EP2578060A1

EP2578060A1 - Synchronization of light sources

Info

Publication number: EP2578060A1
Application number: EP10728951.4A
Authority: EP
Inventors: Vito Carnicelli; Olivier Casagrande
Original assignee: Honeywell International Inc
Current assignee: Honeywell International Inc
Priority date: 2010-05-28
Filing date: 2010-05-28
Publication date: 2013-04-10
Also published as: WO2011149473A1

Abstract

A system includes means for sensing an ambient light intensity at a first location, and means for determining a first time at which the sensed ambient light intensity at the first location crosses a threshold light intensity value. The system also includes means for receiving an offset time value indicative of a time duration by which a measured ambient light intensity at the first location leads or lags a corresponding measured ambient light intensity at a second location. The system further includes means for determining, based on the first time, a default time period, and the offset time value, a second time and causing an illumination state of a first plurality of light sources to change at the second time.

Description

SYNCHRONIZATION OF LIGHT SOURCES

TECHNICAL FIELD

[0001] The disclosure relates to switches for light sources.

BACKGROUND

[0002] Light sources, such as street lamps, may be controlled in response to signals received from a light sensor. For example, the light sensor may sense an ambient light intensity, measured in lux. In some examples, a control unit causes a switch to close to provide electrical power to the light sources when a signal received from the light sensor indicates that ambient light intensity has fallen below a threshold intensity (e.g., at dusk). Additionally, the control unit may cause the switch to open, ceasing delivery of electrical power to the light sources, when a signal received from the light sensor indicates that ambient light intensity has risen above a threshold intensity (e.g., at dawn).

[0003] One control unit may control several light sources (e.g., ten, fifteen, or twenty). When many light sources are installed along a street, two or more control units may be used to control the several light sources. In this case, light sources connected to a first control unit may switch at a different time than light sources connected to a second control unit. This may be due to, for example, different installation conditions of light sensors connected to the first and second control units, respectively, such as the direction the light sensor is facing (e.g., north, south, east, or west), or a presence or absence of buildings, trees, billboards, hills, mountains, or the like, that can cast a shadow on the light sensor. Drivers or pedestrians may prefer that the light sources switch on or off at approximately the same time, or within a short time window such that deviations from a common switching time are minimized.

[0004] One approach to synchronizing the light sources is to provide a wireless receiver with each control unit and synchronize the control units by providing a wireless synchronization signal or clock signal that can be received by the receivers at the control units. In combination with an ephemeris table stored in a memory of the control unit, the switching times of the respective control units, and thus the connected light sources, may be synchronized. However, such a system may be relatively expensive, as the antenna and receiver used to receive the wirelessly-transmitted clock signal may comprise an unduly large portion of the cost of the system (e.g., about 30% of the cost).

SUMMARY

[0005] In general, the disclosure is directed to systems, apparatuses, and techniques for synchronizing switching times for a plurality of light sources, such as street lamps. In some embodiments, the synchronization may be accomplished without connecting to an external time source, such as a wirelessly transmitted time source or indication thereof.

[0006] In one general aspect, the disclosure is directed to a system that includes means for sensing an ambient light intensity at a first location. The system also includes means for determining a first time at which the sensed ambient light intensity at the first location crosses a threshold light intensity value. The system further includes means for receiving an offset time value indicative of a time duration by which a measured ambient light intensity at the first location leads or lags a corresponding measured ambient light intensity at a second location. The system further includes means for determining, based on the first time, a default time period, and the offset time value, a second time and causing an illumination state of a first plurality of light sources to change at the second time.

[0007] In various implementations, the default time period may be twenty-four hours, and the means for determining the second time may determine the second time by adding twenty-four hours and the offset time value to the first time. The first time may be determined during a period of generally decreasing ambient light intensity at the first location, and the illumination state of the first plurality of light sources may be caused to change by a switching means that connects an electricity source to the first plurality of light sources at the third time, thereby causing the first plurality of light sources to illuminate. Alternatively, the first time may be determined during a period of generally increasing ambient light intensity at the first location, and the illumination state of the first plurality of light sources may be caused to change by a switching means that disconnects an electricity source from the first plurality of light sources at the third time, thereby causing the first plurality of light sources to darken.

[0008] The first time may be determined during a period of generally decreasing ambient light intensity at the first location, and the system may also include means for determining, during a period of generally increasing ambient light intensity at the first location, a third time at which the sensed ambient light intensity crosses the threshold light intensity value, and means for determining a nighttime duration based on the first time and the third time. The system may also determine a lights- on duration by adding the offset time value to the nighttime duration, and cause the illumination state of the first plurality of light sources to change a lights-on duration of time following the first time.

[0009] The offset time value may be positive, and the measured ambient light intensity at the first location may lead in time the corresponding measured ambient light intensity at the second location. Alternatively, the offset time value may be negative, and the measured ambient light intensity at the first location may lag in time the corresponding measured ambient light intensity at the second location. The system may further include means for applying a filter to the first time to determine a filtered time, where the filter is used to determine an average time based on the first time and at least another time corresponding to a time at which the ambient light intensity at the first location crossed the threshold light intensity value on a previous day, and wherein the means for determining the second time determines the second time based on the filtered time, the default time period, and the offset time value.

[0010] In another general aspect, the disclosure is directed to a method that includes sensing an ambient light intensity at a first location, and determining a first time at which the sensed ambient light intensity crosses a threshold light intensity value. The method also includes receiving an offset time value indicative of a time duration by which a measured ambient light intensity at the first location leads or lags a corresponding measured light intensity at a second location. The method further includes determining, based on the first time, a default time period, and the offset time value, a second time and causing an illumination state of a first plurality of light sources to change at the second time. [0011] In various implementations, the default time period may be twenty-four hours, and the second time may be determined by adding twenty-four hours and the offset time value to the first time. The first time may be determined during a period of generally decreasing ambient light intensity at the first location, and the method may further include determining, during a period of generally increasing ambient light intensity at the first location, a third time at which the sensed ambient light intensity crosses the threshold light intensity value, determining a nighttime duration based on the first time and the third time, and causing the illumination state of the first plurality of light sources to change a lights-on duration of time following the first time, where the lights-on duration of time is determined by adding the offset time value to the nighttime duration.

[0012] The offset time value may be positive, and the measured ambient light intensity at the first location may lead in time the corresponding measured ambient light intensity at the second location. Alternatively, the offset time value may be negative, and the measured ambient light intensity at the first location may lag in time the corresponding measured ambient light intensity at the second location. The method may further include applying a filter to the first time to determine a filtered time, where the filter is used to determine an average time based on the first time and at least another time corresponding to a time at which the ambient light intensity at the first location crossed the threshold light intensity value on a previous day, and where the second time is determined based on the filtered time, the default time period, and the offset time value.

[0013] The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0014] FIG. 1 is a conceptual diagram illustrating an example of a lighting system for illuminating an outdoor area.

[0015] FIG. 2 is a block diagram that illustrates an example of a control unit for controlling a plurality of light sources. [0016] FIG. 3 is a flow chart illustrating an example of a technique for determining an offset time.

[0017] FIG. 4 is a flow diagram that illustrates an example of a technique that a control unit may implement to determine a switch-on time for a plurality of light sources.

[0018] FIG. 5 is a flow diagram that illustrates an example of a technique that control unit may implement to determine a switch-off time for a plurality of light sources.

[0019] FIG. 6 is a flow diagram of an example of a technique that a control unit may implement to determine both a switch-on time and a switch-off time for a plurality of light sources based on an ambient light level and an offset time.

[0020] FIG. 7 is a flow diagram illustrating an example of a technique according to which a control unit may determine a switch-on time for a plurality of light sources connected to the control unit.

