Abstract:
A method and circuit arrangement dynamically calibrate a photosensitive control for a light source that includes a voltage divider circuit coupled to a resistive light detector and configured to output to a comparison circuit a variable voltage signal that varies with the resistance of the resistive light detector. To calibrate the photosensitive control, a variable impedance circuit in the voltage divider circuit is adjusted to null out any changes in voltage caused by feedback from the controlled light source. The amount of correction is proportional to the amount of light feedback. Based on the amount of correction needed, a new reference voltage is selected that will accurately detect the next dusk to dawn transition while the light feedback is present.

Description:
FIELD OF THE INVENTION  
     The invention is generally directed to the control of a light source responsive to ambient light. 
     BACKGROUND OF THE INVENTION  
     Photosensitive controls are utilized in a number of environments where it is desirable to turn a light source on or off depending upon the amount of ambient light. For example, in landscape lighting applications, it may be desirable to automatically turn lights on at dusk and turn lights off at dawn, or alternatively, after a fixed number of hours after dusk. In addition, it may be desirable in some motion sensing or security applications to sense the amount of ambient light to prevent a motion-sensitive light from turning on during the day. One challenge that is encountered with respect to photosensitive controls, however, results from the feedback of light from a controlled light source to the light detector used in determining the ambient light level. In some photosensitive controls, for example, a light detector output is compared to a static threshold that the light source is turned on when the ambient light falls below that threshold, and turned off when the ambient light rises above that threshold. However, when a light source is turned on, a portion of the generated light may be detected by the light detector, and may cause the detector input to rise above the static threshold, and cause the photosensitive control to turn the light back off. In some instances, the light source may flicker or repeatedly cycle on and off as a result of the feedback of light from an activated light source. 
     Some attempts to minimize the effect of feedback have included shielding a light detector or otherwise placing the light detector in a location that minimizes the amount of light from the controlled light source that is fed back to the detector. However, depending upon where the light source and light detector are installed, surrounding structures such as walls and other reflective surfaces may nonetheless reflect light from the light source back to a light detector. As a result, the amount of light feed back to a light detector may vary from installation to installation, and is thus difficult to eliminate through shielding or placement of the light detector. 
     Additional attempts to minimize the effects of feedback include using hysteresis to set different on and off thresholds, thus requiring a greater amount of ambient light to be detected to turn a light source off than that used to turn the light source on. It has been found, however, that increasing the “window” between on and off thresholds can inhibit accurate dawn detection, particularly on overcast days. 
     Other attempts to minimize the effects of feedback include dynamically setting thresholds based on the amount of ambient light sensed by a light detector. One conventional implementation, for example, monitors the infrared output of a fluorescent light and sets an off threshold based upon the amount of infrared light sensed after the fluorescent light is turned on, typically after waiting until the rate of change of the infrared output has decreased and the output has stabilized. Also, in this implementation, a rate of change of the light detector output may be used along with the absolute output to minimize the effects of rapid changes in the light detector output. 
     One problem associated with the aforementioned implementation, however, is that sensing the rate of change of a light detector output typically requires relatively complex processing. Moreover, sensing the rate of change may limit the overall responsiveness of the light detection circuit. 
     Therefore, what is needed is a simple and responsive photosensitive control that reduces the adverse effects of feedback from a controlled light source. 
     SUMMARY OF THE INVENTION  
     The invention addresses these and other problems associated with the prior art by providing a method and circuit arrangement that dynamically calibrates a photosensitive control for a light source. In particular, a photosensitive control consistent with the invention includes a voltage divider circuit coupled to a resistive light detector and configured to output to a comparison circuit a variable voltage signal that varies with the resistance of the resistive light detector. To calibrate the photosensitive control, a variable impedance circuit in the voltage divider circuit including, for example, a variable resistor, is adjusted to bias the variable voltage signal. 
     These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the Drawings, and to the accompanying descriptive matter, in which there is described exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a block diagram of a photosensitive control for a light source consistent with the invention. 
         FIG. 2  is a flowchart illustrating exemplary steps utilized in a reset routine executed by the photosensitive control of  FIG. 1 . 
         FIG. 3  is a flowchart of the self-calibrate routine referenced in  FIG. 2 . 
