Abstract:
A photocontrol includes a switch that consumes very low power when the switch is turned ON and when it is turned OFF. The photocontrol can provide low-power digital control signals to high-impedance inputs of control devices that control the delivery of power to load devices, such as LED drivers that control the delivery of power to arrays of LEDs. The photocontrol also can provide power signals to control devices that control the delivery of power to light sources, such as electronic transformers that control the delivery of power to fluorescent lamps. The photocontrol may include a comparator that causes the photocontrol to have switching hysteresis.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to the field of photocontrols and, more particularly, low-power photocontrols used with luminaires. 
     2. Description of the Related Art 
     A photocontrol is a device that switches or controls electrical loads based on ambient light levels. As an example, a photocontrol can be used as a switch that provides electrical power to a luminaire only when detected light levels are below a desired level. Photocontrols used for such luminaires may include photosensors that are electrically and operably coupled to switching devices rated for use at relatively high line voltages (e.g., 90 VAC to 600 VAC) and at relatively high currents (e.g., amperes and higher). For example, a photocontrol for a luminaire may include a photosensor that controls an electro-mechanical relay coupled between a source of electrical power and a control device (e.g., a magnetic or electronic transformer) within the luminaire. The electro-mechanical relay may be configured to be in an electrically continuous state unless a signal from the photosensor is present to supply power to the luminaire. If the photosensor is illuminated with a sufficient amount of light, the photosensor outputs the signal that causes the electro-mechanical relay to switch to an electrically discontinuous state such that no power is supplied to the luminaire. 
     Conventional photocontrols used with luminaires suffer from a number of drawbacks. For example, such photocontrols may include small power sources that use “capacitive drop” technology to power a circuit of discrete transistors, integrated circuit operational amplifiers, or comparators. Conventional photocontrols using such technology can consume considerable amounts of power when the luminaire is ON and when the luminaire is OFF. 
     Additionally, a typical electro-mechanical relay used with a photocontrol for a luminaire has a relatively short life span. For example, electro-mechanical relays of conventional photocontrols used with luminaries may be rated to have only 5000 contactor closures with standard loads. Arching caused by high capacitive in-rush currents of electronically ballasted luminaires and inductive “kick back” of magnetically ballasted luminaires can corrode the contactors of the electro-mechanical relays. Additionally, the contactors may include silver or other metal alloys upon which oxides and sulfides may form during normal operation. At line voltage and current, such oxides and sulfides may present a negligible resistance to the passage of current through the contactors. However, at relatively low voltages (e.g., 2V to 24V) and relatively low currents (e.g., microamps) such as those used for digital logic level signaling, the impedance presented by contaminants including oxide and sulfide accumulations can hinder or even prevent the transmission of current through the contactors. Thus, conventional photocontrols for luminaires can be unsuitable for use in applications where the switching of relatively low voltage and relatively low current signals is required, for example, with luminaires that include solid-state light source drivers, for example, light emitting diode (LED) drivers that receive control signals for dimming LED arrays. 
     In response to the increasing emphasis placed on energy efficiency, many luminaires are being retrofitted with more energy efficient light sources. For example, conventional light sources (e.g., incandescent lights) are being replaced with solid-state light sources (e.g., LED arrays). Circuitry that regulates electrical power supplied to such solid-state light source (e.g., LED drivers) may draw relatively high inrush currents when the light sources are switched on. The inrush currents of electrically ballasted light sources may cause more damage to the contactors of electro-mechanical relays than is caused by the kickback currents of magnetically ballasted light sources. Accordingly, when conventional photocontrols having electro-mechanical relays are used with luminaires having solid-state light sources, the electro-mechanical relays may fail or cease to function reliably well before their rated number of contactor closures. 
     There is therefore a need for photocontrols that consume very small amounts of power. Additionally, there is a need for photocontrols that can be used reliably over long periods of time with luminaires having solid-state light sources. 
     BRIEF SUMMARY 
     A photocontrol apparatus to provide a plurality of control signals to a high-impedance controller input used to control the delivery of power to a load device may be summarized as including: a switch including a first node, a second node, and a third node, the first node electrically, communicably, coupled to a source of electrical power and the third node electrically, communicably coupled to the high-impedance controller input; and a photosensor electrically, communicably, coupled between the second node and the third node, the photosensor operable to at least partially cause a voltage level of the second node with respect to the third node to change when the photosensor outputs current in response to being at least partially illuminated with light, wherein when the voltage level of the second node with respect to the third node is greater than a threshold voltage level, the third node outputs a first control signal, and when the voltage level of the second node with respect to the third node is less than the threshold voltage level, the third node outputs a second control signal different from the first control signal. 
     The photosensor may be electrically coupled to at least partially cause the voltage level of the second node with respect to the third node to decrease when the photosensor outputs current in response to being at least partially illuminated with light. The photocontrol apparatus may further include a resistive device electrically, communicably, coupled between the second node and the third node, the resistive device being operable to at least partially cause the voltage level of the second node with respect to the third node to change when the photosensor outputs current in response to being at least partially illuminated with light. The resistive device may be electrically in parallel with the photosensor. The resistance of the resistive device may be adjustable. The resistive device may be a potentiometer. The photocontrol apparatus may further include a housing at least partially enclosing the switch and the photosensor, the housing may include a translucent portion and a shutter coupled to the housing, the shutter being moveable to selectively block and unblock at least part of the ambient light passing through the translucent portion. The switch may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), wherein the first node is a drain node of the MOSFET, the second node is a gate node of the MOSFET, and the third node is a source node of the MOSFET. The switch may be an n-channel depletion mode MOSFET. The switch may be a p-channel enhancement mode MOSFET. The switch may be a low RDS(on) MOSFET. A cathode of the photosensor may be electrically, communicably, coupled to the second node and an anode of the photosensor may be electrically, communicably, coupled to the third node. A voltage level of the first control signal may be greater than a voltage level of the second control signal. The photocontrol apparatus may further include an output conditioner circuit electrically, communicably, coupled between the third node and the high-impedance controller input, the output conditioner circuit being operable to change a voltage level of the first control signal and a voltage level of the second control signal. The photocontrol apparatus may further include a housing and an optical filter disposed within the housing adjacent a light receiving portion of the photosensor, wherein the optical filter transmits only light incident on the optical filter within a predetermined field of view to the light receiving portion of the photosensor. The housing may be transparent. The optical filter may be a film that may be disposed on the light receiving portion of the photosensor. 
     A photocontrol apparatus to permit, when in an electrically continuous state, a source of electrical power to provide a power signal to an input of a control device used to control the delivery of power to a load device may be summarized as including: a switch including a first node, a second node, and third node, the first node electrically, communicably, coupled to the source of electrical power and the third node electrically, communicably, coupled to the input of the control device; and a photosensor electrically, communicably, coupled between the second node and the third node, the photosensor operable to at least partially cause a voltage level of the second node with respect to the third node to change when the photosensor outputs current in response to being at least partially illuminated with light, wherein when the voltage level of the second node with respect to the third node is greater than a threshold voltage level, the photocontrol apparatus outputs the power signal, and when the voltage level of the second node with respect to the third node is less than the threshold voltage level, the photocontrol apparatus does not output the power signal. 
     The photosensor may be operable to at least partially cause the voltage level of the second node with respect to the third node to decrease when the photosensor outputs current in response to being at least partially illuminated with light. The photocontrol apparatus may further include a resistive device electrically, communicably, coupled between the second node and the third node, the resistive device being operable to at least partially cause the voltage level of the second node with respect to the third node to change when the photosensor outputs current in response to being at least partially illuminated with light. The resistive device may be electrically in parallel with the photosensor. A resistance of the resistive device may be adjustable. The resistive device may be a potentiometer. The photocontrol apparatus may further include a housing at least partially enclosing the switch and the photosensor, the housing may include a translucent portion and a shutter coupled to the housing, the shutter being moveable to selectively block and unblock at least part of the translucent portion from ambient light. The switch may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), wherein the first node is a drain node of the MOSFET, the second node is a gate node of the MOSFET, and the third node is a source node of the MOSFET. The switch may be an n-channel depletion mode MOSFET. The switch may be a p-channel enhancement mode MOSFET. The switch may be a low RDS(on) MOSFET. A cathode of the photosensor may be electrically, communicably, coupled to the second node and an anode of the photosensor may be electrically, communicably, coupled to the third node. The third node may output the power signal when the voltage level of the second node with respect to the third node is greater than the threshold voltage level. The photocontrol apparatus may further include a relay device electrically, communicably, coupled between the source of electrical power and the input of the control device, the relay device being operable to switch between an electrically continuous state and an electrically discontinuous state based on a signal output from the third node. The relay device may output the power signal when the voltage level of the second node with respect to the third node is greater than the threshold voltage level. The photocontrol apparatus may further include a housing and an optical filter disposed in the housing adjacent a light receiving portion of the photosensor, wherein the optical filter transmits only light incident on the optical filter within a predetermined field of view to the light receiving portion of the photosensor. The housing may be transparent. The optical filter may be a film that may be disposed on the light receiving portion of the photosensor. 