DETAILED DESCRIPTION

[0021] FIG. 1 is a conceptual diagram illustrating an example of a lighting system 10 for illuminating an outdoor area. For example, lighting system 10 may include a street lamp system for illuminating streets, sidewalks, and/or parks in an outdoor area of a city or other community. In some examples, lighting system 10 is configured to provide light when ambient light conditions (e.g., light conditions due to sunlight, moonlight, starlight, illuminated signs, or other light sources not a part of lighting system 10) are below a first threshold light intensity and to switch off and not provide light when ambient light conditions are above a second threshold light intensity. For example, lighting system 10 may be configured to provide light to the area during a time period between approximately dusk (or twilight) and approximately dawn.

[0022] In the example illustrated in FIG. 1 , lighting system 10 is installed in an outdoor area that includes a first street 12 and a second street 14, which crosses first street 12. Together, first street 12 and second street 14 define two sides of each of a first block 16 and a second block 18. [0023] Lighting system 10 may include a first control unit 24 is installed on first block 16. First control unit 24 is electrically coupled to a first plurality of light sources 26a-26e (collectively "first light sources 26") via a first plurality of electrical connections 28a-28e (collectively "first electrical connections 28"), which may be electrically conductive cables or wires. Although FIG. 1 illustrates first control unit 24 as connected to first light sources 26 in parallel, in other embodiments, first control unit 24 may be connected to first light sources 26 in series.

[0024] Similarly, a second control unit 30 is installed on second block 18. Second control unit 30 is electrically connected to a second plurality of light sources 32a- 32e (collectively "second light sources 32") via a second plurality of electrical connections 34a-34e (collectively "second electrical connections 34"), which may be electrically conductive cables or wires. Although FIG. 1 illustrates second control unit 30 as being connected to second light sources 32 in parallel, in other embodiments, second control unit 30 may be connected to second light sources 32 in series.

[0025] Although FIG. 1 illustrates first control unit 24 and second control unit 30 as connected to five first light sources 26 and five second light sources 32, respectively, in other embodiments, first control unit 24 and/or second control unit 30 may be connected to more or fewer than five first light sources 26. For example, first control unit 24 and second control unit 30 may be connected to about twenty, thirty, forty, or fifty first light sources 26 and second light sources 32, respectively. In other examples, first control unit 24 and/or second control unit 30 may be connected to other numbers of light sources. First control unit 24 and second control unit 30 may be connected to the same number of light sources or a different number of light sources, in various implementations. For simplicity, control units 24 and 30 are shown separated by a single block in FIG. 1, but in other implementations the control units 24 and 30 may be separated by several blocks.

[0026] First control unit 24 controls operation of first light sources 26, and second control unit 30 controls operation of second light sources 32. In particular, first control unit 24 controls connection of first electrical connections 28 to an external electricity source (see FIG. 2) via a switch located within first control unit 24. Similarly, second control unit 30 controls connection of second electrical connections 34 to an external electricity source via a switch located within second control unit 30. In this way, first control unit 24 may control first light sources 26 to switch on and off at a predetermined time, at a predetermined ambient light intensity, or at a predetermined time offset from time at which a particular ambient light intensity occurs, and second control unit 30 may control second light sources 32 in a similar manner.

[0027] In accordance with some embodiments of the disclosure, the switches within first control unit 24 and second control unit 30 may be responsive to a control module coupled to a clock and a light sensor located within or attached to the respective control unit 24 or 30. In some examples of the disclosure, the control module within at least one of first control unit 24 or second control unit 30 may be programmed with an offset time to correct for a lack of synchronization of switching times (switching on and/or off) of first control unit 24 and second control unit 30.

[0028] The lack of synchronization of switching times may be due to, for example, a physical configuration or orientation of the respective light sensors. For example, the physical configuration or orientation of the light sensors may result in the light sensor in first control unit 24 receiving a different ambient light intensity than the light sensor in second control unit 30, even at substantially similar times and even though the control units are in relatively close proximity to one another. Differences in physical configuration or orientation of the light sensors may include, for example, one light sensor being oriented in a first direction (e.g., north) while the other light sensor is oriented in a second, different direction (e.g., west, east, or south). Another difference in physical configuration or orientation may include one light sensor being installed in a location that receives shade from a building, tree, parked cars or trucks, hill, mountain, or the like, while the other light sensor is installed in a location that receives little to no shade. Additionally or alternatively, variances in the sensitivity or calibration of the respective light sensors may cause the light sensors to generate a different signal in response to exposure to substantially the same ambient light intensity. A lack of synchronization between switching times of first control unit 24 and second control unit 30 may be disfavored by pedestrians or drivers utilizing first street 12 and/or second street 14, and may also result in unnecessary energy consumption due to first light sources 26 or second light sources 32 remaining on for longer than they would if the light sensor in first control unit 24 or second control unit 30, respectively, were better controlled to turn off and on at appropriate times.

[0029] In some aspects, first control unit 24 may be programmed with an offset time to correct a lack of switching synchronization between first control unit 24 and second control unit 30. The offset time may allow substantial synchronization between switch-on times of first light sources 26 and second light sources 32 and/or between switch-off times of first light sources 26 and second light sources 32. In various examples, switching times for first light sources 26 and second light sources 32 may be synchronized to occur within about 30 seconds, 1 minute, 90 seconds, or two minutes of one another. In some cases, the lights sources 26, 32 may be switched within about ±1 minute of each other - that is, the first and second light sources 26, 32 may be synchronized to within about one minute.

[0030] In some examples, the offset time may be equal to a difference between a first time at which first control unit 24 switched on first light sources 26, and a second time at which second control unit 30 switched on second light sources 30. In other examples, the offset time may be equal to a difference between a first time at which first control unit 24 switched off the first light sources 26 and a second time at which second control unit 30 switched off second light sources 30. These noted switch-on or switch-off times for the first and second controllers, and upon which the offset time may be based, may correspond to an initial or control switching period. For example, upon deployment, the control units and the corresponding light sources may be monitored to determine when they switch on or off the first time, and to determine when they switch relative to one another. In other examples, the offset time may be equal to a difference between a time at which first control unit 24 or second control unit 30 switched on or off first light sources 26 or second light sources 32, respectively, and a time at which an ephemeris table indicates sunrise or sunset in the general location of first control unit 24 or second control unit 30. [0031] In some examples, the offset time may be positive. A positive offset time may be appropriate when first control unit 24 switched on first light sources 26 early (that is, leading in time) relative to second light sources 32 or the time indicated in the ephemeris table. Incorporating a positive offset time may cause the first control unit 24 to switch on first light sources 26 later than if switching was solely based on the output of the light sensor, which may sense ambient light at the location of the control unit. In some examples, the positive offset time may be set to an amount of time by which the first control unit 24 switched on first light sources 26 in advance of the time that second control unit 30 switched on second light sources 32 during an initial or control period.

[0032] In other examples, the offset time may be negative. A negative offset time may be appropriate when first control unit 24 switched on first light sources 26 late (that is, lagging in time) relative to second light sources 32 or the time indicated in the ephemeris table. A negative offset time may cause the first control unit 24 to switch on first light sources 26 earlier than if switching was solely based on the output of the light sensor. In some examples, the negative offset time may be set to an amount of time by which the first control unit 24 switched on first light sources 26 after the time that second control unit 30 switched on second light sources 32 during an initial or control period. In some examples, the offset time is an integer number of minutes (e.g., an integer between - 15 minutes and 15 minutes, including 0 minutes). While the offset time has been described with reference to first control unit 24, in other examples the offset time may alternatively be applied to second control unit 30.

[0033] The control module in first control unit 24 may utilize the offset time, a clock signal from a clock in first control unit 24, and an illumination signal from a light sensor on or attached to first control unit 24 to determine a time to switch on and/or a time to switch off first light sources 26. For example, on a first day, the control module may receive an illumination signal from the light sensor and may compare the illumination signal to a threshold light intensity value to determine whether the illumination signal indicates that an ambient light intensity has decreased below the threshold light intensity value. In some examples, the threshold light intensity value may be about 4 lux. When the control module determines that the ambient light intensity has decreased below the threshold light intensity value, the control module may note (and store in memory, for example) this time as a first time (e.g., as a first turn-on time value). The control module may then determine a switch-on time for switching on the first light sources 26 on a second, subsequent day by adding 24 hours (based on the clock signal) plus the offset time to the first time. In some examples, the control module may not cause the switch to close and power to be delivered to first light sources 26 based on the illumination signal received from the light sensor indicating the ambient light intensity is below the threshold light intensity value.