         FIG. 4  is a block diagram of an exemplary implementation of a calibration circuit utilized in the photosensitive control of  FIG. 1 . 
         FIG. 5  is a flowchart illustrating exemplary steps utilized in a reset routine executed by the photosensitive control of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION  
     Turning to the drawings, wherein like numbers denote like parts throughout the several views,  FIG. 1  illustrates a photosensitive control  10  consistent with the invention. Photosensitive control  10  is used to control a light source  12 , which may include, for example, one or more incandescent or fluorescent light sources, among other sources of the light. In the alternative, control  10  may be used to power a wireless transmitter such as an RF transmitter for activating a remote light source in response to a logical activation signal. Control  10  includes a light controller or processor  14  to which is coupled a light detector  16 . Light controller  14  may be implemented, for example, as an integrated circuit chip, while light detector  16  may be implemented using any known photosensitive detector or sensor, e.g., a photoconductive sensor such as a cadmium sulfide (CdS) detector, photodiode, phototransistor, etc. 
     In the illustrative embodiment, light detector  16  is implemented as a resistive light detector, wherein the resistance or impedance of the detector varies with the amount of light incident on the detector. Other light detector implementations may be used in the alternative. 
     AC power to photosensitive control  10  is provided via lines  18 ,  20 , with a power supply  22  used to regulated and convert the AC power to DC power for use by controller  14 . The device may also be DC, battery, solar powered, etc. in which case a simpler power supply may be used, or the power supply may be eliminated completely if unnecessary. A power control block  24 , e.g., a relay or other switching device, is coupled between lines  18 ,  20  in series with light source  12 , and is controlled by light controller  14  to selectively power light source  12 . 
     In the illustrative embodiment, photosensitive control  10  additionally has motion sensing capability, whereby one or more motion sensors  26 , e.g., passive infrared (PIR) sensors, are coupled to a network of cascaded amplifiers, e.g., including an external amplifier circuit  28  and additional integrated amplifiers  30  in light controller  14 . 
     It may also be desirable in some implementations to provide a line conditioning circuit  32  for the purpose of providing light controller  14  with a time base from the AC power lines  18 ,  20 . The time base may be used for timing on and off times, as well as for sensing power fluctuations or failures, e.g., due to electrical storms or other power outages, and thereby modify the operation of the photosensitive control based upon such detected fluctuations. 
     Photosensitive control  10  may be used in a wide variety of applications, and may utilize a number of known functions in the control of a light source consistent with the invention. For example, light controller  14  may be configured to activate a light source responsive to motion detected via one of sensors  26 , and thereafter deactivate the light after expiration of a fixed timer. Furthermore, activation of the light source may further be conditioned upon the level of ambient light so that the light source will not be turned on in response to detected motion during the daytime. It may also be desirable to provide a manual override function whereby the light source may be activated irrespective of whether motion is sensed. 
     It will be appreciated that the invention may be utilized in a wide variety of other photosensitive control applications consistent with the invention. For example, the invention may be utilized in any application where it is desirable to control the activation of a light source based upon ambient light level, including non-motion sensing applications. 
     Now turning to  FIG. 2 , an exemplary reset routine  50 , executed by light controller  14  upon initial reset and power up of light controller  14 , is illustrated in greater detail. Routine  50  begins in block  52  by initially setting the controller to a “day” or off state and deactivating the light source. Control then passes to block  54  to set a state transition threshold to a night detect threshold, i.e., a level of detected ambient light below which the photosensitive control will transition from a day (off) to night (on) state. In the illustrative embodiment, it is assumed that with the light source deactivated in such a state, the amount of extraneous ambient light that is not reflective of the time of day will be negligible, so a default, static threshold is selected for the night detect threshold. In other implementations, however, it may be desirable to dynamically generate the night detect threshold instead. 
     Next, control passes to block  56  to monitor the ambient light level with the light detector, and then to block  58  to determine whether the night detect threshold has been met. If the threshold is not met, light controller  14  continues to monitor the ambient light level by returning control to block  56 . 
     Otherwise, if the night detect threshold has been met (e.g., where the ambient light level falls below the night detect threshold), control passes to block  60  to set the controller in a night (on) state and active the light source. Control then passes to block  62  to perform a self-calibrate routine, which dynamically sets a day detect threshold that is used in determining when to switch back to the day (off) state. 
