Patent Description:
The disclosed technology relates generally to detection of a faulted light-emitting diode (LED) of a plurality of LEDs, and more particularly to a mechanism having a plurality of LEDs for illuminating a surface and having the capability of determining whether one of the plurality of LEDs has faulted.

LED based lighting is utilized in a wide array of lighting applications, often with a plurality of LEDs provided in a string. LED strings often are used provide lighting for critical components, such as the back lighting of a television screen or the surface illumination for a sensor or a camera. Failure of one or more LEDs in such a string can lead to subsequent issues with the product being lit, such as unbalanced or wrongly colored lighting where different color LEDs are utilized. Such failure can be difficult to visually recognize where the LED string is hidden or not easily accessible. Failure also can be difficult to discern due to the effect of varying temperature on the LED string or due to minor variations in forward voltages of varying LEDs of the LED string, which aspects may result in a decrease or increase in voltage at a terminal end of the LED string that is difficult to recognize as a respective decrease or increase in voltage as caused by a failed LED.

<CIT> describes a method of identifying a faulty LED which includes receiving a first voltage from a first node associated with a first string of multiple LEDs, receiving a second voltage from a second node associated with a second string of multiple LEDs, and identifying whether at least one of the LEDs has a fault using the first and second voltages.

<CIT> describes an automobile rear light LED assembly which uses a plurality of strings of LEDs connected in parallel so that the LEDs may be directly driven by the automobile's battery voltage. Upon failure of an LED in one of the strings, control circuitry in the LED assembly causes all LEDs in the assembly to be simultaneously disabled. The large current drop is then detected by the conventional bulb burn-out detection circuitry, and the conventional lamp failure alarm is triggered.

The disclosed technology provides an illumination mechanism having a plurality of LEDs for illuminating a surface and having the capability of determining whether one of the plurality of LEDs has faulted while accounting for environment temperature fluctuations and inherent varying forward voltages of same-type LEDs caused by manufacturing. A pair of LED sets is provided, with the sets electrically connected in parallel, such that voltages of the pair of LED sets may be compared to determine whether a difference is representative of a fault of an LED in one of the LED sets of the pair. In some embodiments, connections between the sets may be provided only at terminal connections of each set, such that single LEDs of one set are not otherwise connected to single LEDs of the other set other than via the terminal connections.

The arrangement of the pair of LED sets addresses one or more of the aforementioned issues relating to LED failure. Regardless of at least one of environmental temperature or minor differences in construction of individual LEDs, the mechanism is configured to discern whether at least a single LED of one LED set of the pair has faulted open or closed, by comparison to the other LED set of the pair, which other set does not include the faulted LED. This arrangement has the benefit over more complex arrangements, such as multiplexing of a single LED set.

The arrangement is capable of accounting for the varying environmental temperatures or the varying forward voltages of same-type LEDs which otherwise would require an in-depth understanding of what are often off-the-shelf LEDs having changing tolerances and for which such information is not available. The minimal connections between the two sets further allows for a less complicated and costly circuit. Such configuration is effective in instances where the LED sets are separated from a measurement circuit and/or controller, minimizing the cost, circuit board real estate consumption, and manufacturing complexity associated with additional wiring and connections.

According to the invention, an illumination mechanism includes a first set of LEDs and a second set of LEDs electrically connected in parallel to one another via terminal connections of each of the first and second sets of LEDs, a power source arranged to power the first and second sets of LEDs, a voltage measuring circuitry arranged to separately measure a voltage at each of the first and second sets of LEDs, and a control circuitry in communication with the voltage measuring circuitry. The control circuitry is configured to compare against one another the voltages measured at each of the first and second sets of LEDs and to output a fault indicator where a difference between the measured voltages is discerned as being at least one of less than a minimum predetermined voltage threshold or greater than a maximum predetermined voltage threshold, and wherein the minimum predetermined voltage threshold is a negative of a minimum forward voltage of an LED of the first set of LEDs or the second set of LEDs, and the maximum predetermined voltage is a minimum forward voltage of an LED of the first set of LEDs or the second set of LEDs.

Between the respective terminal connections of the first set of LEDs, each individual LED of the first set of LEDs may be electrically separated from each individual LED of the second set of LEDs disposed between the respective terminal connections of the second set of LEDs and vice versa.