[0034] On the second day, the control module may again receive an illumination signal from the light sensor and may compare the illumination signal to the threshold light intensity value to determine whether the illumination signal indicates that an ambient light intensity has decreased below the threshold light intensity value. When the control module determines that the ambient light intensity has decreased below the threshold light intensity value, the control module may note and store this time as a second time. The control module may then determine a switch-on time for switching on the first light sources 26 on a third, subsequent day by adding 24 hours (based on the clock signal) plus the offset time to the second time.

[0035] In some examples, on the second day, the control module may not cause the switch or switches to close and power to be delivered to first light sources 26 based on the ambient light intensity decreasing below the threshold light intensity value on the second day. Instead, the control module may cause the switch or switches to close and power to be delivered to first light sources 26 in accordance with the switching time the control module determined based on the first time (from the first day), the clock signal, and the offset time. In some examples, the control module may repeat this technique each day: noting a time at which the illumination signal received from the light sensor indicates that the ambient light intensity reached or crossed below a threshold light intensity value and adding 24 hours and the offset time (which may be positive or negative) to this time to determine the switch-on time for the following day. In this way, to determine when to switch on first light sources 26, the control module 24 may utilize the time at which the ambient light intensity reached or crossed a threshold light intensity value on a previous day, the clock signal, and the offset time.

[0036] While the above description has been directed to techniques implemented by first control unit 24 to substantially synchronize a switch-on time of first light sources 26 with a switch-on time of second light sources 32, in some examples, first control unit 24 may utilize a similar technique to substantially synchronize a switch-off time of first light sources 26 with a switch-off time of second light sources 32. For example, the control module of first control unit 24 may receive on a first day an illumination signal representative of the ambient light intensity from the light sensor. The control module may compare the illumination signal to a threshold light intensity value, which may be the same or different than the threshold light intensity value referenced above. For example, the threshold light intensity value may be about 4 lux, or may be between about 6 lux and about 8 lux. When the illumination signal increases above the threshold light intensity value, the control module may note and store the time as a first time value (e.g., a first turn-off time value). The control module may then determine a switching time for switching off first light sources 26 on a second, subsequent day by adding 24 hours (based on the clock signal) plus the offset time to the first time. As described above, in this way, to determine when to switch off first light sources 26, the control module may utilize a time at which the ambient light intensity increased above a threshold light intensity value on a previous day, a clock signal, and an offset time.

[0037] In other examples, instead of determining the switch-off time for first light sources 26 based on the time at which the ambient light intensity increases above a threshold light intensity value on the previous day, the control module may determine the switch-off time of first light sources 26 based on the switch-on time of first light sources 26 and a length of the previous night. For example, during a first day-night cycle, the control module may determine a length of the night based on a difference between a first time at which an ambient light intensity decreased below a first threshold light intensity value (indicating, e.g., dusk) and a second time at which the ambient light intensity increased above a second threshold light intensity value (indicating, e.g., dawn). In some examples, the first and second threshold light intensity values may be the same, while in other examples, the first and second threshold light intensity values may be different. For example, the first threshold light intensity value may be approximately 4 lux and the second threshold light intensity values may be between about 6 lux and about 8 lux. As another example, each of the first and second threshold light intensity values may be about 4 lux (or 5 lux, or 6 lux, e.g.).

[0038] The control module 24 may then determine the switch-on time for the following dusk for first light sources 26 based on the first time, the clock signal (to measure 24 hours) and the offset time, as described above. Additionally, the control module 24 may determine the switch-off time for first light sources 26 by adding the length of the previous night to the determined switch-on time for first light sources 26. The control module may repeat this determination for each subsequent switch-on time and length of night. In this way, the control module may adjust the switch-on time and the switch-off time of first light sources 26 to allow for changing light conditions throughout the year (due to seasonal variations), while maintaining substantial synchronization in switch-on and switch- off times of first light sources 26 and second light sources 32. Similar techniques to those described above could alternatively be performed based on a length of day rather than night, for example. Although the preceding examples have been discussed primarily with respect to first control unit 24 and first light sources 26, similar techniques may be implemented by second control unit 30 to control a switch-on and a switch-off time of second light sources 32 to substantially synchronize with the switch-on and switch-off times with first light sources 26.

[0039] In some examples, switch-on and switch-off times may be substantially synchronized among more than two control units. For example, the switch-on and switch-off times may be substantially synchronized among all control units along a portion of a street (e.g., three, four, or five control units), which may span multiple city blocks, among all control units within a development (e.g., five, eight, or ten control units), among all control units within a neighborhood, or among all control units within a municipality (e.g., fifty, one hundred, two hundred, or a thousand control units), such as a city. Other areas within which the switch-on and switch- off times of multiple control units may be substantially synchronized are also contemplated. In general, any number of control units may be substantially synchronized via application of the techniques described herein.

[0040] FIG. 2 is a block diagram that illustrates an example of a control unit for controlling a plurality of light sources. The control unit shown in FIG. 2 may correspond to first control unit 24 or second control unit 30 of FIG. 1, for example. For simplicity, the control unit 24 shown in FIG. 2 will be associated with first control unit 24 of FIG. 1. As illustrated in FIG. 2, control unit 24 may include a control module 42, a clock module 44, a light sensor 46, an offset module 48, and a switching module 50.

[0041] In general, first control unit 24 includes any suitable arrangement of hardware, alone or in combination with software and/or firmware, to perform the techniques attributed to first control unit 24 and control module 42, clock module 44, light sensor 46, and offset module 48 of first control unit 24. In various examples, first control unit 24 may include one or more processors, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. First control unit 24 also, in various examples, may include a memory (not shown), such as random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, a hard disk, a CD-ROM, a floppy disk, a cassette, magnetic media, or optical media comprising executable instructions for causing the one or more processors to perform the actions attributed to them. Moreover, although control module 42, clock module 44, and offset module 48 are described as separate modules, in some examples, control module 42, clock module 44, and offset module 48 may be functionally integrated. In some examples, control module 42, clock module 44, and offset module 48 correspond to individual hardware units, such as ASICs, DSPs, FPGAs, switches, potentiometers, or other hardware units.

[0042] Control module 42 generally includes logic that directs operation of first control unit 24 and the components thereof and causes first control unit 24 to perform the functions ascribed to it herein. In some examples, control module 42 includes a memory or is electrically connected to a memory that stores instructions for execution by control module 42 in controlling operation of first control unit 24. In some examples, the memory also may store values related to switch-on and/or switch-off times of first light sources 26, such as, for example, the threshold ambient light intensity value(s), one or more offset times, the length of at least one previous night or day, at least one previous time at which the ambient light intensity crossed a threshold ambient light intensity value, or the like.

[0043] User interface 52 may include a button(s), switch(es), or keypad, lights, and/or a display, such as a liquid crystal (LCD), light- emitting diode (LED), or cathode ray tube (CRT). As discussed in this disclosure, control module 42 and/or offset module 48 may present and receive information relating to operation of first control unit 24 via user interface 52. For example, offset module 48 may receive the offset time via user interface 52. The input may be, for example, in the form of pressing one or more buttons on a keypad, turning a rotary dial, actuating a switch or lever, or the like. Additionally or alternatively, control module 42 may present information regarding operation of first control unit 24 via user interface 52. For example, control module 42 may present information regarding a current position of switching module 50 (e.g., open or closed) via a display, an indicator light, or the like. In some examples, control module 42 may output further information regarding operation of first control unit 24, such as the programmed offset time, a currently sensed ambient light level, or the like, via a display for viewing by a user.