       FIG. 3 , for example, illustrates one suitable implementation of self-calibrate routine  62 . In particular, route  62  begins in block  64  by initiating a delay for a predetermined amount of time to allow the light source to reach a relatively steady state, e.g., about 3 to 5 seconds. Next, block  66  detects the ambient light level with the light detector, and thereafter block  68  dynamically generates the day detect threshold based upon the detected ambient light level. 
     Returning to  FIG. 2 , once the day detect threshold has been dynamically generated, control passes to block  70  to monitor the ambient light level with the light detector. Based upon whether the day detect threshold is met, block  72  either returns control to block  70  (if the threshold is not met) or passes control to block  52  (if the threshold is met), the latter condition returning the controller to the day (off) state and deactivating the light source. 
     It will be appreciated that routine  50  may directly active a light source, or in the alternative, may simply enable activation of the light source, where the actual activation of the light source is further conditioned on additional criteria. For example, in a motion sensing implementation, it may be desirable for routine  50  to simply enable and disable activation of a light source during the night and day states, respectively, so that the light source will be turned on in response to motion detected by a motion sensor only when the controller is in the night state. 
     It will also be appreciated that, while self-calibrate routine  62  is shown being executed to dynamically generate a threshold only after the controller transitions from an “off” state to a “on” state, routine  62  may also be executed to generate a threshold in a number of different circumstances. For example, routine  62  may be executed when switching a light source between different luminance levels, e.g., when switching between bright and dim modes. Also, as noted above, routine  62  may be executed upon switching from an “on” state to an “off” state, e.g., as opposed to setting a static threshold as is done in block  54  of routine  50 . Other modifications will be apparent to one of ordinary skill in the art having the benefit of the instant disclosure. 
       FIG. 4  next illustrates one specific implementation of a calibration circuit  80  consistent with the invention.  FIG. 4 , in particular, illustrates an exemplary implementation partially integrated into light controller chip  14 , with the components to the right of dashed line  84  being integrated into light controller chip  14 , and the components to the left of line  84  being disposed external to the chip. Calibration circuit  80  includes a voltage divider circuit  82  coupled to one input of a comparison circuit, e.g., coupled to the positive (+) input of a comparator C 1 . 
     Voltage divider circuit  82  is coupled between power (VDD) and ground, and includes a common node  86  coupled to the positive input of comparator C 1 . Coupled between VDD and common node  86  is a variable impedance circuit  88  comprising a parallel arrangement of a fixed resistor RI and a variable resistor RDAC. Light detector  16 , implemented as a resistive CdS detector (denoted in  FIG. 4  as RCDS), is coupled between common node  86  and ground. As will be discussed in greater detail below, resistors R 1  and RDAC provide a variable impedance capable of biasing a variable voltage signal that varies with the level of ambient light sensed by light detector  16  and that is output to the positive input of comparator C 1 . 
     Coupled to the negative (−) input of comparator C 1  is a reference signal generation circuit  90  comprising an adder Al and a series of switches S 1 –S 6 . Adder A 1  has a positive (+) input coupled to a fixed reference voltage, e.g., VDD/2 volts, or 2.5 volts where VDD=5 volts, for example. The negative (−) input to adder A 1  is coupled to a plurality of discrete offset voltages via switches S 1 –S 6 . Each switch is a digitally-controlled switch which, when closed, passes one of a plurality of offset voltages to adder A 1  and thus decrease the reference voltage output thereby. In the illustrative embodiment, for example, six discrete offset voltages may be selected via switches S 1 –S 6 , including 0.075 volts, 0.100 volts, 0.200 volts, 0.300 volts, 0.500 volts, and 0.800 volts. It will be appreciated that other offset voltage generating circuits, e.g., that generate non-discrete offset voltages, may be used in the alternative. 