Each of the first and second sets of LEDs can have both a cathode terminal connection and an anode terminal connection, and the voltage measuring circuitry can be arranged to measure the voltage at the same of the cathode terminal connection or the anode terminal connection of each of the first and second sets of LEDs.

The voltage that the voltage measuring circuitry is arranged to separately measure at each of the first and second sets of LEDs can be the voltage drop across each of the first and second sets of LEDs.

The voltage measuring circuitry can be configured to simultaneously measure the separate voltages at each of the first and second sets of LEDs.

The control circuitry can be configured to output the fault indicator where an absolute value of the difference is discerned as being greater than a predetermined voltage threshold.

The fault indicator can be a signal output to a display device of the illumination mechanism.

The control circuitry can be configured to further separately compare each of the voltages measured against at least one of a minimum predetermined voltage threshold or a maximum predetermined voltage threshold and to output a fault indicator indicating at least one of the first or the second sets of LEDs as having faulted.

Each of the first and the second sets of LEDs can include a same number of LEDs.

Each of the LEDs of the first set of LEDs can be serially connected to one another, and each of the LEDs of the second set of LEDs can be serially connected to one another.

According to the invention, a method of recognizing a faulted LED of an LED mechanism illuminating a surface includes (a) illuminating a surface with a pair of sets of LEDs each including a same number of connected LEDs, with the sets being electrically connected in parallel at terminal connections of each of the sets of LEDs in an arrangement preventing current from passing between the sets of LEDs physically disposed between the terminal connections; (b) powering the sets of LEDs; (c) separately measuring a voltage at each of the sets of LEDs; (d) comparing against one another the voltages measured at each of the sets of LEDs; and (e) outputting a fault indicator where a difference between the voltages measured is discerned as being at least one of less than a minimum predetermined voltage threshold or greater than a maximum predetermined voltage threshold, and wherein the minimum predetermined voltage threshold is a negative of a minimum forward voltage of an LED of the first set of LEDs or the second set of LEDs, and the maximum predetermined voltage is a minimum forward voltage of an LED of the first set of LEDs or the second set of LEDs.

The method includes comparing the difference between the voltages measured against at least one of a minimum predetermined voltage threshold or a maximum predetermined voltage threshold and outputting a fault indicator wherein the difference is less than the minimum predetermined voltage threshold or greater than the maximum predetermined voltage threshold.

The method can further include outputting a fault indicator where an absolute value of the difference is discerned as being greater than a maximum predetermined voltage threshold.

The foregoing and other features of the disclosed technology are hereinafter described in greater detail with reference to the accompanying drawings.

The annexed drawings, which are not necessarily to scale, show various aspects of the disclosure.

The principles of the present disclosure have general application to detection of a faulted LED of a plurality of LEDs. The principles have more particular application to a mechanism having a plurality of LEDs for illuminating a surface and further having the capability of determining whether at least one of the plurality of LEDs has faulted over a range of environmental temperatures and taking into account an inherent varying forward voltage range of varying LEDs of the same type or same manufacturer.

Generally, an illumination mechanism according to the disclosed technology includes a plurality of LEDs for illuminating the surface. Circuitry of the illumination mechanism is configured such that it can be discerned electronically rather than directly visually (viewing the LED) whether an LED of the plurality of LEDs has faulted, such as faulting open or closed.

By providing LED sets each having a plurality of LEDs, the need to account for differences in inherent varying forward voltages of individual LEDs is reduced or altogether eliminated where the circuitry of the illumination mechanism is arranged to examine a voltage of a set, and not to examine voltages of individual LEDs. Thus, differences in inherent varying forward voltages are averaged out by use of a plurality of LEDs per set.

Likewise, variances in forward voltages due to environmental temperature changes are negated via comparison of voltages of the LED sets where arranged in parallel with one another. This arrangement provides benefits over circuitry not providing comparison of LED sets, regardless of whether multiplexing is utilized, by discounting the need to understand the effect of environmental temperature on the voltage/current flow of any individual LED or non-compared LED set.

The circuitry also is arranged in a manner that provides a minimum of wiring and connections, and thus allows for minimal manufacturing effort, cost and circuit board real estate consumption. For example, where the plurality of LEDs are separated from a respective measurement circuit or controller and/or where the plurality of LEDs are in location that is difficult or not possible to access either visually or manually, such benefits are particularly relevant.

