light  35  will be illuminated when the fan  20  does not rotate due to clogging of the filter and when the motor means  22  fails to generate sufficient electricity to power even the green, yellow or orange light. A backup battery (not shown) is incorporated in the control panel  30  to supply electricity to the light  35  giving a visual indication that the air filter  10  needs urgent replacement.  
         [0019]     The audible warning device  38 , which can be a buzzer or a whistle is likewise connected to the backup battery and sounds an audible signal whenever the supplied electricity is insufficient to illuminate the green, yellow, or orange light emitting devices. It is envisioned that the warning light  35  and the audible warning device  38  will operate contemporaneously giving both visual and audible signals to the homeowner or a technician that no power is coming from the generator  22  and the filter  10  needs to be replaced as soon as possible.  
         [0020]     A relay switch  40  is mounted between the generator  22  and the control panel  30  to properly control the operation of the warning devices mounted on the control panel  30  and to correctly direct the electrical current to the designated light emitting member  34 ,  35  or to the audible warning member  38 . A limit switch  42  is incorporated in the control panel  30  to allow switching between summer and winter modes of operation when the amount of air flow depends on whether a heating or a cooling system operates in a given time. The limit switch  42  allows the user to switch the system to recognize cooling or heating modes of the HVAC and thereby adjust the limits at which the warning system operates.  
         [0021]     The present invention can also help determine whether the AC motor malfunctions by showing that no airflow is passing through the filter  10 . To prevent total damage to the AC motor, the warning devices mounted on the control panel  30  would give an advanced warning of some malfunction in the HVAC system.  
         [0022]      FIG. 2  illustrates an alternative schematic of a clogged filter warning system. As can be seen in the drawing, the sensor  20  and the electrical current generator  22  are similar to the first embodiment. The control panel is somewhat different. In the control panel  50 , a series of light emitting members  52  is spaced in a sequential row, with an indicia under the light members “good,” “change filter,” and “poor.” The light members  52  have different color lenses, changing in hue from green to yellow, from yellow to red, where the light member  54  has a red color lens.  
         [0023]     The limit switch  56  may be formed as a separate device, electrically connected to the control panel  50  and to the sensor  20  through the generator  22  and suitable wiring  57 . Alternatively, the switch  56  may be mounted on the control panel  50 . The on/off limit switch  56  is also connected to by suitable wiring  59  to a wall thermostat  60 . Conventionally, the thermostat  60  is connected by wiring  61  to the HVAC unit (not shown) and to a transformer (not shown)—through suitable wiring  63 .  
         [0024]     Many changes and modifications can be made in the system of the present invention without departing from the spirit thereof. I therefore pray that my rights to the present invention be limited only by the scope of the appended claims.
A warning for identifying clogged air filter condition uses a sensor mounted in the air passage downstream of the air filter. The sensor is connected to an electrical power generator, which transmits the electrical power to a selected one of the light emitting or audio signal emitting members in response to the decrease in the air flow passing across the air filter.