[0044] Under control of control module 42, light sensor 46 may detect an ambient light intensity at a location at which first control unit 24 is installed. Light sensor 46 may include, for example, a photo cell, a photodiode, or a photoresistor. Light sensor 46 may detect an ambient light intensity and output a signal that changes in a known manner as the ambient light intensity changes.

[0045] In some examples, light sensor 46 is installed within first control unit 24 and a sensing element of the light sensor 46 is exposed to the ambient environment via an aperture or window defined in a housing of first control unit 24. In other examples, light sensor 46 may be installed on an exterior surface of a housing of first control unit 24, or may be installed proximate to first control unit 24 but not physically attached to first control unit 24.

[0046] As shown in FIG. 2, light sensor 46 is electrically connected to control module 42. Control module 42 may receive a signal generated by light sensor 46 in response to sensing an ambient light intensity via the electrical connection.

[0047] Additionally, via the electrical connection, control module 42 may control light sensor 46 to initiate sensing of the ambient light intensity or to cease sensing of the ambient light intensity. In some examples, light sensor 46 senses the ambient light intensity substantially continuously, and control module 42 may never control light sensor 46 to cease sensing the ambient light intensity. In other examples, control module 42 may control light sensor 46 to initiate sensing of the ambient light intensity prior to a time at which it is anticipated the ambient light intensity will either increase from less than a threshold light intensity to more than the threshold light intensity or decrease from more than a threshold light intensity to less than the threshold light intensity. Control module 42 also may control light sensor 46 to cease sensing the ambient light intensity subsequent to the time at which it is anticipated the ambient light intensity will cross the threshold.

[0048] By confining the time period during which light sensor 46 is actively sensing the light intensity, control module 42 may reduce a risk that light sensor 46 senses a light event that causes control module 42 to erroneously determine that the ambient light intensity is crossing a threshold light intensity due to dusk or dawn. For example, a decrease in ambient light intensity may be caused not by a setting of the sun, but by an object blocking light sensor 46 (e.g., by an object fully or partially obscuring light from reaching the light sensor). Conversely, an increase in ambient light intensity may be caused not by rising of the sun, but by car headlights, a flashlight, or the like shining on light sensor 46. By confining the time period during which light sensor 46 is actively sensing the ambient light intensity, control module 42 may reduce a risk that such an event will interfere with substantial synchronization of the switch-on and switch-off times of first light sources 26 and second light sources 32 (FIG. 1).

[0049] Clock module 44 includes a time keeping device, such as a counter, which tracks time based on a substantially stable frequency source. In some examples, clock module 44 may track time based on a frequency of the power network to which first control unit 24 is connected. In some embodiments, the frequency of the power network is about 50 Hz. In other embodiments, the frequency of the power network is about 60 Hz. In other embodiments, clock module 44 may track time based on a frequency of another oscillator attached to the counter, such as a crystal oscillator (not shown in FIG. 2).

[0050] In some examples, clock module 44 may not need to keep time that is synchronized with an external clock to be accurate with regard to the time of day. Instead, in some examples, clock module 44 may simply be able to accurately measure 24 hour periods. In this manner, clock module 44 may accurately track 24 hours from a time identified by control module 42. In other examples, clock module 44 may be able to accurately measure time within a minute, such that clock module 44 may track a 24 hour period and adjust for the offset time programmed into control module 42 or offset module 48. In this way, clock module 44 may include a timing device rather than a clock in some implementations.

[0051] Offset module 48 includes hardware, firmware, and/or software that can allow a user, such as a technician, to program and store an offset time for first control unit 24. In some examples, offset module 48 includes a potentiometer, a dual in-line package (DIP) switch, or a coded rotary switch. In such examples, the offset time may be set for offset module 48 using switches, a rotary knob, a screw that is turned using a tool such as a screwdriver, or the like. In other examples, offset module 48 includes circuitry (e.g., transistors and/or memory) connected to control module 42, clock module 44, and/or user interface 52 that receives an input indicating an offset time, stores the offset time in memory, and in response to an instruction received from control module 42, transfers the offset time to control module 42 or clock module 44 for use in calculating the switch-on and/or switch- off time of first light sources 26.

[0052] Switching module 50 may include one or more relays or other switching mechanisms for connecting and disconnecting first electrical connections 28 to external electricity source 54. Control module 42 may control switching module 50 to open or close based on the determined switch-on time and switch-off time. Control unit 24 may additionally include a power supply (not shown in FIG. 2) that receives power from electricity source 54 and provides power at appropriate levels for the various modules, sensors, and electronics of control unit 24.

[0053] In general operation, control module 42 may command switching module 50 to close and connect electricity source 54 to first light sources 26 (see FIG. 1) via first electrical connections 28 based on a signal generated by light sensor 46, an offset time set via offset module 48, and a clock signal received from clock module 44. For example, on a first day, control module 42 may control light sensor 46 to sense an ambient light intensity and may receive an illumination signal from light sensor 46 that represents the ambient light intensity. Control module 42 may compare the illumination signal to a threshold light intensity value. The threshold light intensity value may represent a light intensity that is sufficiently low to suggest desirability of providing illumination via first light sources 26. In some embodiments, the threshold light intensity value may be about 4 lux. In other embodiments, the threshold light intensity value may be between about 6 lux and about 8 lux. Other threshold light intensity values are also contemplated.

[0054] Control module 42 may determine whether the illumination signal has reached or is lower than the threshold light intensity value. However, control module 42 may not control switching module 50 to close the relay or switch to connect electricity source 54 to first light sources 26 in response to determining that the illumination signal is lower than the threshold light intensity value.

Instead, control module 42 may note the time (e.g., as a first time or as a first turn- on time) at which the ambient light intensity reached or decreased below the threshold light intensity value. In some embodiments, control module 42 may cause clock module 44 to begin a timer at the first time. In some embodiments, control module 42 may cause the time at which the ambient light intensity reached or decreased below the threshold light intensity value to be stored in a memory of control module 42 or a separate memory (not shown).

[0055] Control module 42 may determine a time at which to control switching module 50 to connect electricity source 54 to first light sources 26 via first electrical connections 28 based on the time at which the ambient light intensity decreased below the threshold light intensity value, a signal from clock module 44, and an offset time set via offset module 48. For example, in some embodiments, clock module 44 may be a timer. In various embodiments, clock module 44 may not track standard time but rather may track a time delta (e.g., a 24-hour period) from an index time. Control module 42 may control clock module 44 to begin measuring a 24 hour time period when control module determines the ambient light intensity sensed by light sensor 46 has decreased below the threshold light intensity value. Clock module 44 may measure the 24-hour time period, and the period may be adjusted by the offset module 48 to be shorter than 24 hours or longer than 24 hours based on the offset time set via offset module 48. After the time period measured by clock module 44 elapses (where the time period may be supplements positively or negatively by the offset value), control module 42 may receive a signal from clock module 44 indicating that it is time to switch on first light sources 26. Control module 42 may then control the switching module 50 to connect electricity source 54 to first electrical connections 28 to provide power to first light sources 26.

[0056] As another example, control module 42 may cause the time at which the ambient light intensity decreased below the threshold light intensity value to be stored in a memory of control module 42 or a separate memory (not shown). Control module 42 may utilize this time, a time provided by clock module 44, and the offset time to determine the switch-on time for first light sources 26 during the following day. In some embodiments, clock module 44 may measure time in 24 hours cycles. The clock module may be resettable to count from a zero point, or may be free-running and track a time period from a starting value (e.g., track a 24- hour period from a starting, non-zero value). Control module 42 may determine a switch-on time for the following day by adding the offset time to the time at which the ambient light intensity decreased below the threshold light intensity value and saving in memory an instruction to cause switching module 50 to connect electricity source 54 and first electrical connections 28 on the following day at the switch-on time.