While the illumination mechanism is described below with reference to an autonomously driven vehicle, the illumination mechanism can have application with optical systems, such as with sensor or photographing systems. For example, the illumination mechanism can be used for illumination absent a lens, or for backlighting a surface, screen, monitor, etc..

Turning first to <FIG>, an autonomously guided apparatus, and specifically an autonomously guided vehicle, is illustrated at <NUM>. The vehicle <NUM> is autonomously guided using a control system <NUM> that communicates with a ground-facing optical element <NUM>, such as a camera or sensor. In one embodiment, the optical element <NUM> includes both a sensor and a camera. Communication between the control system <NUM> and the optical element <NUM> can be by wire or can be wireless.

The illustrated vehicle <NUM> is an autonomously guided mowing vehicle, which utilizes the control system <NUM> to aid in recognizing obstacles and/or boundaries of an area to be serviced, such as mowed or otherwise maintained. The optical element <NUM> of the illustrated vehicle <NUM> is provided for optically acquiring surface images or frames, which are utilized by the control system <NUM> for directing movement of the vehicle <NUM>, such as for determining a direction and magnitude of movement of the vehicle <NUM>.

An illumination mechanism <NUM> is provided with the optical element <NUM> for illuminating the area to be visualized or imaged, which in the case of the vehicle <NUM>, is the ground. For instance, the vehicle <NUM> can use the illumination mechanism <NUM>, optical element <NUM> and control system <NUM> in the mowing of a bounded area, such as a green at a golf course. In such embodiment, surface images or frames acquired with the optical element <NUM> and illumination mechanism <NUM> can be used to direct movement of the vehicle <NUM>. Alternatively, or additionally, the images or frames can be used to recognize the edge of the area to be mowed for effecting a turning of the vehicle <NUM>, such as in a manner that maintains a cutting apparatus (not shown) of the vehicle <NUM> over the area to be mowed.

In an embodiment, the optical element <NUM> can include an active optical sensor that measures relative motion of the vehicle <NUM>. In general, conventional optical sensors include a light source, e.g. light-emitting diodes (LEDS), disposed in a lower part of the sensor that illuminate a plane below where the optical sensor assembly is positioned. Light is reflected by the plane and focused on a lens of the sensor, The focused light is converted into an electrical signal. The electrical signal corresponding to the image data of the illuminated portion of the plane can be processed by a digital signal processor to determine the moving direction and distance travelled of the optical sensor.

One or more optical odometry modules, including an optical sensor assembly, can be utilized for autonomous device navigation, and can be mounted on a chassis of the autonomous device or vehicle <NUM> facing downward in close proximity to the ground. The optical odometry modules incorporate optical sensors, for example, for measuring the distance and direction traveled by the autonomous device. Data obtained from the optical odometry modules can be combined with other absolute or local positioning data to determine a position and heading of the autonomous device.

Turning now to <FIG>, a portion of the vehicle <NUM> is schematically shown to illustrate aspects of the vehicle <NUM>. The optical element <NUM> and illumination mechanism <NUM> each are coupled to a portion of the vehicle <NUM>, such as a body element <NUM> of the vehicle <NUM>. The illumination mechanism <NUM> includes a plurality of LEDs <NUM> that, in the illustrated embodiment, are vertically, downwardly directed to allow for illumination of the ground or surface <NUM> to be mowed. The plurality of LEDs <NUM> are disposed about a lens <NUM> of the optical element <NUM> to allow for relatively equal lighting circumferentially about the lens <NUM>.

Referring next to <FIG>, a high level schematic diagram of the illumination mechanism <NUM> is illustrated. As shown, the illumination mechanism <NUM> includes a first set of LEDs <NUM>, and second set of LEDs <NUM>, a power source <NUM>, a voltage measuring circuitry <NUM> and a control circuitry <NUM>. These elements are connected and arranged to enable illumination of a surface and determination of a fault of at least one LED <NUM> of one of the sets <NUM>, <NUM> of LEDs (first and second sets <NUM> and <NUM> of LEDs). While the connections of the power source <NUM>, voltage measuring circuitry <NUM> and control circuitry <NUM> are shown in one manner in <FIG>, alternative connection arrangements can be suitable.

The power source <NUM> for providing power to the first set <NUM> and the second set <NUM> can be any suitable source, such as a battery, generator, etc., providing suitable DC power or AC power, and any necessary conversion elements.

