Patent Publication Number: US-2002003214-A1

Title: Filtered photocontroller

Description:
FIELD OF INVENTION  
       [0001] This invention relates to a filtered photocontroller with improved responsiveness for street lights and other electrical devices.  
       BACKGROUND OF INVENTION  
       [0002] Photocontrollers are devices that automatically turn electrical devices on and off in response to the ambient light level. They are used on street lights to automatically turn them off during the day and on at night. They are used on billboard lighting systems to turn the billboard lights on early at night, off late at night during periods of low vehicular traffic, on again during early morning rush hour periods when high traffic levels resume, and then off during the daylight hours. Photocontrollers are also used in reverse, for example, to turn a golf course water fountain on during the day and off at night.  
       [0003] Typical photocontrollers use photosensors as a means to detect the ambient light level. Two common photosensors are cadmium sulphide (CdS) cells and silicon junction devices (hereinafter “silicon sensors”).  
       [0004] Although the spectral response of CdS cells closely approximates the spectral response of a human eye, CdS cells tend to drift suffering an irreversible change toward lower sensitivities and thus higher control turn on and off points. This drift towards lower sensitivities is accelerated by overheating. CdS cells also deteriorate rapidly in areas of high humidity, salt spray, or acidic air pollution again causing a drift toward longer burning hours caused by an earlier turn on and later turn off times. Accordingly, CdS cells must be sealed to protect them resulting in higher cost photocontrollers. CdS cells also raise a potential disposal issue because of perceived cadmium hazards. Because of low initial cost and long history of use and human eye spectral response, CdS is the most widely used light sensor for photocontrols.  
       [0005] Silicon sensors are stable under extreme conditions and are fairly small and inexpensive. Thus, silicon sensors are preferred in some part of the photocontroller industry.  
       [0006] Silicon sensors, however, suffer from a serious shortcoming: they are extremely sensitive to infrared radiation and red light and yet insensitive to the blue and green portions of the light spectrum.  
       [0007] This limitation of silicon sensors causes day to day wandering of turn-on and turn-off times corresponding to variations in the red content of light at both sunset and sunrise. For example, a silicon sensor that turns on at one footcandle (10.8 lux) on a clear night may turn on at 3 or 4 footcandles (32.3-43.0 lux) the next night as the weather changes to cloudy. The same is true in reverse for turn off times.  
       [0008] In general, then, since the spectral response of silicon sensors does not match the spectral response of the human eye, a photocontroller with a silicon sensor will not always turn a street light on and off when required causing a potential vehicle and pedestrian safety traffic hazard on high infrared radiation nights and mornings. That is, on a morning with a high infrared content of radiation versus visible light, the photocontroller “sees” the infrared radiation at sunrise and turns the light off too early. On cloudy days, the clouds attenuate visible but attenuate the infrared radiation at a higher level. As a result, the lights are turned on too early wasting electricity. On nights that have a bright red sunset, the silicon sensor “sees” the high infrared radiation and turns the lights on too late. This causes a safety issue.  
       [0009] Heretofore, the only potential solution was the use of an infrared blocking optical filter made of glass to be placed in front of the silicon sensor to reduce its sensitivity to infrared radiation. This potential solution was never realized, however, because of the cost of glass filters. Typical photocontrollers are sold for about $5.00-$6.00. The cost of the glass filter alone was more than $10.00. In addition, previous glass filters, like KG-1 or BG-3 had to be 0.25″ or more thick to accomplish the desired filtering. The result was a heavy and difficult to mount filter. Therefore, to date, the problems associated with silicon sensor based photocontrollers have not been solved and the expensive glass filters are only used (and re-used) to calibrate the photocontrollers prior to shipment. Because of their high cost, glass filters are not integrated into the actual photocontrollers shipped to customers.  
       SUMMARY OF INVENTION  
       [0010] It is therefore an object of this invention to provide a photocontroller with an inexpensive filter which blocks infrared radiation.  
       [0011] It is a further object of this invention to provide such a filtered photocontroller that reduces potential vehicle and pedestrian safety traffic hazards on evenings and mornings with high infrared radiation and wasted energy on cloudy evenings and mornings.  
       [0012] It is a further object of this invention to provide such a filtered photocontroller which facilitates the use of low cost and yet stable silicon photosensors.  
       [0013] It is a further object of this invention to provide such a filtered photocontroller which does not unduly increase the cost of the photocontroller.  
       [0014] This invention results from the realization that a photocontroller which preferably incorporates a rugged and stable silicon sensor can be improved at a very low cost by placing a unique polymer filter in front of the silicon sensor to attenuate infrared radiation thereby improving the responsiveness of the photocontroller resulting in improved vehicular and pedestrian traffic safety since the responsiveness of the photocontroller now matches the response of the human eye to sunlight.  
       [0015] This invention features a filtered photocontroller comprising a housing including a window portion; a circuit board within the housing including a light sensor behind the window portion of the housing; and a polymer filter which attenuates infrared radiation in front of the light sensor for improving the responsiveness of the photocontroller.  
       [0016] The light sensor is typically a silicon photosensor. The housing includes a base with a pair of upstanding legs proximate the light sensor forming a channel for holding the polymer filter. Alternatively, the polymer filter is integral with the window portion of the housing. The window usually includes a sheet of acrylic material. The preferred housing is an ANSI C136.10 standard housing.  
       [0017] The polymer filter preferably transmits about 70% of radiation between about 510 nm and 570 nm and less than 20% of infrared radiation below about 450 nm and above 740 nm.  
       [0018] The polymer filter may be a thin sheet or a molded structure. One such molded structure includes one surface for receiving the light sensor and one surface in front of the light sensor. Another molded structure is in the form of a cap disposed over the light sensor. 
     