     A photocontrol circuit may be summarized as including: a switch including a first node, a second node, and a third node, the first node being electrically, communicably, coupled to a source of electrical power and the third node electrically, communicably coupled to a controller input; and a photosensor electrically, communicably, coupled between the second node and the third node; a resistive device electrically, communicably, coupled between the second node and the third node, the resistive device being operable to at least partially cause a voltage level of the second node with respect to the third node to change when the photosensor outputs current in response to being at least partially illuminated with light, wherein when the voltage level of the second node with respect to the third node is greater than a threshold voltage level, the third node outputs a first signal, and when the voltage level of the second node with respect to the third node is less than the threshold voltage level, the third node outputs a second signal different from the first signal. 
     The switch may be a depletion mode Metal Oxide Semiconductor Field Effect Transistor (MOSFET). The switch may be an n-channel depletion mode MOSFET. The switch may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), wherein the first node is a drain node of the MOSFET, the second node is a gate node of the MOSFET, and the third node is a source node of the MOSFET. The photosensor may be operable to at least partially cause the voltage level of the second node with respect to the third node to decrease when the photosensor outputs current in response to being at least partially illuminated with light. A voltage level of the first signal may be greater than a voltage level of the second signal. 
     A method of providing an electrical power signal to an input of a control device used to control the delivery of power to a load device may be summarized as including: electrically, communicably, coupling a source of the electrical power signal to a first node of a switch; electrically, communicably, coupling a photosensor between a second node of the switch and a third node of the switch; changing a voltage level of the second node with respect to the third node when the photosensor is at least partially illuminated with light; and providing the electrical power signal to the input of the control device when the voltage level of the second node with respect to the third node is greater than a threshold voltage level. 
     The power signal may not be provided to the input of the control device when the voltage level of the second node with respect to the third node is less than the threshold voltage level. The voltage level of the second node with respect to the third node may be decreased when the photosensor is at least partially illuminated with light. The method may further include electrically, communicably, coupling a resistive device between the second node of the switch and the third node of the switch. The resistive device may be electrically in parallel with the photosensor. The resistance of the resistive device may be adjustable. The method may further include adjusting the resistance of the resistive device. A cathode of the photosensor may be electrically, communicably, coupled to the second node and an anode of the photosensor may be electrically, communicably, coupled to the third node. The method include electrically, communicably, coupling a relay device between the source of the electrical power signal and the input of the control device, and switching the relay device between an electrically continuous state and an electrically discontinuous state based on a signal output from the third node. The relay device may output the power signal when the voltage level of the second node with respect to the third node is greater than the threshold voltage level. The method may further include electrically, communicably, coupling an output conditioner circuit between the third node and the input of the relay device. The method may further include electrically, communicably, coupling an output conditioner circuit between the third node and the input of the control device. 
     The photosensor may output current only when the photocontrol apparatus is at least partially illuminated with wavelengths of light in a range of about 380 nanometers to about 730 nanometers. The photosensor may output current only when the photocontrol apparatus is at least partially illuminated with human visible wavelengths of light. The voltage level of the second node with respect to the third node may be changed only when the photocontrol apparatus is at least partially illuminated with human visible wavelengths of light. The photocontrol apparatus may further include a housing and an optical filter disposed in the housing adjacent a light receiving portion of the photosensor, wherein the optical filter transmits only light incident on the optical filter within a predetermined field of view to the light receiving portion of the photosensor. The housing may be transparent. The optical filter may be a film that may be disposed on the light receiving portion of the photosensor. 
     A photocontrol apparatus may be summarized as including: a switch including a first node, a second node, and a third node, wherein the first node of the switch is electrically, communicably coupled to a source of electrical power and the third node of the switch is electrically, communicably coupled to an input of a control device; a comparator including a first power supply node, a second power supply node, a first input node, a second input node, and a power output node, wherein the power output node is electrically, communicably coupled to the second node of the switch and at least one of the first and the second power supply nodes is electrically, communicably coupled to the third node of the switch; and a photosensor electrically, communicably coupled between the second node of the switch and the third node of the switch, the photosensor operable to at least partially cause a voltage level of the first input node with respect to the second input node to change when the photosensor outputs current in response to being at least partially illuminated with light. 
     When the switch is in a first state and the photosensor causes the voltage level of the first input node to fall below a first threshold voltage level, the comparator may cause the switch to change to a second state, and when the switch is in the second state and the photosensor causes the voltage level of the first input node to rise above a second threshold voltage level, the comparator may cause the switch to change to the first state. The switch may be in the first state when the switch is turned ON, and the switch may be in the second state when the switch is turned OFF. The third node of the switch may output a first control signal when the switch is in the first state, the third node of the switch may output a second control signal when the switch is in the second state. The voltage level of the first control signal may be greater than the voltage level of the second control signal. The photocontrol apparatus may further include a capacitor electrically, communicably coupled between the third node of the switch and the first power supply node. The photocontrol apparatus may further include a reference voltage source electrically, communicably coupled between the third node of the switch and the second input node. The reference voltage source may be a diode. A resistive device may be electrically, communicably coupled between the reference voltage source and the photosensor. The comparator may be an operational amplifier including a negative voltage supply node, a positive voltage supply node, a non-inverting input node, an inverting input node, and a voltage output node, wherein the negative voltage supply node is the first power supply node, the positive voltage supply node is the second power supply node, the non-inverting input node is the first input node, the inverting input node is the second input node, and the voltage output node is the power output node. The photocontrol apparatus may further include: a first resistive device electrically, communicably coupled between the power output node and the first input node; a second resistive device electrically, communicably coupled between the photosensor and the first input node; and a third resistive device electrically, communicably coupled between the first input node and the third node of the switch. The second resistive device and the third resistive device may be included in a potentiometer. The photocontrol apparatus may further include a housing at least partially enclosing the switch and the photosensor, the housing including a translucent portion and a shutter coupled to the housing, the shutter being moveable to selectively block and unblock at least part of the translucent portion from ambient light. The switch may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), wherein the first node of the switch is a drain node of the MOSFET, the second node of the switch is a gate node of the MOSFET, and the third node of the switch is a source node of the MOSFET. The switch may be an n-channel depletion mode MOSFET. The switch may be a p-channel enhancement mode MOSFET. The switch may be a low RDS(on) MOSFET. The photosensor may output current only when the photocontrol apparatus is at least partially illuminated with human visible wavelengths of light. The third node of the switch may output a power signal to the input of the control device when the switch is in the first state, and the third node of the switch may not output the power signal to the input of the control device when the switch is in the second state. The photocontrol apparatus may further include a relay device electrically, communicably coupled between a source of electrical power and the input of the control device, the relay device being operable to switch between an electrically continuous state and an electrically discontinuous state based on a signal output from the third node of the switch. The relay device may output a power signal to the input of the control device when the switch is in the first state, and the relay device may not output the power signal to the input of the control device when the switch is in the second state. The photocontrol apparatus may further include an optical filter adjacent a light receiving portion of the photosensor, the optical filter including a translucent portion and at least one opaque portion disposed between the translucent portion and the light receiving portion of the photosensor, the optical filter transmitting only light incident on the translucent portion that is within a predetermined field of view to the light receiving portion of the photosensor. The optical filter may be a film that may be disposed on the light receiving portion of the photosensor. The photocontrol apparatus may further include a transparent housing enclosing the optical filter and the photosensor. 
     A method may be summarized as including: electrically, communicably, coupling a first node of a switch to a source of electrical power; electrically, communicably, coupling a photosensor between a second node of the switch and a third node of the switch; electrically, communicably coupling the third node of the switch to an input of a control device; electrically, communicably, coupling a comparator between the photosensor and the switch, at least a first power supply node of the comparator being coupled to the third node of the switch; and at least partially causing a voltage level of a first input node of the comparator with respect to a second input node of the comparator to change when the photosensor outputs current in response to being at least partially illuminated with light. 