[0057] In this way, control module 42 may utilize the time at which the ambient light intensity decreases below the threshold light intensity value in determining the switch-on time for first light sources 26 for the following day. In some embodiments, control module 42 may repeat this technique each day, determining the switch-on time for the upcoming day based on the time at which the ambient light intensity decreases below the threshold light intensity value during the current day. This allows control module 42 to change the switch-on time for first light sources 26 to account for seasonal variations in sunset (dusk) times while allowing synchronization of switch-on times between first control unit 24 and other control units, such as second control unit 30.

[0058] In some examples, control module 42 may implement a similar technique to determine a switch-off time of first light sources 26. For example, on a first day, control module 42 may control light sensor 46 to sense an ambient light intensity and may receive an illumination signal from light sensor 46 that represents the ambient light level. Control module 42 may compare the illumination signal to a threshold light intensity level. The threshold light intensity value may represent a light intensity that is sufficiently high to suggest desirability of switching off first light sources 26. The threshold light intensity value may be the same as the threshold light intensity value used in determining the switch-on time of first light sources 26, or may be a different threshold light intensity value. In some embodiments, the threshold light intensity value may be about 4 lux. In other embodiments, the threshold light intensity value may be between about 6 lux and about 8 lux. Other threshold light intensity values are also contemplated.

[0059] When control module 42 determines that the illumination signal has increased above the threshold light intensity value, control module 42 may note the time, e.g., may control clock module 44 to initiate a timer or may save the time in a memory. Control module 42 may then determine a switching time for switching off first light sources 26 on a second, subsequent day by adding 24 hours (based on the signal received from clock module 44) plus the offset time to the time at which the illumination signal increased above the threshold light intensity value.

[0060] In some examples, control module 42 may determine switch-off time of first light sources 26, including when to control switching module 50 to disconnect electricity source 54 from first light sources 26 via first electrical connections 28 based on the switch-on time of first light sources 26 and a length of the previous night. For example, during a first day-night cycle, control module 42 may determine a length of the night based on a difference between a first time at which the ambient light intensity decreased below a first threshold light intensity value (indicating, e.g., dusk) and a second time at which the ambient light intensity increased above a second threshold light intensity value (indicating, e.g., dawn). In some examples, the first and second threshold values may be the same, while in other examples, the first and second threshold values may be different. During the subsequent day -night cycle, control module 24 may control switching module 50 to connect first electrical connections 28 to electricity source 54 (i.e., switch on first light sources 26) based on the first time, a signal received from clock module 44 and the offset time, as described above. Control module 24 then may control switching module 50 to disconnect first electrical connections 28 from electricity source 54 (i.e., switch off first light sources 26) by adding the duration of the previous night to the first time, at which the first light sources 26 were switched on.

[0061] In some embodiments, control module 42 may repeat one of these techniques each day, determining the switch-off time of first light sources 26 for the upcoming day based on the time at which the ambient light intensity increases above the threshold light intensity value during the current day, or based on the time at which the ambient light intensity decreases below the threshold light intensity value during the current day duration of the current night. This can allow control module 42 to control the switch-off time for first light sources 26 to account for seasonal variations in sunset (dusk) times while allowing

synchronization of switch-off times between first control unit 24 and another control unit, e.g., second control unit 30.

[0062] FIG. 3 is a flow chart illustrating an example of a technique for determining an offset time. FIG. 3 will be described with reference to lighting system 10 of FIG. 1 and first control unit 24 of FIG. 2 for purposes of clarity. However, it will be understood that the technique illustrated in FIG. 3 may be implemented in other lighting systems or other control units. For example, the technique illustrated in FIG. 3 may be implemented in a lighting system including more than two control units. In a system with n control units, for example, one control unit may be chosen as a base unit, and offset times may be determined for each of the other (n- 1) control units. [0063] Initially, a technician installs first control unit 24 and second control unit 30, which each include a respective light sensor (e.g., light sensor 46 of FIG. 2) (62). First control unit 24 may be electrically connected to first light sources 26 via first electrical connections 28, and second control unit 30 may be electrically connected to second light sources 32 via second electrical connections 34. Control units 24 and 30 may each be housed in a cabinet or enclosure, which may be mounted on the ground or may be mounted on an object (e.g., a light pole) in various implementations.

[0064] After installation of first control unit 24 and second control unit 30, the technician or another person may observe a first switching time for first control unit 24 and a second switching time for second control unit 30 (64). In some examples, the technician may observe the respective switching times by observing the time at which first light sources 26 and second light sources 32 turn on (e.g., at approximately dusk) and/or turn off (e.g., at approximately dawn). These observations may be made shortly after installation in some examples, such as during the first dusk or dawn period following installation, or shortly thereafter.

[0065] In some examples, a first technician may observe the first switching time and a second technician may observe the second switching time. In some embodiments, the first technician and the second technician may record the first and second switching times as indicated by synchronized clocks (external from the control units), e.g., synchronized wristwatches, cellular phones, personal digital assistants, or the like. The synchronized clocks may provide a common reference point with which to compare the first switching time and the second switching time. In this manner, the technician may determine a time period corresponding to a lead or lag in the turn-on or turn-off time of first light sources 26 compared to the turn-on-or turn-off time of second light sources 32. This time may correspond to an offset time that can be used by the first control unit 24 (or the second control unit 30) for future synchronization of turn-on and turn-off times for the various light sources.

[0066] In some examples, the technician may determine the offset time for first control unit 24 by determining the difference between the second switching time and the first switching time. In such an example, a negative offset time may indicate that first control unit 24 switched later than second control unit 30, and the offset time may be used to cause first control unit 24 to switch earlier (with respect to its initial switching time based solely on ambient light level) on subsequent days, at substantially the same time second control unit 30 will switch.

Conversely, a positive offset time may indicate that first control unit 24 switched earlier than second control unit 30, and the offset time may be used to cause first control unit 24 to switch later on subsequent days (with respect to its initial switching time based solely on ambient light level), at substantially the same time second control unit 30 will switch. In some examples, instead of determining the offset time for first control unit 24, the technician may determine an offset time for second control unit 30, so that future switching times of second control unit 30 can be adjusted to be substantially the same as those of first control unit 24.

[0067] In some implementations, instead of comparing the first switching time and the second switching time to determine the offset time for the first control unit 24 or second control unit 30, the technician may compare the first switching time to an ephemeris table to determine a first offset time, and may compare the second switching time to the ephemeris table to determine a second offset time. The first offset time and the second offset time may each be positive, may each be negative, or one may be positive and the other may be negative. Additionally, the magnitude of the first offset time and the second offset time may be the same or may be different. In some examples, each of the first offset time and the second offset time may include an integer number of minutes, e.g., an integer between about -15 minutes and about +15 minutes. This method may be expanded to any number of control units to synchronize the light sources controlled by each of the control units to turn-on and turn-off at substantially the same times.

[0068] Once the technician has determined the offset time or the first and second offset times (66), the technician may set the relevant offset time(s) via first offset module 48 in first control unit 24 and/or a corresponding offset module in second control unit 30 (68). In some embodiments, offset module 48 may include a potentiometer, a DIP switch, a coded rotary switch, one or more buttons or a keypad, or the like. In various embodiments, the technician may set the offset time using switches, a rotary knob, a screw that is turned using a tool such as a screwdriver, or the like. In other embodiments, offset module 48 includes circuitry (e.g., transistors and/or memory) connected to control module 42, clock module 44, and/or user interface 52, and the technician may set the offset time via user interface 52.

[0069] FIG. 4 is a flow diagram that illustrates an example of a technique that a control unit may implement to determine a switch-on time for a plurality of light sources. For example, first control unit 24 (or second control unit 30) may use the technique of FIG. 4 to determine when to switch on first light sources 24 (or second light sources 32). For purposes of clarity, FIG. 4 will be described with reference to first control unit 24, although it will be understood that second control unit 30 or another control unit may implement the technique illustrated in FIG. 4. When first control unit 24 implements the method illustrated in FIG. 4, offset module 48 may have previously received the offset time, as described above with respect to FIG. 3.