     Also in the illustrative implementation, variable resistor RDAC is implemented as a digitally-controlled resistor, e.g., a resistive digital to analog converter (RDAC) including a R-2R ladder arrangement, as is well known in the art. In the illustrative embodiment, for example, the RDAC may be implemented as a 10-bit converter having a maximum resistance of about 250 KOhm. The impedance of the R-2R ladder may be controlled, for example, by a digital counter. The impedance of the RDAC would then be directly proportional to the count value chosen by this counter. It may also be desirable to implement resistor R 1  with a resistance of about 39 KOhm, such that the effective resistance of the parallel configuration of resistors R 1  and RDAC has a maximum resistance of about 33.7 KOhm. It will be appreciated, however, that other circuitry capable of providing a variable impedance to bias the variable voltage signal generated by light detector  16 , e.g., using various combinations of other resistors, capacitors, inductors, current sources, active components, etc., may be used as an alternative to the parallel arrangement of resistors R 1  and RDAC. 
     During normal daytime operation, the impedance of RDAC would be set to maximum and switches S 1  through S 6  would be open. As the ambient light levels decrease, the impedance of RCDS will increase and eventually the voltage at node  86  will rise above the reference voltage V DD/2 , or for example, 2.5V and the output of comparator C 1  will change states. Assuming that the external light source is turned on at the time, light feedback will decrease the impedance of RCDS and the voltage at node  86  will decrease to an extent that depends directly upon the amount of light that is fed back. In general, calibration circuit  80  operates by first adjusting resistor RDAC to bias the variable voltage signal at node  86  until the variable voltage is greater than or equal to 2.5 volts. This action effectively cancels the error caused by any light feedback. Thereafter, the reference voltage is generated based upon the count in the RDAC used to bias the variable voltage signal. The offset is selected for different ranges of count values, although alternate formulas or algorithms may be utilized in the alternative. For example, it may be desirable to provide an offset of 0.075 volts for a count value below 21, an offset voltage of 0.100 volts for a count between 21 and 30, an offset voltage of 0.200 volts for a count between 31 and 50, an offset voltage of 0.300 volts for a count value between 51 and 100, an offset voltage of 0.500 volts for count value between 101 and 225, and an offset voltage of 0.800 volts for a count value greater than 226. 
     As such, calibration circuit  80  generally provides a variable threshold based upon the sensed ambient light. Of note, this variable threshold may also be considered to be a variable window between the switch off and switch on thresholds. 
     It will be appreciated that the profile of such a variable window may vary in different implementations of the invention. Generally, it is desirable in many implementations to set the comparator offset voltage to be large enough to provide adequate head room when the feedback luminance is relatively small, but is desirably is kept as small as possible to minimize errors when the feedback luminance is relatively large. 
       FIG. 5  next illustrates an exemplary reset routine  100  that may be executed by light controller  14  upon initial power up when the calibration circuit of  FIG. 4  is utilized in a photosensitive control consistent with the invention. It will be appreciated that routine  100  may be implemented at least partially in software or via other programmable circuitry. 
     Routine  100  begins in block  102  by setting the controller to a day state and deactivating the light source. Thereafter, a delay is implemented in block  104  to allow the light source to fully shut off. Next, block  106  adjusts the RDAC resistor to its maximum (default) resistance, and block  108  sets the comparator reference voltage to 2.5 volts, i.e., with no offset voltage. Blocks  106  and  108  therefore have the functionality of setting for the light controller a default night detect threshold. 
     Next, block  110  waits until the comparator output goes high, indicating that the ambient light level has fallen below the detect threshold. Control then passes to block  112  to set the controller to night state, and activate the light source. 
     Next, block  114  waits a predetermined time period, e.g., about 3 to 5 seconds, and block  116  then progressively adjusts the RDAC to bias the variable voltage input at comparator C 1  to the largest value at which the variable voltage is about 2.5 volts (e.g., the last value before the comparator changes state). Block  118  then latches the count value for the RDAC resistor, and based upon this latched value, block  120  selects the comparator reference offset value as described above, and activates the appropriate switch S 1 –S 6 . 
     Block  122  then waits until the comparator output goes low, indicating that ambient light level has increased above the threshold dynamically generated in blocks  116 – 120 . Once the comparator output is detected at low, block  122  then passes control to block  102  to set the light controller to day state and deactivate the light source, as described above. 
     Various additional modifications may be made to the illustrated embodiments without departing from the spirit and scope of the invention. The invention is therefore defined in the claims hereinafter appended.