     When the switch is in a first state and the photosensor causes the voltage level of the first input node to rise above the voltage level of the second input node, the switch may be caused to change to a second state; and when the switch is in the second state and the photosensor causes the voltage level of the first input node to fall below the voltage level of the second input node, the switch may be caused to change to the first state. The switch may be in the first state when the switch is turned ON, and the switch may be in the second state when the switch is turned OFF. The method may further include outputting a first control signal from the third node of the switch when the switch is in the first state; and outputting a second control signal from the third node of the switch when the switch is in the second state. The voltage level of the first control signal may be greater than the voltage level of the second control signal. The method may further include electrically, communicably coupling a capacitor between the third node of the switch and a first power supply node of the comparator. The method may further include electrically, communicably coupling a reference voltage source between the third node of the switch and the second input node of the comparator. The reference voltage source may be a diode. The method may further include electrically, communicably coupling a resistive device between the reference voltage source and the photosensor. The comparator may be an operational amplifier including a negative voltage supply node, a positive voltage supply node, a non-inverting input node, an inverting input node, and a voltage output node, and wherein the negative voltage supply node of the operational amplifier is the first power supply node of the comparator, the positive voltage supply node of the operational amplifier is a second power supply node of the comparator, the non-inverting input node of the operational amplifier is the first input node of the comparator, the inverting input node of the operational amplifier is the second input node of the comparator, and the voltage output node of the operational amplifier is a power output node of the comparator. The method may further include: electrically, communicably coupling a first resistive device between a power output node of the comparator and the first input node of the comparator; electrically, communicably coupling a second resistive device between the photosensor and the first input node of the comparator; and electrically, communicably coupling a third resistive device between the first input node of the comparator and the third node of the switch. The second resistive device and the third resistive device may be included in a potentiometer. The switch may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), the first node of the switch is a drain node of the MOSFET, wherein the second node of the switch is a gate node of the MOSFET, and the third node of the switch is a source node of the MOSFET. The switch may be an n-channel depletion mode MOSFET. The switch may be a p-channel enhancement mode MOSFET. The switch may be a low RDS(on) MOSFET. The voltage level of the first input node of the comparator with respect to the second input node of the comparator may be changed only when the photocontrol apparatus is at least partially illuminated with human visible wavelengths of light. The third node of the switch may output a power signal to the input of the control device when the switch is in the first state, and the third node of the switch may not output the power signal to the input of the control device when the switch is in the second state. The method may further include electrically, communicably coupling a relay device between the source of electrical power and the input of the control device, the relay device being operable to switch between an electrically continuous state and an electrically discontinuous state based on a signal output from the third node of the switch. The relay device may output a power signal to the input of the control device when the switch is in the first state, and the relay device may not output the power signal to the input of the control device when the switch is in the second state. The method may further include providing an optical filter adjacent a light receiving portion of the photosensor, the optical filter including a translucent portion and at least one opaque portion disposed between the translucent portion and the light receiving portion of the photosensor, the optical filter transmitting only light incident on the translucent portion that is within a predetermined field of view to the light receiving portion of the photosensor. The optical filter may be a film that may be disposed on the light receiving portion of the photosensor. The method may further include enclosing the optical filter and the photosensor in a transparent housing. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
         FIG. 1  is a block diagram of a control system, according to one illustrated embodiment. 
         FIG. 2A  is a detailed electrical schematic diagram of a photocontrol that can be used in the control system shown in  FIG. 1 , according to one illustrated embodiment. 
         FIG. 2B  is a detailed electrical schematic diagram of another photocontrol that can be used in the control system shown in  FIG. 1 , according to one illustrated embodiment. 
         FIG. 3  is a hypothetical plot showing output voltage as a function of time when the photocontrol shown in  FIG. 2A  is used in a first environmental setting, according to one non-limiting illustrated embodiment. 
         FIG. 4  is a hypothetical plot showing output voltage as a function of time when the photocontrol shown in  FIG. 2A  is used in a second environmental setting, according to one non-limiting illustrated embodiment. 
         FIG. 5  is a perspective view of a photocontrol assembly, according to one illustrated embodiment. 
         FIG. 6  is a block diagram of a control system, according to one illustrated embodiment. 
         FIG. 7  is a detailed electrical schematic diagram of a photocontrol that can be used in the control system shown in  FIG. 6 , according to one illustrated embodiment. 
         FIG. 8  is a detailed electrical schematic diagram of another photocontrol that can be used in the control system shown in  FIG. 6 , according to one illustrated embodiment. 
         FIG. 9  is a detailed electrical schematic diagram of a photocontrol, according to one illustrated embodiment. 
         FIG. 10  is a detailed electrical schematic diagram of a photocontrol, according to one illustrated embodiment. 
         FIG. 11  is a hypothetical plot showing output voltage as a function of time when the photocontrol shown in  FIG. 10  is used in a third environmental setting, according to one non-limiting illustrated embodiment. 
         FIG. 12  is a hypothetical plot showing output voltage as a function of time when the photocontrol shown in  FIG. 10  is used in a fourth environmental setting, according to one non-limiting illustrated embodiment. 
         FIG. 13A  is a hypothetical graph showing output voltage of the photocontrol shown in  FIG. 2A  as a function of photosensor current, according to one non-limiting illustrated embodiment. 
         FIG. 13B  is a hypothetical graph showing output voltage of the photocontrol shown in  FIG. 10  as a function of photosensor current, according to one non-limiting illustrated embodiment. 
         FIG. 14  is a block diagram of a potentiometer, according to one illustrated embodiment. 
         FIG. 15A  is a block diagram of a photosensor, according to one illustrated embodiment. 
         FIG. 15B  is a block diagram of the photosensor shown in  FIG. 15A  arranged with an optical sensor, according to one illustrated embodiment. 
         FIG. 16  is a plot showing the relative sensitivity of an amorphous visible-light photosensor, according to one non-limiting illustrated embodiment. 
         FIG. 17  is a detailed electrical schematic diagram of a photocontrol that can be used in the control system shown in  FIG. 6 , according to one illustrated embodiment. 
         FIG. 18A  is top plan view of an optical filter and a photosensor according to one illustrated embodiment. 
         FIG. 18B  is side plan view of the optical filter and the photosensor shown in  FIG. 18A . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with lighting systems, for example power converters, thermal management structures and subsystems, and/or solid state lights have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     As used in the specification and the appended claims, references are made to a “node” or “nodes.” It is understood that a node may be a pad, a pin, a junction, a connector, a wire, or any other point recognizable by one of ordinary skill in the art as being suitable for making an electrical connection within an integrated circuit, on a circuit board, in a chassis or the like. 
     The headings and Abstract of the Disclosure provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
       FIG. 1  shows a control system  100 , according to one illustrated embodiment. The description of  FIG. 1  provides an overview of the structure and operation of the control system  100 . A power source  102  (e.g., mains power) provides electrical power to a photocontrol apparatus  104  and a control device  106 , for example, using electrically conductive wires. More particularly, the power source  102  provides line voltage from node  108 , which is electrically coupled to node  112  of the photocontrol apparatus  104  and to node  118  of the control device  106 . Node  110  of the power source  102  is electrically coupled to system neutral and to node  120  of the control device  106 . Node  114  of the photocontrol apparatus  104  is electrically coupled to node  116  of the control device  106 . The photocontrol apparatus  104  provides control signals to the control device  106  via the nodes  114  and  116 . As will be explained below, the control device  106  uses the control signals provided by the photocontrol apparatus  104  to control the supply of electrical power to a load device (not shown). 
     If the photocontrol apparatus  104  is not illuminated with ambient light (e.g., during nighttime), the photocontrol apparatus  104  outputs a first control signal to the control device  106 . When the first control signal is received by the control device  106 , the control device  106  causes electrical power to be supplied to the load device. For example, the control device  106  causes electrical power to be supplied to an array of LEDs such that the array of LEDs produces a maximum rated amount of light. If the photocontrol apparatus  104  is illuminated with a sufficient amount of ambient light (e.g., during daytime), the photocontrol apparatus  104  outputs a second control signal to the control device  106 . When the second control signal is received by the control device  106 , the control device  106  reduces the amount of electrical power supplied to the load device. For example, the control device  106  reduces or stops the supply of electrical power to an array of LEDs such that the array of LEDs produces less light, or no light at all. 
       FIG. 2A  is a detailed electrical schematic diagram of the photocontrol apparatus  104  shown in  FIG. 1 , according to one illustrated embodiment. The photocontrol apparatus  104  includes a photocontrol circuit  122  and an output conditioner circuit  124 . In one embodiment, the photocontrol apparatus  104  also includes an input conditioner circuit (not illustrated) coupled between node  112  and the photocontrol circuit  122  that provides overvoltage and/or current surge protection for the photocontrol apparatus  104 . 
     The photocontrol circuit  124  includes a switch M 1 , a photosensor  126 , and a resistive device R 1 . In the illustrated embodiment, the switch M 1  is a low threshold, n-channel, depletion mode (normally-on) Metal Oxide Semiconductor Field Effect Transistor (MOSFET), such as transistor model DN2540 from Supertex Inc. In one embodiment, the switch M 1  is a low threshold, p-channel, enhancement mode (normally-off) MOSFET. In one embodiment, the switch M 1  is a low RDS(on), n-channel depletion mode MOSFET. 
     The switch M 1  includes a drain node D, a gate node G, and a source node S. The drain node D is electrically coupled to the node  112 , which can be electrically coupled to the node  108  of the power source  102  shown in  FIG. 1 . The photosensor  126  includes photodiodes PD 1 , PD 2 , PD 3 , and PD 4  arranged electrically in series. The photosensor  126  is electrically coupled between the gate node G and the source node S of the switch M 1 . More particularly, the photodiodes PD 1 , PD 2 , PD 3 , and PD 4  are arranged such that the cathode (i.e., more negative end) of photodiode PD 1  is electrically coupled to the gate node G of the switch M 1 , and the anode (i.e., more positive end) of photodiode PD 4  is electrically coupled to the source node S of the switch M 1 . When the photosensor  126  produces a sufficient amount of current, the voltage level of the gate node G with respect to the source node S of the switch M 1  becomes more negative until the switch M 1  turns OFF, as will be explained below. 