Generally, the first set <NUM> and the second set <NUM> each include a plurality of LEDs <NUM>, with each of the first set <NUM> and the second set <NUM> being an electrical copy of the other of the first set <NUM> or the second set <NUM>. For example, the LEDs <NUM> of each of the first set <NUM> and the second set <NUM> include the same number of LEDs <NUM> and the same number of each type of LED <NUM> where multiple types are utilized in some embodiments. Each of the first set <NUM> and the second set <NUM> are identically electrically arranged, having the same electrical circuit or portion of electrical circuit. These aspects are provided to enable efficient comparison between the first set <NUM> and the second set <NUM>, such as with respect to a voltage or current at each of the first set <NUM> and the second set <NUM>.

As illustrated, the first set <NUM> of LEDs <NUM> includes a plurality of LEDs <NUM> connected to one another, which connection is illustrated as a series connection of each of the plurality of LEDs <NUM>. The second set <NUM> of LEDs <NUM> likewise includes a plurality of LEDs <NUM> connected to one another, which connection also is illustrated as a series connection of each of the plurality of LEDs <NUM>. Each of the first set <NUM> and the second set <NUM> include a total of six LEDs <NUM> connected in series, labeled D1-D6 of the first set <NUM> and D7-D12 of the second set <NUM>.

In some embodiments, fewer or additional LEDs <NUM> per set can be used, while maintaining a same number of LEDs and same number of each type of LED per set. In some embodiments, a known different number of LEDs of the same type can be included in different sets. In some embodiments, the plurality of LEDs can be arranged other than all in series, while maintaining an identical circuit arrangement of each set. In some embodiments, three or more sets of LEDs can be included, such as being parallel arranged.

Generally, the first set <NUM> of LEDs and the second set <NUM> of LEDs are electrically connected in parallel to one another via terminal connections <NUM> and <NUM> of each of the first and second sets <NUM> and <NUM> of LEDs. Individual LEDs <NUM> of one set are not otherwise connected to single LEDs <NUM> of the other set other than via the terminal connections <NUM> and <NUM>. Multiplexing of the two sets <NUM> and <NUM> is not necessary, but can be used in other embodiments. Rather, between the respective terminal connections <NUM> and <NUM> of the first set <NUM> of LEDs, each individual LED <NUM> of the first set <NUM> is electrically separated from each individual LED <NUM> of the second set <NUM> of LEDs disposed between the respective terminal connections <NUM> and <NUM> of the second set <NUM> of LEDs, and vice versa.

For example, the first and second sets <NUM> and <NUM> are connected at one terminal end at a common anode <NUM> and have constant current sinks. At an opposite terminal end, each of the first and second sets <NUM> and <NUM> includes an individual or separated cathode <NUM>. In some embodiments, the electrical arrangement of the illumination mechanism <NUM> can be suitably arranged such that the first and second sets <NUM> and <NUM> instead have a common cathode and constant current sources with separated anodes.

Provision of the sets <NUM> and <NUM> of LEDs in parallel and comparison between the sets <NUM> and <NUM> reduces or altogether eliminates issues caused by differences in inherent varying forward voltages of individual LEDs and differences in voltages measured at different environmental temperatures. By comparing voltages of the sets <NUM> and <NUM> which are arranged in parallel, differences in inherent varying forward voltages are averaged out by use of a plurality of LEDs per set. Likewise, variances in forward voltages due to environmental temperature changes are negated via allowing for a comparison of voltages of the sets, rather than evaluating a voltage measured at a single LED set, requiring in-depth understanding of the effect of temperature on respective LEDs.

The measuring and comparison of voltages at each of the sets <NUM> and <NUM> of LEDs is accomplished via the measurement circuitry <NUM>, also herein referred to as a measurement circuit or measurement device, and the control circuitry <NUM>, also herein referred to as a controller. While the illustrated measurement circuitry <NUM> is shown separate from the control circuitry <NUM>, the measurement circuitry <NUM> and control circuitry <NUM> can be combined in other embodiments, such as being part of a single integrated element or circuit.

The measurement circuitry <NUM> is configured to measure a voltage of at least one of the sets <NUM> and <NUM>. According to the invention, the illustrated measurement circuitry <NUM> is configured to measure a voltage of each of the sets <NUM> and <NUM>.

The measurement circuitry <NUM> can be configured to measure the voltages of the sets <NUM> and <NUM> simultaneously or consecutively.