[0070] Control module 42 may control light sensor 46 to detect an ambient light intensity at or near first control unit 24. In some examples, control module 42 controls light sensor 46 to detect the ambient light intensity substantially continuously throughout each day (e.g., substantially 24 hours per day). In other examples, control module 42 may control light sensor 46 to detect the ambient light intensity only during predetermined periods of time. For example, control module 42 may control light sensor 46 to detect the ambient light intensity during a period of time that includes a predicted time at which the ambient light intensity will decrease from above a threshold light intensity value to below the threshold light intensity value (e.g., a period of time that includes a predicted sunset time).

[0071] In some examples, the period(s) of time during which control module 42 controls light sensor 46 to detect the ambient light intensity may be based on a time at which the ambient light intensity decreased below a threshold light intensity value during the previous day. For example, control module 42 may delay 23 hours and 30 minutes from the time at which the ambient light intensity decreased below the threshold light intensity value and may then control light sensor 46 to detect the ambient light intensity for about 1 hour. The period of time for which control module 42 controls light sensor 46 to detect the ambient light intensity may be any suitable time duration, such as, for example, between about 15 minutes and about 2 hours, such as about 30 minutes or about 1 hour. Other time periods can also be used, such as about 90 minutes, 2 hours, 3 hours, or 4 hours.

[0072] Light sensor 46 may generate an illumination signal based on a detected ambient light intensity and may transmit the illumination signal to control module 42. Control module 42 may receive the illumination signal and compare the illumination signal to a threshold light intensity value (72) to determine whether the ambient light intensity has decreased from above the threshold light intensity value to at or below the threshold light intensity value. As described above, the threshold light intensity value may be any suitable light intensity value. In some examples, the threshold light intensity value is below about 10 lux, such as between about 2 lux and about 8 lux, about 4 lux, or between about 6 lux and about 8 lux.

[0073] When control module 42 determines based on the illumination signal that the ambient light intensity has not decreased from above the threshold light intensity value to below the threshold light intensity value (the "NO" branch of decision block 72), control module 42 continues to receive the illumination signal from light sensor 46 and compare the illumination signal to the threshold light intensity value (72). However, when control module 42 determines based on the illumination signal that the ambient light intensity has decreased from above the threshold light intensity value to below the threshold light intensity value (the "YES" branch of decision block 72), control module 42 proceeds to determine a switch-on time for first light sources 26 based on the offset time and the time at which the ambient light intensity decreased below the threshold light intensity value (74).

[0074] As described above, control module 42 may determine the switch-on time by controlling clock module 44 to initiate measuring or timing when control module 42 determines that the ambient light intensity has decreased from above the threshold light intensity value to below the threshold light intensity value. Clock module 44 may include a timer which is able to accurately measure time periods of about 24 hours, and clock module 44 may proceed to measure a time period of 24 hours plus the offset time (which may be positive or negative), which clock module 44 may receive from offset module 48. When clock module 44 finishes timing the time period of 24 hours plus the offset time, clock module 44 may transfer a signal to control module 42 that indicates the time period is complete. Control module 42 may receive the signal and in response may control switching module 50 to electrically connect electrical source 54 with first light sources 26 via electrical connections 28 (76), which may cause first light sources 26 to receive electrical power, turn on, and provide light.

[0075] In cases where the offset time is positive, the time measured or counted by clock module 44 will exceed 24 hours. In cases where the offset value is negative, the time measured or counted by clock module 44 will be less than 24 hours. For example, for an offset time of one minute, the time measured will be 24 hours and one minute for a positive offset time, and 23 hours and 59 minutes for a negative offset time.

[0076] In some embodiments, control module 42 may cause the time at which the ambient light intensity decreased below (or reached) the threshold light intensity value to be stored in a memory. Control module 42 may utilize this time, a time provided by clock module 44, and the offset time to determine the switch-on time for first light sources 26 during the following day. In some embodiments, clock module 44 may measure time in 24 hours cycles, i.e., may include a clock. Control module 42 may determine a switch-on time for the following day by adding the offset time to the time at which the ambient light intensity decreased below the threshold light intensity value and saving in memory an instruction to cause switching module 50 to connect electricity source 54 and first electrical connections 28 on the following day at the switch-on time (76). In some cases, the clock module may provide a timeout signal when a period of 24 hours plus the offset time (which may be positive or negative) expires, and the control module 42 may cause switching module 50 to actuate at that time.

[0077] In some embodiments, control module 42 may repeat this technique each day, determining the switch-on time for the upcoming day based on the time at which the ambient light intensity decreases below the threshold light intensity value during the current day. This may allow control module 42 to change the switch-on time for first light sources 26 to account for seasonal variations in sunset (dusk) times while allowing synchronization of switch-on times between first control unit 24 and another control unit, e.g., second control unit 30. The timing of the computation to determine switch-on (or switch-off) time can be varied. For example, as described above, the time for the upcoming day may be determined during the current day. Alternatively, the switching time may be determined at some predetermined time following the last switching time or the last ambient light threshold indication (e.g., 21, 22 or 23 hours from one of these times).

[0078] FIG. 5 is a flow diagram that illustrates an example of a technique that a control unit may implement to determine a switch-off time for a plurality of light sources. First control unit 24 and/or second control unit 30 may implement the technique of FIG. 5 to determine a switch-off time for first light sources 26 and/or second light sources 32, respectively. Similar to FIG. 4, when first control unit 24 and/or second control unit 30 implements the technique illustrated in FIG. 5, offset module 48 has already received the offset time, as described above with respect to FIG. 3. For purposes of clarity, FIG. 5 will be described with reference to first control unit 24, although it will be understood that second control unit 30 or another control unit may implement the technique illustrated in FIG. 5.

[0079] Control module 42 controls light sensor 46 to detect an ambient light intensity at or near first control unit 24. In some examples, control module 42 controls light sensor 46 to detect the ambient light intensity substantially continuously throughout each day (e.g., substantially 24 hours per day). In other examples, control module 42 may control light sensor 46 to detect the ambient light intensity only during predetermined periods of time. For example, control module 42 may control light sensor 46 to detect the ambient light intensity during a period of time that includes a predicted time at which the ambient light intensity will increase from below a threshold light intensity value to above the threshold light intensity value (e.g., a period of time that includes a predicted sunrise time).

[0080] In some examples, the period(s) of time during which control module 42 controls light sensor 46 to detect the ambient light intensity may be based on a time at which the ambient light intensity increased above a threshold light intensity value during the previous day. For example, control module 42 may delay 23 hours and 30 minutes from the time at which the ambient light intensity increased above the threshold light intensity value and may then control light sensor 46 to detect the ambient light intensity for about 1 hour. The period of time for which control module 42 controls light sensor 46 to detect the ambient light intensity may be any suitable time duration, such as, for example, between about 15 minutes and about 2 hours, such as about 30 minutes or about 1 hour.

[0081] Light sensor 46 may generate an illumination signal based on the detected ambient light intensity and may transmit the illumination signal to control module 42. Control module 42 may receive the illumination signal and compare the illumination signal to a threshold light intensity value (82) to determine whether the ambient light intensity has increased from below the threshold light intensity value to above the threshold light intensity value. As described above, the threshold light intensity value may be any suitable light intensity value. In some examples, the threshold light intensity value is below about 10 lux, such as between about 2 lux and about 8 lux, about 4 lux, or between about 6 lux and about 8 lux.

[0082] When control module 42 determines based on the illumination signal that the ambient light intensity has not increased from below the threshold light intensity value to above the threshold light intensity value (the "NO" branch of decision block 82), control module 42 continues to receive the illumination signal from light sensor 46 and compare the illumination signal to the threshold light intensity value (82). However, when control module 42 determines based on the illumination signal that the ambient light intensity has increased from below the threshold light intensity value to above the threshold light intensity value (or has reached the value) (the "YES" branch of decision block 82), control module 42 proceeds to determine a switch-off time for first light sources 26 based on the offset time and the time at which the ambient light intensity increased above the threshold light intensity value (84). Control module 42 may implement similar techniques to determine the switch-off time for first light sources 26 as described with respect to FIG. 4 for determining the switch-on time for first light sources 26. When the switch-off time arrives, control module 42 may control switching module 50 to electrically disconnect electrical source 54 from first light sources 26 via first electrical connections 28 (86), which may cause first light sources 26 to stop receiving electrical power, turn off, and stop providing light.

