Abstract:
A light emitting diode (LED) signal with an LED light output monitoring sensor. If the LED light output drops below a pre-set level, an output signal of the sensor via a comparator and deactivation circuit, deactivates one or more LEDs creating a visible fault display aspect, thereby visually indicating signal failure but allowing a continued use of the signal prior to replacement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to signals, in particular, Light Emitting Diode (LED) Signals. More specifically, the present invention relates to an LED traffic signal having a LED light output degradation sensing circuit that activates a visible fault mode. 
     2. Description of the Related Art 
     LED traffic signals present numerous advantages over common incandescent lamp traffic signals. Use of LED&#39;s provides a power consumption savings and extremely long life in comparison to common incandescent light sources. The long life span creates improved reliability and sharply lowered maintenance costs. 
     LED signals have an extremely long service life that has increased with each new generation of LEDs. Incandescent lamps, while having a much shorter service life, have relatively constant light output until a total failure occurs, i.e. burnout of the light filament. LED signals, over an extended period, have gradually diminishing light output. Further, LED light output is negatively affected by temperature. In extreme climate or during unnaturally warm periods LED light output diminishes during the day and then returns to a normal level during cooler periods at night. 
     Because of the difficulty, time and expense of accurately determining when an LED signal has permanently dropped below the acceptable light output limit, it is customary for consumers to automatically replace LED signals upon expiration of the warranty (for example, five years). This may result in years of useful service life being unnecessarily wasted, reducing the cost effectiveness of using LED signals. 
     U.S. patent application Ser. No. 09/543,240, assigned to Gelcore LLC, incorporated herein by reference, discloses monitoring circuits for an LED signal that shut off the signal if the power supply or LED arrays change their voltage and/or current characteristics. Unfortunately, LED light output may degrade without a change in the LED signal&#39;s voltage or current characteristics. 
     U.S. patent application Ser. No. 09/827,429, assigned to Gelcore LLC, filed Apr. 6, 2001, incorporated herein by reference, discloses light output monitoring circuits for an LED signal which disables the signal when a low light output threshold has been reached. Unfortunately, disabling an LED signal, for example a traffic signal, can have an extremely disruptive effect that will last until materials and personnel become available and the failed signal is replaced. 
     Therefore, the present invention has the objective of providing a cost effective and automatic means for detecting when an LED signal&#39;s light output has fallen below an acceptable level, and then initiating a visible failure mode while still continuing to operate. 
     Further objects will be realized by one skilled in the art, through review of the following description and appended claims. 
     SUMMARY OF THE INVENTION 
     A light sensor, mounted within an LED signal housing, senses a light output level. When the light level falls below a preset level, a short circuit is created that disables a selected group of LEDs. The disabled LEDs creating an altered display aspect that alerts users that replacement is necessary, but permits signal operation until replacement occurs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of an LED signal. 
     FIG. 2 is an electrical schematic of one embodiment of LED light degradation sensor circuitry. 
     FIG. 3 is an example of an LED signal display aspect in visible failure mode. 
     FIG. 4 is another example of an LED signal display aspect in visible failure mode. 
     FIG. 5 is another example of an LED signal display aspect in visible failure mode. 
     FIG. 6 is an example of an LED turn signal display aspect in visible failure mode. 
     FIG. 7 is an isometric view of a light sensor baffle. 
     FIG. 8 is an isometric view of another embodiment of a light sensor baffle. 
     FIG. 9 is an isometric view of another embodiment of a light sensor baffle. 
     FIG. 10 is an isometric view of another embodiment of a light sensor baffle. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an exploded view of an energy efficient LED signal  1  designed to retrofit into an existing signal housing originally designed for an incandescent light source. The LED signal  1  has a plurality of LED(s)  21  mounted in the signal on a PCB  28  which also carries the electrical components comprising the signal&#39;s power supply and visual failure mode circuitry. 
     A light sensor PH 1  is mounted in the LED signal  1  on the PCB  28 . The light sensor PH 1  may be, for example, a photo diode, a photo transistor, a photo cell or other device capable of outputting a signal with respect to the light level sensed. Light sensor PH 1  is an input for a comparator circuit which, for example, may compare the input to a reference voltage. If the input does not exceed the preset level, a disabling/deactivating function is initiated, for example short circuit across a selected group of LEDs, causing the signal display aspect to change, indicating failure of the signal but still allowing the signal&#39;s operation until it is replaced. Where the light sensor PH 1  is a photo transistor, a common voltage comparator circuit may be used. The reference voltage, set by selection of a resistor in the voltage comparator circuit, determines the light level at which the LED short circuit will be initiated. The short circuit may be created by, for example, a MosFET switch which closes a transistor. 
     One embodiment of the electrical circuitry is shown in FIG.  2 . Light sensed by a photo transistor PH 1  creates a proportional current output which, transformed by resistor R 18  and filtered by capacitor C 11  is seen as a voltage level input passed through current limiting resistor R 20  to the REF pin of comparator integrated circuit U 1 , for example a TL431 adjustable precision shunt regulator. The selected value of resistor R 18  sets the voltage level proportional to the desired light level which the comparator circuit U 1  will compare to its internal reference voltage. As the LED light output degrades over time, PH 1  senses less and less light, lowering its output. When the voltage at the U 1  Ref Pin falls below the U 1  internal voltage, U 1  opens the short circuit between pins A and C causing the Q 6  gate to go high, closing MosFET Q 6 . In normal operation, LED light in the housing will be sensed by PH 1  creating an output high enough so that the voltage at the U 1  Ref pin is higher than the U 1  internal reference voltage. As long as the U 1  Ref Pin is at a higher voltage than the U 1  internal reference voltage, U 1  pins A and C will be shorted causing the Q 6  gate to be grounded, which in turn maintains Q 6  in a blocked state. An RC network comprising resistor R 19  and capacitor C 12  provides a transient suppression effect to prevent a false energized state in the Q 6  gate from momentarily occurring and falsely causing a disabling short circuit. 
     When Q 6  is closed Q 7 , a PNP transistor for example type MPSA56, closes, creating a short circuit across a selected group of LEDs  21  (D 3 -D 5 , D 9 -D 11 ) establishing a bias current into Q 7 . The shorted LEDs are selected to cause a visible change in the display aspect. For example as shown in FIGS. 3-5, the center polygon(s) are darkened areas of the LED signals display aspect, thereby visually indicating the failure mode of the LED signal. FIG. 6 shows a directional arrow signal display aspect, interrupted in the middle to indicate a failure mode. 
     To ensure that the light sensor PH 1  is reading the aggregate light output level of the LED signal and not just the output of the closest LED(s)  21  a baffle, shroud or blinder  27  as shown in FIGS. 7-10 may be used so that the light sensed is a reflection off of the optical elements and/or the housing  2  side walls and not dependent just on the output of the closest LED(s)  21 . Alternately, a light sensor PH 1  with a high open/large viewing angle may be used. 
     The present invention may be used with any form of signals, displays, illumination or control circuits utilizing LED&#39;s as a light source that requires monitoring for light output degradation, the degradation indicated by a visual failure mode. 
     When used with an LED signal with the LEDs close to the outer cover, it may not be possible to locate the light sensor PH 1  where it can sense the aggregate light output of all the LEDs. In this case, the light sensor PH 1  may be located between a representative group of the LEDs and the reference voltage adjusted accordingly. 
     The light sensor is in operation whenever the LED signal is energized. During daylight use, external light levels may influence the light sensor PH 1  into a false reading that LED  1  output levels are normal even though they have in actuality degraded below the acceptable level. This is not a problem as the degradation in output levels occurs over a period of years. As the cut-off level approaches, a difference of an additional 12 hours (for night time or other transient interruption of the external light to occur) is immaterial. A capacitor, resistor combination or other timed delay can be used to create a known delay period during which the input must be below the reference level or the circuit will reset and be forced to pass through the entire delay period again before triggering the visual failure mode. This feature prevents line voltage transients or extreme heat that may temporarily lower light output or create a false output at the MosFET Q 6  from activating the visual failure mode. 
     Further, although particular components and materials are specifically identified herein, one skilled in the art may readily substitute components and/or materials of similar function without departing from the invention as defined in the appended claims. 
     The present invention is entitled to a range of equivalents, and is to be limited only by the following claims.