In some embodiments, the measurement circuitry <NUM> can be configured to measure a current of at least one of the sets <NUM> and <NUM>, such as to aid in determining whether an LED <NUM> has faulted open.

Generally, the measurement circuitry <NUM> is configured to measure the voltage at the same of the cathode terminal connection <NUM> or the anode terminal connection <NUM> of each of the sets <NUM> and <NUM> of LEDs. For example, the illustrated measurement circuitry <NUM> is arranged in the circuit with the sets <NUM> and <NUM>, to measure an "output" voltage of each of the sets <NUM> and <NUM> at the individual cathode terminal connections <NUM>. In other embodiments, the measurement circuitry <NUM> can be arranged in the circuit to measure voltage at each of the anode and cathode terminal connections <NUM> and <NUM>, or to measure the voltage drop across each of the sets <NUM> and <NUM>.

The measurement circuitry <NUM> can include any one or more suitable elements for measuring and/or comparing measured voltages of the sets <NUM> and <NUM> of LEDs, such as a comparator, null detector, digital logic gate, application specific integrated circuit, programmable logic device, analog-to digital converter, processor or memory, which list is not meant to be limiting. Where the measurement circuitry <NUM> includes a processor and memory, the memory can be provided for storing instructions, the instructions being executable by the processor to carry out measurement of voltages of each of the first and second sets <NUM> and <NUM> of LEDs.

The measurement circuitry <NUM> further is configured to communicate the results of the measuring with the control circuitry <NUM>. Communication between the measurement circuitry <NUM> and the control circuitry <NUM> involves one or more output voltages being provided by the measurement circuitry <NUM>, whether initiated by the measurement circuitry <NUM> or upon request by the control circuitry <NUM>.

Communication between the measurement circuitry <NUM> and the control circuitry <NUM> can include an electrical signal being transferred wired or wirelessly, through any suitable communication connection, such as Wi-Fi, Ethernet, Bluetooth, token ring, Zigbee, CAN Bus, I2C, SPI, UART, etc..

Generally, the control circuitry <NUM> is configured to communicate with the measurement circuitry <NUM> and to analyze the output of the measurement circuitry <NUM>, such as analyzing voltages measured by the measurement circuitry <NUM> and outputting a fault indication after the analysis. To provide these functions, the control circuitry <NUM> can include any suitable element or elements, such as a processor, memory, application specific integrated circuit or programmable logic device, which list is not meant to be limiting. Where the control circuitry <NUM> includes a processor and memory, the memory can be provided for storing instructions, the instructions being executable by the processor to carry out one or more analysis steps to be described herein.

In use, the control circuitry <NUM> is configured to output a fault indicator after analysis of the voltages measured by the measurement circuitry <NUM>. The control circuitry <NUM> analyzes the voltages measured and at least discerns a difference between the measured voltages, which difference is indicative of a faulted LED, whether faulted short or open. A fault indicator is subsequently output, indicating that a difference, and particularly a non-zero difference, between the measured voltages was found. Accordingly, in the illustrated embodiment, where at least one LED <NUM> of one of the sets <NUM> and <NUM> of LEDs has faulted, a difference will be discerned between the voltages measured at the cathodes <NUM>.

According to the invention, the control circuitry <NUM> is configured to output a fault indicator where the difference is discerned as being at least one of less than a minimum predetermined voltage threshold or greater than a maximum predetermined voltage threshold. The minimum predetermined voltage threshold is a negative of the minimum forward voltage of an LED <NUM> and the maximum predetermined voltage is a minimum forward voltage of an LED <NUM>. In one example, where the difference is indicative of a minimum forward operating voltage of a single LED <NUM> of the sets <NUM> and <NUM> of LEDs, the difference is thus indicative of a faulted LED, and specifically a faulted short LED.

In some embodiments, the control circuitry <NUM> is configured to output a fault indicator where an absolute value of a difference between the measured voltages is discerned as being greater than a predetermined voltage threshold. For example, the predetermined voltage threshold can be a minimum forward voltage of an LED <NUM>. The control circuitry <NUM> can be configured to output an absolute value of the difference between the voltages measured.

The control circuitry <NUM> can be configured to analyze the voltages measured on demand by a user or can be configured to analyze the voltages upon completion of a certain event, such as at startup of the illumination mechanism <NUM>. Alternatively, the control circuitry <NUM> can include a timing element and can be configured to analyze the voltages measured a predetermined number of times per time unit, such as once per second, or analysis can occur in conjunction with pulse-width-modulation of the sets <NUM> and <NUM>, such as timed with startup.