[0083] In some embodiments, control module 42 may repeat the technique illustrated in FIG. 5 each day, determining the switch-off time for the upcoming day based on the time at which the ambient light intensity increases above the threshold light intensity value during the current day. This may allow control module 42 to change the switch-off time for first light sources 26 to account for seasonal variations in sunrise (dawn) times while allowing synchronization of switch-off times between first control unit 24 and another control unit, e.g., second control unit 30.

[0084] Although the embodiments illustrated in FIGS. 3, 4, and 5 have been described separately, in some examples, the embodiments may be implemented together by first control unit 24, second control unit 30, and/or another control unit. For example, first control unit 24 may receive an offset time via offset module 48, as described with respect to FIG. 3, may determine a switch-on time according to an embodiment described with respect to FIG. 4, and may determine a switch-off time according to an embodiment described with respect to FIG. 5. In some cases, separate time offsets for switch-on and switch-off may be used. For example, a switch-on time offset may be used with the technique of FIG. 4, and a different switch-off time offset may be used with the technique of FIG. 5. The time offsets may be determined using the technique of FIG. 3 as described above, according to some examples.

[0085] FIG. 6 is a flow diagram of an example of a technique that a control unit may implement to determine both a switch-on time and a switch-off time for a plurality of light sources based on an ambient light level and an offset time. When first control unit 24 and/or second control unit 30 implements the method illustrated in FIG. 6, offset module 48 may have already received the offset time, as described above with respect to FIG. 3. Although FIG. 6 will be described with reference to first control unit 24 of FIG. 2, it will be understood that second control unit 30 or another control unit may implement the technique illustrated in FIG. 6.

[0086] Initially, as described in further detail with respect to FIG. 4, control module 42 controls light sensor 46 to detect an ambient light intensity at or near first control unit 24. Light sensor 46 may generate an illumination signal based on the detected ambient light intensity and may transmit the illumination signal to control module 42. Control module 42 may receive the illumination signal and compare the illumination signal to a first threshold light intensity value (72) to determine whether the ambient light intensity has decreased from above the first threshold light intensity value to below the first threshold light intensity value.

[0087] The first threshold light intensity value may be any suitable light intensity value. In some examples, the first threshold light intensity value is below about 10 lux, such as between about 2 lux and about 8 lux, about 4 lux, or between about 6 lux and about 8 lux.

[0088] When control module 42 determines based on the illumination signal that the ambient light intensity has not decreased from above the first threshold light intensity value to below the first threshold light intensity value (the "NO" branch of decision block 72), control module 42 continues to receive the illumination signal from light sensor 46 and compare the illumination signal to the first threshold light intensity value (72). However, when control module 42 determines based on the illumination signal that the ambient light intensity has decreased from above the first threshold light intensity value to below the first threshold light intensity value (the "YES" branch of decision block 72), control module 42 may cause the first threshold time to be stored in a memory of first control unit 24 for later use in determining a duration of the night (92). Additionally, control module 42 may determine a switch-on time for first light sources 26 for the following day (74) based on the first threshold time and the offset time, as described above with respect to FIG. 4.

[0089] Control module 42 may again control light sensor 46 to detect the ambient light intensity at or near first control unit 24, as described above with respect to FIG. 5. Light sensor 46 may generate an illumination signal based on the detected ambient light intensity and may transmit the illumination signal to control module 42. Control module 42 may receive the illumination signal and compare the illumination signal to a second threshold light intensity value (82) to determine whether the ambient light intensity has increased from below the second threshold light intensity value to above the second threshold light intensity value. [0090] The second threshold light intensity value may be any suitable light intensity value. In some examples, the second threshold light intensity value may be the same as the first threshold light intensity value. In other examples, the second threshold light intensity value may be different than the first threshold light intensity value. In some examples, the second threshold light intensity value is below about 10 lux, such as between about 2 lux and about 8 lux, about 4 lux, or between about 6 lux and about 8 lux. In one embodiment, both the first threshold light intensity value and the second threshold light intensity value is about 4 lux.

[0091] When control module 42 determines based on the illumination signal that the ambient light intensity has not increased from below the threshold light intensity value to above the threshold light intensity value (the "NO" branch of decision block 82), control module 42 continues to receive the illumination signal from light sensor 26 and compare the illumination signal to the threshold light intensity value (82). However, when control module 42 determines based on the illumination signal that the ambient light intensity has increased from below the threshold light intensity value to above the threshold light intensity value (the "YES" branch of decision block 82), control module 42 proceeds to determine a length of night based on a difference between the second threshold time and the first threshold time (94). In other embodiments, clock module 44 may start a counter when ambient illumination falls below a threshold value (e.g., at dusk) and stop the counter when ambient illumination rises above the same or a different threshold value (e.g., at dawn) to measure a length of the night.

[0092] Control module 42 may then wait to receive a signal from clock module 44 indicating that the switch-on time determined in (74) has occurred. In response to the signal from clock module 44, control module 42 may control switching module 50 to electrically connect electrical source 54 to first light sources 26 via first electrical connections 28 (76), which may cause first light sources 26 to receive electrical power, turn on, and provide light.

[0093] Control module 42 may then cause clock module 44 to begin measuring a time period equal to the determined length of the previous night. When clock module 44 has measured a time equal to the length of the previous night, clock module 44 may send a signal to control module 42, which may control switching module 50 to disconnect electrical source 54 from first light sources 26 via first electrical connections 28 to turn off the lights (86).

[0094] In some embodiments, control module 42 may repeat the technique illustrated in FIG. 6 each day, determining the switch-on time for the following day based on the time at which the ambient light intensity decreases below the threshold light intensity value during the current day, and determining the switch- off time for the following day based on the switch-on time and the duration of the current night. This may allow control module 42 to change the switch-on time and switch-off time for first light sources 26 to account for seasonal variations in sunrise and sunset times while allowing synchronization of switch-on and switch- off times between first control unit 24 and another control unit, e.g., second control unit 30.

[0095] FIG. 7 is a flow diagram that illustrates an example of a technique that first control unit 24 and/or second control unit 30 may implement to determine a switch-on time of first light sources 26 and/or second light sources 32, respectively. When first control unit 24 and/or second control unit 30 implements the method illustrated in FIG. 7, offset module 48 may already have received the offset time, as described above with respect to FIG. 3. For purposes of clarity, FIG. 7 will be described with reference to first control unit 24, although it will be understood that second control unit 30 or another control unit may implement the technique illustrated in FIG. 7.

[0096] Initially, as described above with respect to FIG. 4, control module 42 controls light sensor 46 to detect an ambient light intensity at or near first control unit 24. Light sensor 46 may generate an illumination signal based on the detected ambient light intensity and may transmit the illumination signal to control module 42. Control module 42 may receive the illumination signal and compare the illumination signal to a first threshold light intensity value (72) to determine whether the ambient light intensity has decreased from above the first threshold light intensity value to below the first threshold light intensity value.

[0097] When control module 42 determines based on the illumination signal that the ambient light intensity has not decreased from above the first threshold light intensity value to below the first threshold light intensity value (the "NO" branch of decision block 72), control module 42 continues to receive the illumination signal from light sensor 46 and compare the illumination signal to the first threshold light intensity value (72). However, when control module 42 determines based on the illumination signal that the ambient light intensity has decreased from above the first threshold light intensity value to below the first threshold light intensity value (the "YES" branch of decision block 72), control module 42 may cause a first time, corresponding to the time where the ambient light intensity reached or crossed the threshold, to be stored in a memory of first control unit 24 (102).