     The resistive device R 1  is electrically coupled between the gate node G and the source node S of the switch M 1  such that the resistive device R 1  is electrically in parallel with the photosensor  126 . The value of resistance of the resistive device R 1  can control the turn-on/off set point of the photocontrol apparatus  104 . That is, the voltage level of the gate node G with respect to the source node S can be controlled by the voltage drop across the resistive device R 1 , as will be explained below. In one embodiment, the value of resistance of the resistive device R 1  is adjustable so that the turn-on/off set point of the photocontrol apparatus  104  can be adjusted. In one embodiment, the resistive device R 1  is a manually adjustable potentiometer. In another embodiment, the resistive device R 1  is an electronically adjustable potentiometer. In one embodiment, the photocontrol  206  does not include the resistive device R 1  and the voltage level of the gate node G with respect to the source node S can be controlled by the voltage drop across the intrinsic parallel resistance of the photodiodes PD 1 , PD 2 , PD 3 , and PD 4 . 
     If the voltage level of the gate node G with respect to the source node S is greater than a threshold value (e.g., −1.5 volts), the resistance between the drain node D and the source node S becomes relatively small and a relatively high current can flow from the drain node D to the source node S of the switch M 1  (i.e., the switch M 1  turns ON). For example, if the voltage level of the gate node G with respect to a reference voltage level is 220 volts and the voltage level of the source node S with respect to the reference voltage level is 220 volts, the voltage level of the gate node G with respect to the source node S is 0 volts, which is greater than a threshold voltage level of −1.5 volts and thus the switch M 1  is turned ON. If the voltage level of the gate node G with respect to the source node S is less than or equal to the threshold value, the resistance between the drain node D and the source node S becomes relatively high (i.e., the switch M 1  turns OFF). For example, if the voltage level of the gate node G with respect to the reference voltage level is 218.4 volts and the voltage level of the source node S with respect to the reference voltage level is 220 volts, the voltage level of the gate node G with respect to the source node S is −1.6 volts, which is less than or equal to the threshold voltage level of −1.5 volts and thus the switch M 1  is turned OFF. 
     When light strikes the photosensor  126 , photons are absorbed by the photodiodes PD 1 , PD 2 , PD 3  and PD 4  and a current is produced that flows through the resistive device R 1 . When a sufficient amount of current flows through the resistive device R 1 , the voltage level of the gate node G with respect to the source node S becomes more negative until the switch M 1  turns OFF. While the switch M 1  is turned OFF, only a relatively small leakage current can flow through the resistive device R 3  and the control signal provided to the control device  106  has a relatively low voltage level. 
     When no light strikes the photosensor  126 , no current flows through the resistive device R 1  and the voltage level of the gate node G with respect to the source node S becomes more positive until the switch M 1  turns ON. While the switch M 1  is turned ON, a relatively high current can flow through the resistive device R 3  and the control signal provided to the control device  106  can have a relatively high voltage level. 
     The output conditioner circuit  124  includes a diode D 1 , a capacitor C 1 , and resistive devices R 2  and R 3 . The output conditioner circuit  124  is coupled between the source node S of the switch M 1  and the node  114  (i.e., output node of the photocontrol apparatus  104 ). In one embodiment, the capacitor C 1  has a value of 4.7 microfarads, the resistive device R 2  has a value of 1 Mohm, and the resistive device R 3  has a value of 100 Kohms. The values of the components of the output conditioner circuit  124  may be selected such that the control signals output by the photocontrol apparatus  104  have voltage and current levels that are compatible with the control device  106 . For example, the values of the components of the output conditioner circuit  124  may be selected such that the control signals output by the photocontrol apparatus  104  are compatible with an enable control input or a dimming control input of an LED driver incorporated within the AreaMax™ LED Area Light provided by the Evluma division of Express Imaging Systems, LLC. 
       FIG. 2B  is a detailed electrical schematic diagram of a photocontrol apparatus  104 ′ that can be used in place of the photocontrol apparatus  104  shown in  FIG. 1 , according to one illustrated embodiment. The photocontrol apparatus  104 ′ includes the photocontrol circuit  122  described above in connection with  FIG. 2A , but does not include the output conditioner circuit  124 . 
       FIG. 3  is a hypothetical plot showing the voltage level of a control signal output by the photocontrol apparatus  104  (i.e., the voltage across the resistive device R 3  that is output by the node  114 ) as a function of time when the photocontrol apparatus  104  is used in a first environmental setting, according to one non-limiting illustrated embodiment. In the first environmental setting, the photosensor  126  is exposed to a relatively low level of ambient light that causes the photosensor  126  to generate 10 microamps of current, which is not sufficient to turn the switch M 1  OFF (i.e., the switch M 1  is ON). As shown in  FIG. 3 , the voltage level of the control signal is about 2.7 volts after about 1 second of exposure to the relatively low level of ambient light in the first environmental setting. The photocontrol apparatus  104  may provide a control signal having the voltage shown in  FIG. 3  via the nodes  114  and  116  to the control device  106  to indicate that it is nighttime. By way of example, when the control device  106  receives the control signal shown in  FIG. 3 , the control device  106  controls power to an array of LEDs such that a maximum rated amount of light is output by the array of LEDs. 
       FIG. 4  is a hypothetical plot showing the voltage level of a control signal output by the photocontrol apparatus  104  as a function of time when the photocontrol apparatus  104  is used in a second environmental setting, according to one non-limiting illustrated embodiment. In the second environmental setting, the photosensor  126  is exposed to a relatively high level of ambient light that causes the photosensor  126  to generate 100 microamps of current, which is sufficient to turn the switch M 1  ON. As shown in  FIG. 4 , the voltage level of the control signal is about negative 18 millivolts after about 1 second of exposure to the relatively high level of ambient light in the second environmental setting. The photocontrol apparatus  104  may provide a control signal having the voltage shown in  FIG. 4  via the nodes  114  and  116  to the control device  106  to indicate that it is daytime. By way of example, when the control device  106  receives the control signal shown in  FIG. 4 , it controls power to an array of LEDs such that a reduced amount of light (e.g., no light) is output by the array of LEDs. 
       FIG. 5  is a perspective view of a photocontrol assembly  500 , according to one illustrated embodiment. The photocontrol assembly  500  includes a housing  502  having a translucent portion or window  504 , a bottom portion  508 , and contacts  510   a ,  510   b , and  510   c  extending from the bottom portion  508 . The photocontrol circuit  122  and the output conditioner circuit  124  may be mounted on a printed circuit board that is attached to the upper surface of the bottom portion  508  such that ambient light is able to pass through the window  504  and at least partially illuminate the photosensor  126 . 
     In one embodiment, the nodes  112  and  114  of the photocontrol apparatus  104  are electrically coupled to the contacts  510   a  and  510   b  of the housing assembly  500 , respectively. In one embodiment, the contacts  510   a ,  510   b , and  510   c  are arranged to form a twist-locking type of connector defined by the National Electrical Manufacturers Association (NEMA). 
     In one illustrated embodiment, the resistive device R 1  is a potentiometer and the housing assembly  500  includes an adjustment knob  506  having a rotatable portion  506   a  with a recess  506   b  formed therein. The potentiometer R 1  is manually coupled to the rotatable portion  506   a , for example, using a rigid piece of plastic that extends from the rotatable portion  506   a  to an adjustment knob of the potentiometer. When the tip of a screwdriver, for example, is inserted into the recess  506   b  and rotated, the value of resistance of the potentiometer changes. Accordingly, the adjustment knob  506  and the potentiometer enable the on/off set point of the photocontrol apparatus  104  to be adjusted when the photocontrol apparatus  104  is enclosed in the housing  502 . In one embodiment, the resistive device R 1  is an electronically controllable potentiometer, the resistance of which may be controlled remotely using wireless control signals (e.g., Bluetooth signals). 
     In one illustrated embodiment, the photocontrol assembly  500  includes a track  510  and an opaque portion or shutter  512 . The track  510  is formed on the housing  502  around the window  504  and includes raised portions  510   a  and  510   b  disposed on opposite sides of a center portion  510   c . The shutter  512  includes side portions  512   a  and  512   b  each having an inwardly facing projection (not shown) that fits snugly between the raised portions  510   a  and  510   b  of the track  510  on opposite sides of the window  504 . The shutter  512  may be moved upwardly and downwardly along the track  510  to selectively permit and block ambient light from passing through the window  504  to the photosensor  126 . 
     For example, if the photocontrol apparatus  104  is mounted inside the housing  500  and is deemed to provide the control signal indicating that it is daytime too early in the morning, the shutter  512  may be moved downwardly along the track  510  to block a portion of the window  504 . When the shutter  512  blocks a portion of the window  504 , a higher intensity of ambient light typically found later in the morning can be required to cause the photosensor  126  to produce sufficient current to turn the switch M 1  OFF, which causes the photocontrol apparatus  104  to produce the control signal indicating that it is daytime later in the morning. Similarly, if the photocontrol apparatus  104  is deemed to provide the control signal indicating that it is daytime too late in the morning, the shutter  512  may be moved upwardly along the track  510  to block a smaller portion (or no portion) of the window  504 . When the shutter  512  blocks less of the window  504 , a lower intensity of ambient light typically found earlier in the morning can be sufficient to cause the photosensor  126  to produce enough current to turn the switch M 1  OFF, which causes the photocontrol apparatus  104  to produce the control signal indicating that it is daytime earlier in the morning. Accordingly, the shutter  512  enables the on/off set point of the photocontrol apparatus  104  to be adjusted by selectively varying the degree to which the window  504  enables ambient light to pass to the photosensor  126 . 
     In one embodiment, the shutter  512  is provided inside the housing  502 , and the shutter  512  is coupled to an adjustment knob (not illustrated) provided on the outside of the housing  502  that enables the shutter  512  to be moved to selectively block and unblock at least part of the ambient light passing through the window  504 . For example, the shutter  512  may be in the form of a louver blind with horizontal or vertical slats that can be moved using a knob or lever provided on the outside of the housing  502 . 
       FIG. 6  shows a control system  600 , according to one illustrated embodiment. The description of  FIG. 6  provides an overview of the structure and operation of the control system  600 . A power source  602  (e.g., mains power) provides electrical power to a photocontrol apparatus  604 . More particularly, the power source  602  provides line voltage from node  608  to node  612  of the photocontrol apparatus  604 . Node  610  of the power source  602  is electrically coupled to system neutral and to node  614  of the photocontrol apparatus  604  and to node  620  of the control device  606 . Node  616  of the photocontrol apparatus  604  is electrically coupled to node  618  of the control device  606 . The photocontrol apparatus  604  selectively provides electrical power to the control device  606  via the nodes  616  and  618 . 
       FIG. 7  is a detailed electrical schematic diagram of the photocontrol apparatus  604  shown in  FIG. 6 , according to one illustrated embodiment. The photocontrol apparatus  604  includes a photocontrol circuit  622 , an output conditioner circuit  624 , and a relay device  628 , which includes a relay coil  630  and a single pole, single throw switch  632 . The photocontrol circuit  622  includes a switch M 1 , a photosensor  626 , and a resistive device R 1 . The photosensor  626  includes photodiodes PD 1 , PD 2 , PD 3 , PD 4 , PD 5 , PD 6 , PD 7 , and PDB. In the illustrated embodiment, the switch M 1  is a low threshold, p-channel, enhancement mode (normally-off) MOSFET. For example, the switch M 1  is a model TP2640 transistor from Supertex, Inc. In one embodiment, the switch M 1  is a low threshold, n-channel, depletion mode (normally-on) MOSFET. In one embodiment, the switch M 1  is a low RDS(on), n-channel depletion mode MOSFET. 
     The output conditioner circuit  624  includes a diode D 1 , a capacitor C 1 , and resistive devices R 2  and R 3 . The output conditioner circuit  624  is coupled between the source node S of the switch M 1  and the relay coil  630  of the relay device  628 . The values of the components of the output conditioner circuit  624  may be selected such that the control signals output by the output conditioner circuit  624  have voltage and current levels that are compatible with the relay coil  630 . 
     The resistive device R 3  of the output conditioner circuit  624  is electrically coupled to the relay coil  630 . By default, the switch  632  is closed (i.e., the switch  632  is in an electrically continuous state). When the switch M 1  outputs a control signal indicating that it is nighttime, the switch  632  remains closed. When the switch M 1  outputs a control signal indicating that it is daytime, the relay coil  630  causes the switch  632  to open (i.e., the switch  632  transitions to an electrically discontinuous state). The switch  632  remains open until the switch M 1  outputs the control signal indicating that it is nighttime to the relay coil  630 , which causes the switch  632  to close. 
     More particularly, when light strikes the photosensor  626 , photons are absorbed by the photodiodes PD 1 , PD 2 , PD 3 , PD 4 , PD 5 , PD 6 , PD 7 , and PD 8  and a current is produced that flows through the resistive device R 1 . When a sufficient amount of current flows through the resistive device R 1 , the voltage level of the gate node G with respect to the drain node D becomes more negative until it is greater than or equal to a threshold voltage level (e.g., −5 volts) and the switch M 1  turns ON. For example, if the voltage level of the gate node G with respect to a reference voltage level is −5.5 volts and the voltage level of the drain node D with respect to the reference voltage level is 0 volts, the voltage level of the gate node G with respect to the drain node D is −5.5 volts, which is less than or equal to a threshold voltage level of −5 volts and the switch M 1  turns ON. While the switch M 1  is turned ON, a relatively high current can flow through the relay coil  630 , which causes the switch  632  to turn OFF. While the switch  632  is turned OFF, an electrical power signal from the power source  602  is not able to flow to the control device  606 . 
     When relatively little light strikes the photosensor  626 , a relatively small current flows through the resistive device R 1  and the voltage level of the gate node G becomes closer to the drain node D until the switch M 1  turns OFF. For example, if the voltage level of the gate node G with respect to the reference voltage level is 216 volts and the voltage level of the drain node D with respect to the reference voltage is 220 volts, the voltage level of the gate node G with respect to the drain node D is −4 volts, which is greater than the threshold voltage level of −5 volts and the switch M 1  turns OFF. While the switch M 1  is turned OFF, only a relatively small leakage current can flow through the relay coil  630 , which causes the switch  632  to turn ON. While the switch  632  is turned ON, the electrical power signal from the power source  602  is able to flow to the control device  606 . 
       FIG. 8  is a detailed electrical schematic diagram of a photocontrol apparatus  604 ′, according to one illustrated embodiment. The photocontrol apparatus  604 ′ can be used in place of the photocontrol apparatus  604  shown in  FIG. 6 . The photocontrol apparatus  604 ′ includes a photocontrol circuit  902  and the output conditioner circuit  624 . The photocontrol circuit  902  includes a switch M 1 , a photosensor  626 ′, and a resistive device R 1 . In the illustrated embodiment, the switch M 1  is a low RDS(on), n-channel depletion mode MOSFET. For example, the switch M 1  is a transistor model IXTP6N100D2 from IXYS Corp. Preferably the resistance from the drain node D to the source node S when the switch M 1  is turned ON is between 2 and 5 ohms, and more preferably between 100 and 500 milliohms. In one embodiment, the switch M 1  is a low threshold, p-channel, enhancement mode (normally-off) MOSFET. In one embodiment, the switch M 1  is a low threshold, n-channel, depletion mode (normally-on) MOSFET. 
     If no light strikes the photosensor  626 ′, the switch M 1  is turned ON. While the switch M 1  is turned ON, a power signal from the power source  602  is able to flow from the node  608  to the node  612  and through the switch M 1  to the control device  606 . More particularly, the power signal from the power source  602  flows through the drain node D to the source node S of the switch M 1 , through the output conditioner circuit  624 , and then to the node  616 , which is electrically coupled to the node  618  of the control device  606 . When light strikes the photosensor  626 ′, current flows through the resistive device R 1  and the voltage level of the gate node G with respect to the source node S becomes more negative until the switch M 1  turns OFF. If the switch M 1  is turned OFF, the power signal from the power source  602  is not able to flow through the switch M 1  to the control device  606 . 
       FIG. 9  is a detailed electrical schematic diagram of a photocontrol apparatus  900 , according to one illustrated embodiment. The photocontrol apparatus  900  includes a photocontrol circuit  902 , an input node  904 , and an output node  906 . The photocontrol apparatus  900  can be used in place of the photocontrol apparatus  104  shown in  FIG. 1 . That is, the input node  904  of the photocontrol apparatus  900  can be electrically coupled to the node  108  of the power source  102  and the output node  906  of the photocontrol apparatus  900  can be electrically coupled to the node  118  of the control device  106 . 
     The photocontrol circuit  902  includes a switch M 1 , a photosensor  908 , a comparator U 1 , a capacitor C 1 , a diode D 1 , and resistive devices R 1 , R 2 , R 3 , and R 4 . In one embodiment, the value of the resistive device R 1  is 20 megaohms, the value of the resistive device R 2  is 20 megaohms, the value of the resistive device R 3  is 2 megaohms, the value of the resistive device R 4  is 20 megaohms, and the value of the capacitor C 1  is 10 microfarads. The switch M 1  includes a drain node D, a gate node G, and a source node S. In one embodiment, the switch M 1  is a low threshold, n-channel, depletion mode (normally-on) Metal Oxide Semiconductor Field Effect Transistor (MOSFET), such as transistor model DN2540 from Supertex Inc. The photosensor  908  includes photodiodes PD 1 , PD 2 , PD 3 , PD 4 , PD 5 , PD 6 , PD 7 , and PDB. 
     The comparator U 1  includes a positive supply voltage node PS, a negative supply voltage node NS, an inverting input node I, a non-inverting input node N, and an output node O. In one embodiment, the comparator U 1  is a model LT6003, 1.6V, 1 μA precision rail-to-rail input and output operational amplifier from Linear Technology Corporation. 
     The capacitor C 1  is electrically coupled between the negative supply voltage node NS of the comparator U 1  and the source node S of the switch M 1 . The source node S of the switch M 1  is electrically coupled to the positive supply voltage node PS of the comparator U 1 . The output node O of the comparator U 1  is electrically coupled to the gate node G of the switch M 1 . The resistive device R 1  is electrically coupled between the output node O of the comparator U 1  and the non-inverting input node N of the comparator U 1 . The resistive device R 2  is electrically coupled between the non-inverting input node N of the comparator U 1  and the cathode of the photosensor  908 . The resistive device R 3  is electrically coupled between the non-inverting input node N of the comparator U 1  and the source node S of the switch M 1 . The resistive device R 4  is electrically coupled between the inverting input node I of the comparator U 1  and the cathode of the photosensor  908 . The cathode of the diode D 1  is electrically coupled to the inverting input node I of the comparator U 1 , and the anode of the diode D 1  is electrically coupled to the source node S of the switch M 1 . The cathode of the photosensor  908  is electrically coupled to the negative supply voltage node NS of the comparator U 1 , and the anode of the photosensor  908  is electrically coupled to the source node S of the switch M 1 . 
     The resistive device R 1  provides positive feedback to the comparator U 1 , and causes the photocontrol circuit  902  to have switching hysteresis. The resistive devices R 2  and R 3  form a voltage divider that controls the voltage level V+ at the non-inverting input node N of the comparator U 1 . In one embodiment, the resistive devices R 2  and R 3  are included in a trimming potentiometer.  FIG. 14  is a block diagram of a potentiometer  1400  according to one embodiment. The potentiometer  1400  includes a first node  1402 , a second node  1404 , and a third node  1406 . In one embodiment, the first node  1402  is electrically coupled to the cathode of the photosensor  908 , the second node  1404  is electrically coupled to the non-inverting input node N of the comparator U 1 , and the third node  1406  is electrically coupled to the source node S of the switch M 1 . 
     The forward voltage of the diode D 1  provides a reference voltage at the inverting input node I of the comparator U 1 . In one embodiment, the diode D 1  is a model MMSD4148 diode from Fairchild Semiconductor. The diode D 1  may have a temperature coefficient similar to that of the photosensor  908 , or a temperature coefficient that is higher or lower than that of the photosensor  908 . In one embodiment, the diode D 1  is a red light emitting diode (LED). In another embodiment, an integrated circuit reference voltage is used on place of the diode D 1 . 
     When the switch M 1  is ON and the photosensor  908  is not producing current, the output of the comparator U 1  is the same as the voltage level of the source node S. As a result, the voltage level of the gate node G of the switch M 1  is the same as the voltage level of the source node S of the switch M 1  and the switch M 1  remains ON. If the photosensor  908  produces enough current to cause the voltage level V+ at the non-inverting input node N of the comparator U 1  to fall below the voltage level V− at the inverting input node N of the comparator U 1 , the comparator U 1  outputs the voltage level provided to the negative power supply node NS of the comparator U 1 . As result, the voltage level of the gate node G of the switch M 1  drops sufficiently below the voltage level of the source node S of the switch M 1  to cause the switch M 1  to turn OFF. 
     When the switch M 1  is OFF and the photosensor  908  stops producing enough current to cause the voltage level V+ at the non-inverting input node N of the comparator U 1  to be below the voltage level V− at the inverting input node N of the comparator U 1 , the comparator U 1  outputs the voltage level provided to the positive power supply node PS of the comparator U 1 . As result, the voltage level of the gate node G of the switch M 1  is no longer sufficiently below the voltage level of the source node S of the switch M 1  to keep the switch M 1  turned OFF and the switch turns ON. Similarly, when the switch M 1  is OFF and the photosensor  908  stops producing enough current to keep the voltage level at the negative and/or the positive power supply nodes NS and NP above a minimum operating voltage level to keep the comparator U 1  operational, the voltage level of the gate node G of the switch M 1  not sufficiently below the voltage level of the source node S of the switch M 1  to keep the switch M 1  turned OFF and the switch turns ON. 
       FIG. 10  is a detailed electrical schematic diagram of a photocontrol apparatus  1000  according to one illustrated embodiment. The photocontrol apparatus  1000  includes the photocontrol circuit  902  described above in connection with  FIG. 9 , an input node  1004 , an output node  1006 , and an output conditioner circuit  1008 . In one embodiment, the output conditioner circuit  1008  is the same as the output conditioner circuit  124  described above in connection with  FIG. 2A . The photocontrol apparatus  1000  can be used in place of the photocontrol apparatus  104  shown in  FIG. 1 . That is, the input node  1004  of the photocontrol apparatus  1000  can be electrically coupled to the node  108  of the power source  102  and the output node  1006  of the photocontrol apparatus  1000  can be electrically coupled to the node  118  of the control device  106 . 
       FIG. 11  is a hypothetical plot showing the voltage level of a control signal output by the photocontrol apparatus  1000  as a function of time when the photocontrol apparatus  1000  is used in place of the photocontrol apparatus  104  shown in  FIG. 1 . The photocontrol apparatus  1000  is operated in a third environmental setting in which the photosensor  908  is exposed to a level of ambient light that causes the photosensor  908  to generate a current having a magnitude that is not sufficient to cause the switch M 1  to turn OFF (i.e., the switch M 1  is ON). As shown in  FIG. 11 , the voltage level of the control signal is about 3.5 volts after about 0.2 seconds of exposure to the relatively low level of ambient light in the third environmental setting. The photocontrol apparatus  1000  may provide the control signal shown in  FIG. 11  via the nodes  1006  and  116  to the control device  106  to indicate that it is nighttime. For example, when the control device  106  receives the control signal shown in  FIG. 11 , the control device  106  controls power to an array of LEDs such that a maximum rated amount of light is output by the array of LEDs. 
       FIG. 12  is a hypothetical plot showing the voltage level of a control signal output by the photocontrol apparatus  1000  as a function of time when the photocontrol apparatus  1000  is used in place of the photocontrol apparatus  104  shown in  FIG. 1 . The photocontrol apparatus  1000  is operated in a fourth environmental setting, in which the photosensor  908  is exposed to a level of ambient light that causes the photosensor  908  to generate a current of sufficient magnitude to cause the switch M 1  to turn OFF. As shown in  FIG. 12 , the voltage level of the control signal is about negative 25 millivolts after about 1.8 seconds of exposure to the relatively high level of ambient light in the fourth environmental setting. The photocontrol apparatus  1000  may provide the control signal shown in  FIG. 12  via the nodes  1006  and  116  to the control device  106  to indicate that it is daytime. For example, when the control device  106  receives the control signal shown in  FIG. 12 , it controls power to an array of LEDs such that a reduced amount of light (e.g., no light) is output by the array of LEDs. 
     As will be explained below, the photocontrol circuit  902  of the photocontrol apparatus  1000  can prevent the photocontrol apparatus  1000  from outputting a signal that causes the control device  106  to turn OFF the load device during nighttime when the photosensor  908  of the photocontrol apparatus  1000  is illuminated with a relatively low level of light. For example, the photocontrol circuit  902  of the photocontrol apparatus  1000  can prevent stray light emitted by a light source controlled by the control device  106  from causing the photocontrol apparatus  1000  to output a control signal that causes the control device  106  to turn the light source OFF. 
     The operation of the photocontrol apparatus  1000  will now be compared to the operation of the photocontrol apparatus  104  with reference to  FIGS. 13A and 13B .  FIG. 13A  is a hypothetical graph showing a voltage V O  output by the photocontrol apparatus  104  shown in  FIG. 2A  as a function of a current I P  generated by the photosensor  126 . The switching point of the photocontrol circuit  122  is determined by a threshold current level I T  that causes the switch M 1  to turn ON and OFF. When the magnitude of the current I P  generated by the photosensor  126  is less than the threshold current level I T , the switch M 1  is ON and the magnitude of the output voltage V O  of the photocontrol apparatus  104  is V HIGH . When the magnitude of the current Ip generated by the photosensor  126  is greater than the threshold current level I T , the switch M 1  turns OFF and the magnitude of the output voltage V O  of the photocontrol apparatus  104  is V LOW . 
     When the magnitude of the current I P  generated by the photosensor  126  is near the threshold current level I T , relatively small fluctuations in the intensity of light that illuminates the photosensor  126  may cause the magnitude of the current I P  generated by the photosensor  126  to fluctuate above and below the threshold current level I T . When the magnitude of the current I P  generated by the photosensor  126  rapidly fluctuates above and below the threshold current level I T , the level of the output voltage V O  of the photocontrol apparatus  104  rapidly between V Low  and V HIGH . If the photocontrol apparatus  104  provides such an output voltage V O  as input to the control device  106  shown in  FIG. 1 , the control device  106  would rapidly switch a load device (e.g., a light source) ON and OFF. For example, when the output voltage V O  of the photocontrol apparatus  104  is provided as input to a controller that controls a light source, relatively small changes in the intensity of light that illuminates the photosensor  126  that occur frequently may cause the light source to rapidly turn ON and OFF. 