The fault indicator output by the control circuitry <NUM> can be a signal transmitted to any of a display screen, an LED, a device outputting an audible signal, or another controller, any of which can be included as part of the illumination mechanism <NUM> or can be separate from the illumination mechanism <NUM>. Transmission of the fault indicator can be wired or wireless, through any suitable communication connection, such as Wi-Fi, Ethernet, Bluetooth, token ring, Zigbee, CAN Bus, I2C, SPI, UART, etc..

In some embodiments, the measurement circuitry <NUM> and the control circuitry <NUM> can be integrated into a single element or circuit. In such case the combined control element/circuit will be configured to measure a voltage at each of the first set <NUM> and the second set <NUM>, to at minimum determine whether there is a difference between the measured voltages, and to output a fault indicator where a difference is discerned.

In summary, an illumination mechanism <NUM> has a plurality of LEDs <NUM> for illuminating a surface <NUM> and the capability of determining whether one of the plurality of LEDs <NUM> has faulted. A pair of LED sets <NUM> and <NUM> is provided with the sets <NUM> and <NUM> electrically connected in parallel via terminal connections <NUM> and <NUM> of each set <NUM> and <NUM> of LEDs. Regardless of at least one of environmental temperature or minor differences in minimum forward voltages of individual LEDs <NUM>, the illumination mechanism <NUM> is configured to discern whether at least a single LED <NUM> of one set of LEDs <NUM> of the pair has faulted. In instances where the sets <NUM> and <NUM> of LEDs are separated from a controller <NUM> or voltage measurement circuitry <NUM>, the cost, circuit board real estate consumption, and manufacturing complexity associated with additional wiring and connections are minimized.

The present disclosure also provides a method of recognizing a faulted LED <NUM> of an LED mechanism <NUM> illuminating a surface <NUM>. The method includes the step of (a) illuminating a surface <NUM> with a pair of sets <NUM> and <NUM> of LEDs <NUM> each including a same number of connected LEDs <NUM>, with the sets <NUM> and <NUM> being electrically connected in parallel at terminal connections <NUM> and <NUM> of each of the sets <NUM> and <NUM> of LEDs in an arrangement preventing current from passing between the sets <NUM> and <NUM> of LEDs physically disposed between the terminal connections <NUM> and <NUM>. The method further includes the steps of (b) powering the sets <NUM> and <NUM> of LEDs, (c) separately measuring a voltage at each of the sets <NUM> and <NUM> of LEDs, (d) comparing against one another the voltages measured at each of the sets <NUM> and <NUM> of LEDs, and (e) outputting a fault indicator where a difference is discerned between the voltages measured.

The method includes comparing the difference between the voltages measured against at least one of a minimum predetermined voltage threshold or a maximum predetermined voltage threshold and outputting a fault indicator where the difference is less than the minimum predetermined voltage threshold or greater than the maximum predetermined voltage threshold.

The method can include outputting a fault indicator where an absolute value of the difference is discerned as being greater than a predetermined voltage threshold.

Claim 1:
An illumination mechanism, comprising:
a first set (<NUM>) of light-emitting diodes (LEDs) and a second set (<NUM>) of LEDs electrically connected in parallel to one another via terminal connections (<NUM>, <NUM>) of each of the first (<NUM>) and second sets of LEDs;
a power source arranged to power the first (<NUM>) and second (<NUM>) sets of LEDs;
a voltage measuring circuitry (<NUM>) arranged to separately measure a voltage at each of the first (<NUM>) and second (<NUM>) sets of LEDs; characterized by:
a control circuitry (<NUM>) in communication with the voltage measuring circuitry and configured to compare against one another the voltages measured at each of the first (<NUM>) and second (<NUM>) sets of LEDs and to output a fault indicator where a difference between the measured voltages is discerned as being at least one of less than a minimum predetermined voltage threshold or greater than a maximum predetermined voltage threshold, and
wherein the minimum predetermined voltage threshold is a negative of a minimum forward voltage of an LED of the first set (<NUM>) of LEDs or the second set (<NUM>) of LEDs, and
the maximum predetermined voltage is a minimum forward voltage of an LED of the first set (<NUM>) of LEDs or the second set (<NUM>) of LEDs.