    
    
     DISCLOSURE OF PREFERRED EMBODIMENT  
     [0019] Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:  
     [0020]FIG. 1 is a schematic partially exploded view of the filtered photocontroller of the subject invention;  
     [0021]FIG. 2 is a graph showing the response of the human eye, the response of a typical CdS photocell, and the response of a typical silicon sensor to various radiation wavelengths;  
     [0022]FIG. 3 is a graph showing the relative sensitivity of a typical silicon photosensor based on certain radiation wavelengths;  
     [0023]FIG. 4 is a graph showing the relative sensitivity of the filtered silicon sensor in accordance with the subject invention as compared to an unfiltered silicon sensor;  
     [0024]FIG. 5 is a wiring diagram showing the primary components of the circuitry of the filtered photocontroller of this invention; and  
     [0025]FIGS. 6 and 7 are schematic views of molded filter configuration in accordance with this invention. 
    
    
     [0026] Filtered photocontroller  10 , FIG. 1, includes housing cap  12  and housing base  14  typically meeting the requirements of ANSI C136.10 standard. In cap  12  is window portion  16  typically made of a clear acrylic material. Another typical housing is the ANSI C136.24 housing. In foreign countries, typical housings made according to the BSI and JIS standards. In some cases, the window is the whole top section of the housing cap.  
     [0027] Circuit board  18  on housing base  14  and covered by housing cap  12  includes the circuitry required to turn the light on at night and off in the day. Included in this circuitry is silicon sensor  20  which is responsive to the ambient light level, and, unfortunately, as discussed in the Background of the Invention above, infrared radiation. The result is that on high infrared radiation mornings when the light should be turned off later than on a clear morning, sensor  20  still “sees” the infrared radiation at sunrise and turns the light off too early. On high infrared radiation evenings, sensor  20  still “sees” the radiation at sunset and turns the light on too late.  
     [0028] In this invention, however, rectangular polymer filter  22  in front of sensor  20  attenuates infrared radiation thereby improving the responsiveness of photocontroller  10  rendering it nearly the same as the responsiveness of the human eye so that photocontroller  10  turns the street light it is connected to on and off at the correct time independent of inelement weather conditions such as cloudy evenings or cloudy mornings or high infrared radiation sunrises or sunsets.  
     [0029] Base portion  14  of the housing preferably includes upstanding legs  24  and  26  forming a C-shaped channel for receiving a rectangular about 0.040 to 0.25″ thick by 1″ to 1¾″ by 1″ sheet of polymer material  22  which filters out infrared radiation. Polymer material  22  is available from Uniroyal Technology Corporation, Glasflex Division, Four Stirling Road, Stirling, N.J. 07980. This material, in a different form, was previously used in goggles to protect a person&#39;s eyes from harmful laser or other infrared radiation. For the subject invention, the previous material was modified by Uniroyal as specified by the inventors hereof to develop a material that met the specific infrared absorption characteristics shown in FIG. 4. The previously available Uniroyal material was modified to more closely match the spectral response of a typical KG-1 glass filter and thus to have a peak spectral response (&gt;70%) between 510 nm and 570 nm sloping off quickly so that the spectral response is less than 20% below 450 nm and above 740 nm.  
     [0030] Alternatively, filter  22  could be secured to cover the inside (or outside) of window portion  16  of cap  12  and thus integral with the photocontroller housing. Window portion  16  could include a sheet of acrylic and filter  22  be located behind this sheet of acrylic. Alternatively, the reverse could be true. Filter  22  could also be molded directly into window portion  16  or molded into a support structure placed just in front of or over the sensor.  
     [0031] Silicon sensor  20  unfortunately responds to infrared radiation as shown at  50 , FIGS. 2 and 3. The result is a potential vehicle and pedestrian safety traffic hazard on cloudy or high infrared radiation evenings and mornings.  
     [0032] CdS photocells respond more closely to the response of the human eye as shown at  51 , FIG. 2 but, unfortunately CdS photocells drift as discussed in the Background of the Invention above.  
     [0033] Filter  22 , FIG. 1, however, filters infrared radiation as shown at  60 , FIG. 4.  
     [0034] Thus, the combination of silicon sensor  20 , FIG. 1, and filter  22  enjoys a more accurate response as shown at  70 , FIG. 4. The result is a photocontroller which incorporates the rugged and stable silicon sensor and which has improved responsiveness at a very low cost due to placement of a polymer filter in front of the silicon sensor to attenuate infrared radiation. The result is improved vehicle and pedestrian traffic safety since the responsiveness of the photocontroller now matches the responsiveness of the human eye to sunlight. Filter  22  transmits less than 20% of radiation at or below about 450 nm and above 740 nm as shown in FIG. 4. Filter  22  has a peak transmitting 70% of radiation at about 540 mn.  