       FIG. 13B  is a hypothetical graph showing a voltage V O  output by the photocontrol apparatus  1000  shown in  FIG. 10  as a function of a current I P  generated by the photosensor  908 . The switching points of the photocontrol circuit  902  are determined by a lower threshold current level I LT  and an upper threshold current level I UT  that cause the switch M 1  to turn ON and OFF, depending on the state of the photocontrol circuit  902  (e.g., the state of the comparator U 1  or the switch M 1 ). When the switch M 1  is ON and the magnitude of the current I P  generated by the photosensor  908  rises above the upper threshold current level I UT , the switch M 1  turns OFF and the magnitude of the output voltage V O  of the photocontrol apparatus  1000  becomes V LOW . When the switch M 1  is OFF and the magnitude of the current I P  generated by the photosensor  908  falls below the lower threshold current level I LT , the switch M 1  turns ON and the magnitude of the output voltage V O  of the photocontrol apparatus  1000  becomes V HIGH . 
     Because the photocontrol apparatus  1000  has two threshold current levels that depend on the state of the photocontrol circuit  902 , when the value of the current I P  generated by the photosensor  908  is close to either of the threshold current levels, relatively small fluctuations in the intensity of light that illuminates the photosensor  908  do not cause the switch M 1  to turn ON and OFF. For example, when the output voltage level V O  of the photocontrol apparatus  1000  is provided as input to a controller that controls a light source, relatively small changes in the intensity of light that illuminates the photosensor  908  that occur frequently do not cause the light source to turn ON and turn OFF for short periods of time. 
     The voltage levels at the non-inverting input node N and the inverting input node I of the comparator U 1  depend the state of the switch M 1  and the magnitude of the current I P  generated by the photosensor  908 . When the switch M 1  is ON and the voltage level at the non-inverting node N of the comparator U 1  falls below a first value of the voltage level at the inverting node I of the comparator U 1  (i.e., an upper threshold voltage level V UT ), the comparator U 1  causes the voltage level at the output node O to be such that the switch M 1  turns OFF. When the switch M 1  is OFF and the voltage level at the non-inverting node N of the comparator U 1  rises above a second value of the voltage level at the inverting node I of the comparator U 1  (i.e., a lower threshold voltage level V ST ), the comparator U 1  cause the voltage level at the output node O to be such that the switch M 1  turns ON. 
       FIG. 15A  is a block diagram of a photosensor  1500 , according to one illustrated embodiment. The photosensor  1500  includes a light receiving surface  1502  that, when illuminated with light, causes the photosensor  1500  to generate a current having a magnitude that is proportional to the intensity of the light.  FIG. 15B  is block diagram showing an optical filter  1504  disposed between a translucent portion  1506  (e.g., the translucent portion  504  of the housing  502  shown in  FIG. 5 ) and the light receiving surface  1502  (not labeled in  FIG. 15B ) of the photosensor  1500 . The optical filter  1504  may prevent one or more predetermined ranges of wavelengths of light passing through the translucent portion  1506  from reaching the light receiving surface  1502  of the photosensor  1500 . That is, the optical filter  1504  may transmit only one or more predetermined ranges of wavelengths of light passing through the translucent portion  1506  to the light receiving surface  1502  of the photosensor  1500 . The optical filter  1504  may be formed on the translucent portion  1506  or on the light receiving surface  1502  of the photosensor  1500 . In one embodiment, the optical filter  1504  is integrally formed with the translucent portion  1506 . 
     The optical filter  1504  may transmit to the light receiving surface  1502  of the photosensor  1500  only human visible wavelengths of light, for example, wavelengths of light between about 400 nanometers and 700 nanometers. In one embodiment, the optical filter  1504  transmits only wavelengths of light between about 380 nanometers and 750 nanometers to the light receiving surface  1502  of the photosensor  1500 . The optical filter  1504  may transmit only wavelengths of light that corresponding to one or more colors. For example, the optical filter  1504  may transmit only wavelengths of light in a range of about 380 to 450 nanometers (i.e., violet light), in a range of about 450 to 495 nanometers (i.e., blue light), in a range of about 495 to 570 nanometers (i.e., green light), in a range of about 570 to 590 nanometers (i.e., yellow light), in a range of about 590 to 620 nanometers (i.e., orange light), and/or in a range of about 620 to 750 (i.e., red light). The values of wavelengths listed above are approximate and preferably do not deviate from the listed values by more than 10%. More preferably, the values of wavelengths listed above do not deviate from the listed values by more than 5%. The optical filter  1504  may take the form of an absorptive filter, a dichroic filter, a resonance filter, a mesh filter, and/or a polarizer. 
     Alternatively or additionally, the photosensor  1500  may be an amorphous silicon photosensor having a relative sensitivity similar to that of the human eye. In one embodiment, the photosensor  1500  is a model AM-5308 photosensor from SANYO Amorton Co., Ltd having the relative sensitivity shown in  FIG. 16 . For example, the photosensor  1500  may output a current only when illuminated with human visible wavelengths of light in a range of about 400 nanometers to 700 nanometers, or only when illuminated with wavelengths of light in a range of about 380 nanometers to 730 nanometers. The values of wavelengths listed above are approximate and preferably do not deviate from the listed values by more than 10%. More preferably, the values of wavelengths listed above do not deviate from the listed values by more than 5%. 
       FIG. 17  is a detailed electrical schematic diagram of a photocontrol apparatus  1704 , according to one illustrated embodiment. The photocontrol apparatus  1704  includes a first node  1712 , a second node  1714 , and third node  1716 . The photocontrol apparatus  1704  can be used in place of the photocontrol apparatus  604  shown in  FIG. 6 . That is, the first node  1712  of the photocontrol apparatus  1704  can be electrically coupled to the node  608  of the power source  602 , the second node  1714  of the photocontrol apparatus  1704  can be electrically coupled to the node  610  of the power source  602  and the node  620  of the control device  606 , and the third node  1716  of the photocontrol apparatus  1704  can be electrically coupled to the node  618  of the control device  606 . 
     The photocontrol apparatus  1704  includes the photocontrol circuit  902  described above in connection with  FIGS. 9 and 10 , an output conditioner circuit  1724 , and a relay device  1728 , which includes a relay coil  1730  and a single pole, single throw switch  1732 . The output conditioner circuit  1724  includes a diode D 2 , a capacitor C 2 , and resistive devices R 5  and R 6 . The output conditioner circuit  1724  is coupled between the source node S of the switch M 1  and the relay coil  1730  of the relay device  1728 . The values of the components of the output conditioner circuit  1724  may be selected such that the control signals output by the output conditioner circuit  1724  have voltage and current levels that are compatible with the relay coil  1730 . 
     The resistive device R 6  of the output conditioner circuit  624  is electrically coupled to the relay coil  1730 . During normal operation, the switch  1732  is closed (i.e., the switch  1732  is in an electrically continuous state). When the switch M 1  outputs a control signal indicating that it is nighttime, the switch  1732  remains closed. When the switch M 1  outputs a control signal indicating that it is daytime, the relay coil  1730  causes the switch  1732  to open (i.e., the switch  1732  transitions to an electrically discontinuous state). The switch  1732  remains open until the switch M 1  outputs the control signal indicating that it is nighttime to the relay coil  1730 , which causes the switch  1732  to close. 
       FIG. 18A  is top plan view of an optical filter  1802  and a photosensor  1804  according to one embodiment. The optical filter  1802  includes a first transparent portion  1806 , a first partially opaque portion  1808 , a second partially opaque portion  1810 , and a second transparent portion  1812 . The first partially opaque portion  1808  and the second partially opaque portion  1810  are disposed between the first transparent portion  1806  and the second transparent portion  1812 . The first partially opaque portion  1808  and the second partially opaque portion  1810  include light absorbing louvers that prevent some of the light incident on the first transparent portion  1806  from reaching the second first transparent portion  1814 . The photosensor  1804  includes a light receiving portion  1814  adjacent the second transparent portion  1812 . Thus, the first partially opaque portion  1808  and the second partially opaque portion  1810  prevent some of the light incident on the first transparent portion  1806  from reaching the light receiving portion  1814  of the photosensor  1804 . 
     The first transparent portion  1806  and the second transparent portion  1812  may be formed from a polycarbonate material. The first partially opaque portion  1808  and the second partially opaque portion  1810  may be formed from acrylic resin and carbon black materials. In one embodiment, the optical filter  1802  is a light control film that is disposed on the light receiving portion  1814  of the photosensor  1804 . The optical filter  1802  may be secured to the light receiving portion  1814  of the photosensor  1804  with an adhesive. In one embodiment, the optical filter  1802  is formed from the Advanced Light Control Film ALCF-P ABR2 available from the 3M Company. 
     Reference arrows  1816 ,  1818 , and  1820  are shown extending from the first transparent portion  1806  of the optical filter  1802 . The reference arrow  1816  is perpendicular to the first transparent portion  1806  of the optical filter  1802  and may be perpendicular to the light receiving portion  1814  of the photosensor  1804 . The reference arrows  1816  and  1818  form an angle α therebetween. The reference arrows  1816  and  1820  form an angle β therebetween. The angles α and β define a first field of view of the optical filter  1802  and thus the light receiving portion  1814  of the photosensor  1804 . 
     The first partially opaque portion  1808  prevents light, which is incident on the first transparent portion  1806  of the optical filter  1802  that is not within the first field of view from reaching the light receiving portion  1814  of the photosensor  1804 . For example, the first partially opaque portion  1808  may prevent light rays incident on the first transparent portion  1806 , which originate from the right side of the reference arrow  1816  and form an angle greater than the angle β with the reference arrow  1816 , from reaching the light receiving portion  1814  of the photosensor  1804 . Additionally, the first partially opaque portion  1808  may prevent light rays incident on the first transparent portion  1806 , which originate from the left side of the reference arrow  1816  and form an angle greater than the angle α with the reference arrow  1816 , from illuminating the light receiving portion  1814  of the photosensor  1804 . Accordingly, the first partially opaque portion  1808  may enable only light incident on the first transparent portion  1806  that is within the first field of view defined by angles α and β to illuminate the light receiving portion  1814  of the photosensor  1804 . 
     In one embodiment, the first partially opaque portion  1808  enables only light that is within the angles α and β plus and minus a predefined tolerance to illuminate the light receiving portion  1814  of the photosensor  1804 . For example, if each of the angles α and β is equal to 30 degrees and the tolerance is 4 degrees, the first partially opaque portion  1808  may enable only light that is within a field of view that extends from −34 degrees to 34 degrees with respect to the reference arrow  1816  to illuminate the light receiving portion  1814  of the photosensor  1804 . 
       FIG. 18B  is side plan view of the optical filter  1802  and the photosensor  1804 . Reference arrows  1822 ,  1824 , and  1826  are shown extending from the first transparent portion  1806  of the optical filter  1802 . The reference arrow  1822  is perpendicular to the first transparent portion  1806  of the optical filter  1802  and may be perpendicular to the light receiving portion  1814  of the photosensor  1804 . The reference arrows  1822  and  1824  form an angle γ therebetween. The reference arrows  1822  and  1826  form an angle δ therebetween. The angles γ and δ define a second field of view of the optical filter  1802  and thus the light receiving portion  1814  of the photosensor  1804 . 
     The second partially opaque portion  1810  prevents light, which is incident on the first transparent portion  1806  of the optical filter  1802  that is not within the second field of view from reaching the light receiving portion  1814  of the photosensor  1804 . For example, the second partially opaque portion  1810  prevents light rays incident on the first transparent portion  1806 , which originate from the right of the reference arrow  1822  and form an angle greater than the angle δ with the reference arrow  1822 , from illuminating the light receiving portion  1814  of the photosensor  1804 . Additionally, the second partially opaque portion  1810  prevents light rays incident on the first transparent portion  1806 , which originate from the left of the reference arrow  1822  and form an angle greater than the angle γ with the reference arrow  1822 , from illuminating the light receiving portion  1814  of the photosensor  1804 . That is, the second partially opaque portion  1810  enables only light incident on the first transparent portion  1806  that is within the second field of view defined by angles γ and δ to illuminate the light receiving portion  1818  of the photosensor  1804 . 
     In one embodiment, the second partially opaque portion  1810  enables only light that is within the angles γ and δ plus and minus a predefined tolerance to illuminate the light receiving portion  1814  of the photosensor  1804 . For example, if each of the angles γ and δ is equal to 30 degrees and the tolerance is 4 degrees, the second partially opaque portion  1810  may enable only light that is within a field of view that extends from −34 degrees to 34 degrees with respect to the reference arrow  1822  to illuminate the light receiving portion  1814  of the photosensor  1804 . 
     Accordingly, the optical filter  1802  may transmit only light incident on the first transparent portion  1806  that is within the first field of view and also within the second field of view to the light receiving portion  1814  of the photosensor  1804 . In one embodiment, the optical filter  1802  and the photosensor  1804  are included in a housing that is transparent. For example, the optical filter  1802  and the photosensor  1804  may be included in the housing  502  shown in  FIG. 5 , wherein the entire housing  502  is transparent and thus the window  504  may be omitted. 
     The various embodiments described above can be combined to provide further embodiments. To the extent that they are not inconsistent with the specific teachings and definitions herein, all of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including but not limited to U.S. Provisional Patent Application No. 61/052,924, filed May 13, 2008; U.S. Patent Publication No. US 2009/0284155, published Nov. 19, 2009; U.S. Provisional Patent Application No. 61/051,619, filed May 8, 2008; U.S. Pat. No. 8,118,456, issued Feb. 12, 2012; U.S. Provisional Patent Application No. 61/088,651, filed Aug. 13, 2008; U.S. Patent Publication No. US 2010/0090577, published Apr. 15, 2010; U.S. Provisional Patent Application No. 61/115,438, filed Nov. 17, 2008; U.S. Provisional Patent Application No. 61/154,619, filed Feb. 23, 2009; U.S. Patent Publication No. US2010/0123403, published May 20, 2010; U.S. Provisional Patent Application No. 61/174,913, filed May 1, 2009; U.S. Patent Publication No. US2010/0277082, published Nov. 4, 2010; U.S. Provisional Patent Application No. 61/180,017, filed May 20, 2009; U.S. Patent Publication No. US2010/0295946, published Nov. 25, 2010; U.S. Provisional Patent Application No. 61/229,435, filed Jul. 29, 2009; U.S. Patent Publication No. US2011/0026264, published Feb. 3, 2011; U.S. Provisional Patent Application No. 61/295,519 filed Jan. 15, 2010; U.S. Provisional Patent Application No. 61/406,490 filed Oct. 25, 2010; U.S. Patent Publication No. US2011/0175518, published Jul. 21, 2011; U.S. Provisional Patent Application Ser. No. 61/333,983, filed May 12, 2010; U.S. Patent Publication No. US2010/0295454, published Nov. 25, 2010; U.S. Provisional Patent Application Ser. No. 61/346,263, filed May 19, 2010, U.S. Patent Publication No. US2010/0295455, published Nov. 25, 2010; U.S. Provisional Patent Application Ser. No. 61/357,421, filed Jun. 22, 2010; U.S. Patent Publication No. US2011/0310605, published Dec. 22, 2011; U.S. Patent Publication No. 2012/0262069, published Oct. 18, 2012; U.S. Non-Provisional patent application Ser. No. 13/212,074, filed Aug. 17, 2011; U.S. Provisional Patent Application Ser. No. 61/527,029, filed Aug. 24, 2011; U.S. Non-Provisional patent application Ser. No. 13/592,590 filed Aug. 23, 2012; U.S. Provisional Patent Application Ser. No. 61/534,722, filed Sep. 14, 2011; U.S. Non-Provisional patent application Ser. No. 13/619,085, filed Sep. 14, 2012; U.S. Provisional Patent Application Ser. No. 61/567,308, filed Dec. 6, 2011; U.S. Provisional Patent Application Ser. No. 61/561,616, filed Nov. 18, 2011; U.S. Provisional Patent Application Ser. No. 61/641,781, filed May 2, 2012; U.S. Non-Provisional patent application Ser. No. 13/411,321 filed Mar. 2, 2012; U.S. Provisional Patent Application Ser. No. 61/640,963, filed May 1, 2012; U.S. Non-Provisional patent application Ser. No. 13/558,191 filed Jul. 25, 2012; U.S. Provisional Patent Application Ser. No. 61/692,619, filed Aug. 23, 2012; U.S. Provisional Patent Application Ser. No. 61/694,159, filed Aug. 28, 2012; U.S. Non-Provisional patent application Ser. No. 13/604,327 filed Sep. 5, 2012; U.S. Provisional Patent Application Ser. No. 61/723,675, filed Nov. 7, 2012; U.S. Non-Provisional patent application Ser. No. 13/679,687, filed Nov. 16, 2012; U.S. Provisional Patent Application Ser. No. 61/728,150, filed Nov. 19, 2012; U.S. Provisional Patent Application Ser. No. 61/764,395, filed Feb. 13, 2013; and U.S. Provisional Patent Application Ser. No. 61/849,841, filed Jul. 24, 2013 are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments. 
     For example, a photocontrol apparatus according to the present disclosure can provide control signals to a control device that receives additional control signals from other environmental sensors, for example, a motion sensor, a proximity sensor, and an occupancy sensor. Such a control device can be programmed to control a luminaire based on the control signals received from two or more of the environmental sensors and a current time of day. For example, the control device can cause the luminaire to produce a signal indicating a security breach and to illuminate an array of LEDs if, during a time period specified for night operations, the photocontrol indicates that a detected light level is above a desired level and a motion sensor indicates that motion has been detected. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.