[0098] In some embodiments, control module 42 then may apply a filter to the first time (104) prior to determining a switch-on time for first light sources 26 based on the offset time and the time at which the ambient light intensity decreased below the threshold light intensity value (74). In some examples, the filter may include determining an average of the first time and a time at which the ambient light intensity decreased from above the threshold light intensity value to below the threshold light intensity value from at least one previous day. For example, control module 42 may retrieve one or two (or three, four, five, seven, ten, or the like) previously stored times, which may correspond to first times stored on earlier days, from a memory of first control unit 24 and may average the previous time or times with the most recent time to determine an average time value. Control module 42 may then determine a switch-on time for the following day for first light sources 26 based on the average time value, the offset value, and a signal output from clock module 44, as described with respect to FIG. 4.

[0099] By determining an average time, control module 42 may reduce an effect that an anomalous light intensity measurement from light sensor 46. For example, a weather event, such as a thunderstorm, snowstorm, fog, rainstorm, or the like, may result in an ambient light intensity that is less or even significantly less than the light intensity would be at a similar time on a substantially clear (sunny) day. This may cause light sensor 46 to detect a lower ambient light intensity at a given time than if the weather was clear. Accordingly, if control module only considers such a time, the switch-on time for the following day may be abnormally early. However, by determining an average time as described above, control module 42 may reduce these effects. In some examples, control module 42 may determine the average time based on at least three times at which the ambient light intensity decreased from above the threshold light intensity value to below the threshold light intensity value.

[0100] Once control module 42 determines the average time, control module 42 may determine the switch-on time for the following day (74) according to any of the methods described herein, but using the average time instead of a measurement for a single day. In this way, a running average may be kept or determined, and used to determine switch-on (or off) times. On the following day at the determined switch-on time, control module 42 may control switching module 50 to electrically connect electrical source 54 with first light sources 26 via first electrical connections 28 (76) to switch on first light sources 26.

[0101] In other examples, control module 42 may implement another filter at step (102). For example, control module 42 may monitor the illumination signal received from light sensor 46 for a period of time until the observed intensity satisfies the condition for a predetermined period of time to increase a likelihood that the change in ambient light intensity is being caused by, e.g., a setting of the sun, instead of an event such as an object or person temporarily blocking light sensor 46. For example, control module 42 may monitor the illumination signal received from light sensor 46 for about 5 minutes, 10 minutes, 20 minutes, or 30 minutes after determining that the ambient light intensity decreased from above the threshold light intensity value to below the threshold light intensity value. If control module 42 detects that the illumination signal indicates that the ambient light intensity has increased above the threshold light intensity value during this time, control module 42 may interpret the earlier decrease in ambient light intensity to below the threshold value as anomalous and may disregard the crossing of the threshold. If predetermined light sensor watch periods are used as described above, these can be adjusted to account for the filter.

[0102] If, however, control module 42 does not detect that the illumination signal indicates during this time period that the ambient light intensity has returned to above the threshold light intensity value, control module 42 may utilize the initial time at which the ambient light intensity decreased below the threshold light intensity value in determining the switch-on time for first light sources 26 for the following day. Control module then may determine the switch-on time for the following day (74) according to any of the methods described herein, e.g., the methods described with respect to FIG. 4. On the following day at the determined switch-on time, control module 42 may control switching module 50 to electrically connect electrical source 54 to first light sources 26 via first electrical connections 28 (76) to switch on first light sources 26. Although not depicted in FIG. 7, in some examples, control module 42 may implement one or more filters when determining a switch-off time for first light sources 26, such as in the technique illustrated with respect to FIG. 5.

[0103] The techniques described in this disclosure, including those attributed to first control unit 24, second control unit 30, or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more

microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term "processor" or "processing circuitry" may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry.

[0104] In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components.

[0105] When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, EEPROM, FLASH memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed, for example by one or more processors, to support one or more aspects of the functionality described in this disclosure.

[0106] Various embodiments of the disclosure have been described. These and other embodiments are within the scope of the following claims.

Claims

CLAIMS:

1. A system, comprising:

means for sensing an ambient light intensity at a first location;

means for determining a first time at which the sensed ambient light intensity at the first location crosses a threshold light intensity value;

means for receiving an offset time value indicative of a time duration by which a measured ambient light intensity at the first location leads or lags a corresponding measured ambient light intensity at a second location; and

means for determining, based on the first time, a default time period, and the offset time value, a second time and causing an illumination state of a first plurality of light sources to change at the second time.

2. The system of claim 1 , wherein the default time period is twenty-four hours, and wherein the means for determining the second time determines the second time by adding twenty-four hours and the offset time value to the first time.

3. The system of claim 1 or 2, wherein the first time is determined during a period of generally decreasing ambient light intensity at the first location, and wherein the illumination state of the first plurality of light sources is caused to change by a switching means that connects an electricity source to the first plurality of light sources at the third time, thereby causing the first plurality of light sources to illuminate.

4. The system of claim 1 or 2, wherein the first time is determined during a period of generally increasing ambient light intensity at the first location, and wherein the illumination state of the first plurality of light sources is caused to change by a switching means that disconnects an electricity source from the first plurality of light sources at the third time, thereby causing the first plurality of light sources to darken.

5. The system of claim 1 or 2, wherein the first time is determined during a period of generally decreasing ambient light intensity at the first location, and further comprising:

means for determining, during a period of generally increasing ambient light intensity at the first location, a third time at which the sensed ambient light intensity crosses the threshold light intensity value; and

means for determining a nighttime duration based on the first time and the third time.

6. The system of claim 5, further comprising determining a lights-on duration by adding the offset time value to the nighttime duration, and causing the illumination state of the first plurality of light sources to change a lights-on duration of time following the first time.

7. The system of claim 1 , wherein the offset time value is positive, and wherein the measured ambient light intensity at the first location leads in time the corresponding measured ambient light intensity at the second location.

8. The system of claim 1, wherein the offset time value is negative, and wherein the measured ambient light intensity at the first location lags in time the corresponding measured ambient light intensity at the second location.

9. The system of claim 1 or 2, further comprising means for applying a filter to the first time to determine a filtered time, wherein the filter is used to determine an average time based on the first time and at least another time corresponding to a time at which the ambient light intensity at the first location crossed the threshold light intensity value on a previous day, and wherein the means for determining the second time determines the second time based on the filtered time, the default time period, and the offset time value.

10. A method, comprising:

sensing an ambient light intensity at a first location;

determining a first time at which the sensed ambient light intensity crosses a threshold light intensity value;

receiving an offset time value indicative of a time duration by which a measured ambient light intensity at the first location leads or lags a corresponding measured light intensity at a second location; and

determining, based on the first time, a default time period, and the offset time value, a second time and causing an illumination state of a first plurality of light sources to change at the second time.

11. The method of claim 10, wherein the default time period is twenty-four hours, and wherein the second time is determined by adding twenty-four hours and the offset time value to the first time.

12. The method of claim 10 or 1 1, wherein the first time is determined during a period of generally decreasing ambient light intensity at the first location, and further comprising:

determining, during a period of generally increasing ambient light intensity at the first location, a third time at which the sensed ambient light intensity crosses the threshold light intensity value;

determining a nighttime duration based on the first time and the third time; and

causing the illumination state of the first plurality of light sources to change a lights-on duration of time following the first time, wherein the lights-on duration of time is determined by adding the offset time value to the nighttime duration.

13. The method of claim 10, wherein the offset time value is positive, and wherein the measured ambient light intensity at the first location leads in time the corresponding measured ambient light intensity at the second location.

14. The method of claim 10, wherein the offset time value is negative, and wherein the measured ambient light intensity at the first location lags in time the corresponding measured ambient light intensity at the second location.

15. The method claim 10 or 11, further comprising applying a filter to the first time to determine a filtered time, wherein the filter is used to determine an average time based on the first time and at least another time corresponding to a time at which the ambient light intensity at the first location crossed the threshold light intensity value on a previous day, and wherein the second time is determined based on the filtered time, the default time period, and the offset time value.