     [0035] Photocontroller  10 , FIG. 1 thus offers a serious advantage over non-filtered photocontrollers which do not match the spectral response of the human eye and thus will not always turn a street light on and off when required causing a potential vehicular and pedestrian traffic safety hazard on cloudy or high infrared radiation evenings and cloudy mornings. Photocontroller  10 , FIG. 1 also offers a serious cost advantage over any attempt to use glass infrared blocking optical filters which would prohibitively increase the cost of the photocontroller.  
     [0036] A photocontroller with a heavy glass filter would cost approximately $15-$16 and the glass filter alone costs about $10. The photocontroller of this invention with the polymer filter costs about $5-$6 and the polymer filter alone costs about $0.10-$0.30. Thus, if 750,000 photocontrollers are sold in a one year period, the cost savings of the subject invention is approximately $7,500,000. In general, the cost of the silicon sensor combined with the cost of the filter is approximately $0.15-$0.35. This cost is similar to the cost of a CdS cell which suffers from the degradation problems discussed in the Background of the Invention above. Thus, the subject invention results in an infrared insensitive photocontroller incorporating a silicon photocell which does not drift or suffer from degradation at approximately the same cost as an infrared insensitive photocontroller with a CdS photocell which does drift. At the same time, the weight of the polymer filter is significantly less than a glass filter and is easier to mount and adhere within the photocontroller than a glass filter. In addition, the polymer filter can be molded into specific shapes easier than a glass filter as discussed inra.  
     [0037] Typical additional components of circuit board  18 , FIG. 1, are shown in the block diagram of FIG. 5. MOV1  71  is used for surge suppression caused primarily by lightning. Resistor  72  is a current limiting resistor for the entire controller. Diodes  74 ,  76 ,  78  and  80  constitute bridge rectifier  81 . Bridge rectifier  81  rectifies the AC line voltage to DC for use by switching circuit  83  and trigger circuit  86 . Switching circuit  83  includes resistor  82  which is a current limiting resistor for relay  84 . Relay  84  is used to switch the power to the lamp. Diode  87  is used as a backswing diode whose purpose is to suppress the voltage spikes caused by the collapse of the magnetic field in the coil of relay  84 . Relay  84  is controlled by trigger circuit  86 . Trigger circuit  86  includes resistor  88  which is the current limiting resistor for the trigger circuit. Resistor  88  and capacitor  90  are used to filter out any AC ripple. Zener diode  92  is used as a voltage regulator and secondary surge suppressor. Resistors  94  and  96  are in parallel and are used to calibrate the light level that causes the trigger circuit to be activated. The amount of light that sensor  20  is exposed to determines the amount of current that flows through resistors  94  and  96 . As the light increases, more current is allowed to flow through resistors  94  and  96 . This causes the voltage drop across the resistors to increase and for the voltage drop across sensor  20  to decrease. When it gets dark, the opposite is true resulting in the voltage level across sensor  20  increasing.  
     [0038] Diode  100 , capacitor  102 , resistor  104 , and resistor  106  make up a time delay feature so that the control does not turn off the lamp in response to extraneous light (e.g. lightning). Capacitor  102  and resistor  104  make up the time constant of the time delay. Block diode  100  prevents the time constant from discharging though sensor  20 . Resistor  106  and transistor  110  are used to drive Schmitt trigger circuit  122 . Diode  100  also makes time delay unilateral. Capacitor  112 , resistor  114 , resistor  116 , and transistors  118  and  120  make up Schmitt trigger  122 . These components could also be implemented with any other device containing a Schmitt trigger such as an integrated circuit.  
     [0039] Filter  22 , however, could be used in conjunction with other photocontroller circuit designs including those sold by Dark to Light, 590 Washington Street, Pembroke, Mass. 02359, and other manufacturers. See also U.S. Pat. No. 5,195,016 incorporated herein by this reference.  
     [0040] As discussed above, a heavy and relatively thick glass infrared filter is difficult to mount within a standard photocontroller housing and is not easily molded. As shown in FIG. 1, polymer filter  22  is relatively thin and small and thus easily mounted within photocontroller  10 . In addition, the polymer filter material of this invention can be molded resulting in the filter structures shown in FIGS. 6 and 7.  
     [0041] In FIG. 6, filter  22 ″ includes surface  130  with orifice  132  for receiving the base of a standard silicon sensor therethrough, and leg  134  is secured directly to circuit board  18 , FIG. 1. Leg  136  includes surface  138  which is then disposed in front of the silicon sensor.  
     [0042] In FIG. 7, filter  22 ″ is in the form of a cap which is disposed over the silicon sensor and then mounted to circuit board  18 , FIG. 1. Other possible molded configurations are possible.  
     [0043] Therefore, although specific features of this invention are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. And, other embodiments will occur to those skilled in the art and are within the following claims: