Light-sensitive switch structure and method with inverse off/on ratio
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 828,370 filed February 11, 1986 U.S. Pat. No. 4,771,186. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to condition-sensitive switching structures and methods and refers more specifically to a light-sensitive electrical switch and method useful for controlling street lamps or decorative and/or safety lighting so that they are energized during the hours of darkness. 
     2. Description of the Prior Art 
     The structures commonly used at the present time for controlling street lights include photo-sensitive elements. These structures are simple, inexpensive, and operate automatically and reliably for a number of years. They generally consist of a photo-sensitive element the resistance of which varies inversely as the light striking it, a time delay relay with normally closed contacts to control the load and, occasionally, some resistors to control the current through the operating coil of the relay. The time delay feature is needed to prevent the control from responding to transient changes in light level such as may be caused by lightning flashes or automobile headlights. The normally closed contacts provide &#34;fail-on&#34; operation of the street lights in response to the most likely failure modes of the control. 
     Adjustment of the turn-on light level in accordance with prior methods for controlling street lights is made by adjusting the drop-out current through the relay. This may be accomplished by controlling the light which strikes the photo-sensitive element, limiting the relay current with resistors, and/or mechanically adjusting the force required to operate the relay contacts. Once the turn-on light level has been selected, the turn-off light level is automatically fixed also. And, since the relay&#39;s pull-in current is always higher than the drop-out current, the turn-off light level will always be higher than the turn-on light level. This turn-off light level to turn-on light level ratio, typically, varies anywhere from 2:1 to 5:1. 
     For a number of reasons, it is desirable to turn on the street lights in the evening earlier than when they are actually needed. These reasons have to do with the physiological effects that the waning evening light has on the human eye and also with the fact that some street lights presently in use require a period of warm-up time before they reach full brilliance. However, this means that the lights are turned off in the morning at a light level which is from two to five times higher than the turn-on level in the evening. In fact, it is satisfactory to turn the lights off in the morning at a light level which may be three or four times less than the turn-on light level in the evening, depending on its value. Thus a light-sensitive switch with a turn-off to turn-on ratio of less than 1:1 would be desirable. Ratios of 1:3 or 1:4 would be ideal if relatively high turn-on values are required in the evening. Such a control would promptly turn off the lighting load when it was no longer needed, thereby saving substantial amounts of energy. 
     The prior art discloses efforts to address this problem by designing light-sensitive switches with off/on ratios as close to 1:1 as possible (see U.S. Pat. No. 3,351,762 and U.S. Pat. No. 4,095,100 for a description of some of these efforts). The difficulty with these controls is that their operation may become unstable. When the light level is near the operating point in the evening or in the morning, slight variations in light intensities, such as may be caused by clouds, may cause the control to cycle on and off. This mode of operation is disturbing to the viewer and may also cause damage to some of the lights presently in use. 
     SUMMARY OF THE INVENTION 
     The present invention is a light-sensitive switch for and method of turning a load, such as a street lamp, on or off in response to a varying condition such as light light level. The structure and method of the invention provide for automatically turning the load off in the morning at a light level which is lower than the light level at which the load was turned on in the evening, thus providing an inverse turnoff to turn-on ratio. Such operation is accomplished in accordance with the method of the invention by using an auxiliary relay to increase, during the hours of darkness, the current through the relay which controls the load. The stability of the control is assured by providing the biasing current upon reaching a light level which is lower than the turn-off light level in the morning. Furthermore, the amount of biasing current supplied will cause the total current through the load control relay to be close, but not equal, to the load control relay pull-in value. The load control relay pull-in value will only be reached when the current is increased in response to the increasing light level in the morning. Such operation may also be accomplished in accordance with the method of the invention by using electronic components, such as operational amplifiers, to compare the voltage level established by the photo-resistor element in conjunction with a another resistor and reference voltage levels established by settable voltage dividers and performing the desired switching based on these comparisons. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of light-sensitive switch structure constructed in accordance with the invention for practicing the method of the invention shown connected to a source of alternating current electrical energy and to a load. 
     FIGS. 2 through 8 are graphs useful in explaining the operation of the circuit of FIG. 1 in accordance with the method of the invention showing, as a function of time, the evening and morning variations in light level, the corresponding variations in the resistance of the photo-sensitive element of the circuit shown in FIG. 1, the current through the operating coil of the load control relay shown in FIG. 1 and the status (open or closed) of its normally closed contacts, the current through the operating coil of the auxiliary relay shown in FIG. 1 and the status of the normally open and normally closed contacts of the auxiliary relay, respectively. 
     FIG. 9 is a schematic diagram of a second embodiment of the invention. 
     FIG. 10 is a schematic diagram of a third embodiment of the invention. 
     FIG. 11 is a schematic diagram of a fourth embodiment of the invention utilizing electronic components to switch the load and including a load control relay with normally closed contacts. 
     FIG. 12 is a schematic diagram of a fifth embodiment of the invention utilizing electronic components to switch the load and including a load control relay with normally open contacts. 
    
    
     DETAILED DESCRIPTION 
     The embodiment shown in FIG. 1 is useful in explaining in detail the method of operation of the present invention. This embodiment consists of a light-sensitive switch 10 including a photo-sensitive element or photo-resistor 11, whose resistance varies in inverse proportion to the light striking it, a load control relay 12, an auxiliary relay 13, and two current limiting resistors 14 and 15. 
     In this embodiment the load control relay 12 and the auxiliary relay 13 are are both time delay thermostatic switches each having a bimetallic actuating arm in thermal contact with their respective heating coils 19 and 16. The load control relay 12 also has a set of normally closed contacts 20, while the auxiliary relay 13 has a set of normally open contacts 17 and a set of normally closed contacts 18. 
     The components of the switch 10 are intended to be assembled and mounted in a plug unit which is known in the prior art and is not illustrated here (see U.S. Pat. No. 3,048,833). This unit is equipped with three contacts or prongs protruding through the base and designed to mate with a corresponding socket on the street light control fixture in accordance with the requirements of American National Standard ANSI C136.10-1979 (it can also be assembled in different configurations to satisfy other applications). These prongs are identified as P1, P2, and P3 in the schematic diagram of FIG. 1 and are used for connecting the switch 10 to the alternating current (AC) electrical energy source 21 and to the load 22. Spark gaps and other components may also be added to provide protection against lightning and other line surges. 
     As illustrated in FIG. 1, the photo-sensitive element 11 is connected on one side to the prong, or terminal, P1 which, when the switch 10 is plugged into its mating socket, will connect the photo-sensitive element 11 to the line side of the AC energy source 21. On the other side the photo-sensitive element 11 is connected to one side of two parallel branch circuits. 
     The first of the parallel branch circuits consists of current limiting resistor 14 connected in series with the heating coil 16 of the auxiliary relay 13. The second of the parallel branch circuits consists of another current limiting resistor 15 connected in series with the heating coil 19 of the load control relay 12. 
     The other side of each of the two parallel branch circuits is, in turn, connected to prong P3. Thus, when the switch 10 is plugged into its mating socket, the connection of the photo-sensitive element 11 and parallel branch circuits to the AC energy source 21 is completed. 
     The normally open contacts 17 of the auxiliary relay 13 are connected in parallel with current limiting resistor 14. Thus the resistor 14 will be short-circuited when the auxiliary relay 13 is energized. 
     The normally closed contacts 18 of the auxiliary relay 13 are connected in parallel with current limiting resistor 15. Therefore, resistor 15 will be short-circuited when the auxiliary relay 13 is de-energized. 
     The normally closed contacts 20 of the load control relay 12 are connected between prongs P1 and P2 of the switch 10. When the switch 10 is plugged into the mating socket, the prongs will connect the load 22 to the AC energy source 21 provided the contacts 20 are closed which, in turn, will occur when the load control relay 12 is de-energized. 
     The graphs of FIGS. 2 through 8 in conjunction with the schematic diagram of FIG. 1, will be used to describe the daily sequence of operation of the light-sensitive switch 10 in accordance with the method of the invention. 
     During the daylight hours, when the light level is high, the resistance of the photo-sensitive element 11 is at its minimum. Therefore, the currents through the operating coils 16 and 19 of the auxiliary relay 13 and the load control relay 12, respectively, are higher than the pull-in values and relays 13 and 12 are energized. At this time current limiting resistor 14 is short-circuited while current limiting resistor 15 is in the circuit in series with heating coil 19 of the load control relay 12. Also, normally closed contacts 20 are in the open position and the load 22 is de-energized. 
     As the light drops to a predetermined level in the evening, the increase in the resistance of the photo-sensitive element 11 will cause the load control relay 12 to drop out as shown in FIG. 4, contacts 20 to close as shown in FIG. 5, and the load 22 to be energized. These events are indicated at time T1 in FIGS. 3, 4, and 5. Auxiliary relay 13 has not yet dropped out as shown in FIGS. 7 and 8 because its operating current is still above the drop-out level, as shown in FIG. 6, owing to the fact that its current limiting resistor 14 is short-circuited. 
     As the light level declines further in the evening, the resistance of the photo-sensitive element 11 will continue to increase as shown in FIG. 3, and the operating currents through the relays 12 and 13 will continue to decrease as shown in FIGS. 4 and 6, respectively. Eventually the drop-out current will be reached for auxiliary relay 13 also. This event is indicated at time T2 in FIGS. 2, 3, 4, 6, 7, and 8. 
     When auxiliary relay 13 drops out, contacts 17 will open and contacts 18 will close. This will insert current limiting resistor 14 in series with the operating coil 16 of the auxiliary relay 13, thus further reducing its operating current as shown in FIG. 6. It will also short-circuit current limiting resistor 15 which is in series with the operating coil 19 of the load control relay 12, thus increasing its operating current as shown in FIG. 4. 
     The values of the components in circuit 10 must be chosen so that the operating current of the load control relay 12 is biased towards, but is not permitted to reach, the pull-in value of the load control relay 12 at this time T2 as shown in FIG. 4. This will permit the load control relay 12 to remain de-energized thus keeping the load 22 energized through the hours of darkness. 
     As the light level begins to increase in the morning, the resistance of the photo-sensitive element 11 will decrease, and the operating currents through both relays 12 and 13 will increase as shown in FIGS. 4 and 6, respectively. The load control relay 12, whose operating current is not being limited by current limiting resistor 15 at this time, will reach the pull-in value first, causing it to open contacts 20 and deenergize the load 22. This event is indicated at time T3 in FIGS. 2, 3, 4, and 5. 
     Although the pull-in current for both the auxiliary relay 13 and the load control relay 12 is two to five times higher than their respective drop-out currents, the biasing effect on the load control relay 12 brought about by auxiliary relay 13, and described above, will permit the load 22 to be switched off at a light level lower than that at which the load 22 was switched on the evening before. 
     As the light level increases further, the operating current through the auxiliary relay 13 will reach the pull-in value. This event is indicated at time T4 in FIGS. 2, 3, 6, 7, and 8. Current limiting resistor 14 will be short-circuited by the closing of contacts 17, causing the operating current through auxiliary relay 13 to increase as shown in FIG. 6. Current limiting resistor 15 will be re-inserted in series with the operating coil 19 of load control relay 12 by the opening of contacts 18 as shown in FIG. 8. This will cause the operating current through load control relay 12 to decrease as shown in FIG. 4. However, the component values in circuit 10 are such that this current will not reach the drop-out value of load control relay 12 as shown in FIG. 4 so as not to re-energize the load 22. 
     The components have now completed their daily cycle of operation in accordance with the method of the invention and are now set to repeat it again the following evening. 
     As indicated above it is recognized that for proper operation of the switch 10, it is necessary to coordinate the selection of the various component values and operating characteristics with each other. These values however are either readily available commercially or can be incorporated into the design and manufacture of the components easily within the skill of the art. Furthermore, some of the operating characteristics or component values, such as the values of the current limiting resistors 14 and 15, may be made variable, as shown in FIG. 1, so the user of the device can select the operating characteristics of the switch 10 to meet specific requirements. 
     While one embodiment of the structure and method of the invention has been considered in detail above, it will be understood that other embodiments and modifications of the invention are contemplated. 
     FIG. 9 depicts, schematically, a light-sensitive switch 26 intended to simplify the circuit of FIG. 1 and some of the components described therein. In this embodiment, the auxiliary relay 23 has been simplified to have only a set of normally closed contacts 25 in addition to its operating coil 24. The current limiting resistor 14 and normally open contacts 17 of FIG. 1 have been eliminated. This is made possible by designing the auxiliary relay 23 with operating characteristics that will cause it to drop out at T2 and pull in at T4, as shown in FIG. 6, without biasing its own operating current. 
     FIG. 10 depicts schematically another light-sensitive switch 29 which utilizes the simplified auxiliary relay 23 described above but without the stringent requirements placed on the design of its operating characteristics. This is accomplished by using current limiting resistors 27 and 28 in conjunction with the normally closed contacts 25 of auxiliary relay 23 to bias both its own operating current and the operating current of the load control relay 12. 
     The switch 29 of FIG. 10 consists of the series connection, between P1 and P3, of the photo-sensitive element 11, the operating coil 24 of the auxiliary relay 23, and the operating coil 19 of the load control relay 12. The operating coil 24 of the auxiliary relay 23 is shunted by the current limiting resistors 27 and 28 connected in series with each other. Current limiting resistor 28 is, in turn, shunted by the normally closed contacts 25 of the auxiliary relay 23. The required biasing function occurs when the normally closed contacts 25 drop to the closed position at time T2, FIGS. 4 and 6. The closing of these contacts 25 causes the total resistance of the parallel combination of the current limiting resistors 27 and 28 and the operating coil 24 of the auxiliary relay 23 to be decreased. This will increase the current through the operating coil 19 of the load control relay 12. It will also reduce the resistance of the path shunting the operating coil 24 of the auxiliary relay 23, thus reducing its operating current. The process is reversed at time T4 so that the operation of this switch 29 is the same as that of switch 10 shown in FIG. 4. 
     FIG. 11 depicts, schematically, the presently preferred embodiment of the invention. The switch 30 controls the load 22 by means of a relay 32 equipped with normally closed contacts 50. The operating coil 49 of the load control relay 32 is energized or de-energized, in accordance with the method of the invention, by the electronic circuitry depicted in the schematic and operating as described below. 
     The circuit consists essentially of voltage dividers to establish the signal voltage and the reference voltages at which the switching will occur and operational amplifiers connected as voltage comparators to perform the desired switching. 
     The power supply 31, represented only in symbolic form, is connected across the source of alternating current (AC) electric energy 21 whenever the switch 30 is installed in its mating socket by prongs P1, P2, and P3 on, for example, a street light. It provides a suitable direct current (DC) voltage between the terminal V1 and ground G necessary to energize the electronic circuitry of the switch 30. The actual connections to the corresponding terminals of the operational amplifiers 34 and 35 have not been shown in FIG. 11 but have been indicated by also labeling the terminals on operational amplifiers 34 and 35 as V1 and G. 
     The voltage at terminal V1 with respect to terminal G may be any voltage within the acceptable operating range of the operational amplifiers 34 and 35 and would typically be between 5 volts and 24 volts. The power requirements of the power supply 31 would be determined primarily by the requirements of the operating coil 49 of the load control relay 32. The power supply may be designed, as in FIG. 11, to have a second output with respect to G, in addition to the one indicated above. This output, identified as V2 in FIG. 11, is designed to energize only the operating coil 49 of the load control relay 32. This arrangement permits greater freedom in selecting the operating characteristics of the load control relay 32. 
     The switching signal or voltage at V in FIG. 11, is established by the voltage divider consisting of resistor 36 and the photo-resistive element 11. However, the switching voltage connected to the inputs of amplifiers 34 and 35 is taken from the node between capacitor 48 and resistor 47 at Vs in FIG. 11. Capacitor 48 and resistor 47 are intended to form a time delay circuit that will prevent the voltage Vs from reacting to sudden changes in the resistance of photo-resistive element 11 such as may be caused by lightning flashes or automobile headlights. Otherwise, as long as light level changes occur only gradually, the signal voltage Vs will be essentially the same as voltage V. 
     The value of voltage V, and thus Vs, is affected not only by the values of photo-resistance 11 and resistor 36 but also by the output of operational amplifier 34 through resistor 40. When the output is high (near V1) V will be raised, and when the output is low (near G) V will be lowered. The amount of change in V will depend on the resistance value of resistor 40 with respect to the values of photo-resistor 11 and resistor 36. 
     The signal voltage Vs is connected to the inverting input of each operational amplifier 34 and 35, having the symbols (-) in FIG. 11. The non-inverting input of each operational amplifier 34 and 35, designated (+) in FIG. 11, is connected to the midpoint of another voltage divider. Operational amplifier 34 has its divider constructed with resistors 37 and 38, and operational amplifier 35 has its divider constructed with resistors 41 and 42. In addition, operational amplifier 34 has resistor 39 connected between its output and its non-inverting input (+), and operational amplifier 35 has resistor 43 connected between its output and its non-inverting input. The purpose of these two feedback resistors 39 and 43 is to provide hysteresis in the operation of each amplifier 34 and 35 in order to prevent unstable operation of the control. 
     The output of operational amplifier 35 controls the operation of load control relay 32 by means of driver circuit 33. The driver circuit consists of a resistive divider constructed with resistors 44 and 45 and transistor 46. The purpose of this circuit is to reduce the loading placed on the output of operational amplifier 35 and on the low voltage output V1 of the power supply 31. 
     The functional interaction of the various components described above and illustrated in FIG. 11 is as follows. 
     During the daylight hours, when the light level is high, the resistance of photo-resistor 11 is low and the voltage V (and therefore Vs) is low. The outputs of operational amplifiers 34 and 35 are both high since Vs is lower than their respective reference voltages. Since the output voltage Vo of the operational amplifier 35 is high, transistor 46 is conducting thus energizing the operating coil 49 of load control relay 32. Therefore, normally closed contacts 50 are open and the load 22 is de-energized. 
     As the light wanes in the evening, the resistance of photo-resistor 11 increases thereby increasing the signal voltage Vs. When the signal voltage Vs equals the reference voltage established at the non-inverting input (+) of operational amplifier 35 by resistors 41 and 42 (with resistor 43 connected to Vo which is in the high state) the operational amplifier 35 will switch its output voltage Vo to the low state. This will shut off transistor 46 which will in turn de-energize the load control relay 32, cause its contacts 50 to close, and turn on the load 22. 
     This will also cause the reference voltage for operational amplifier 35 to be decreased because resistor 43, which has one side connected to the non-inverting input (+), will have the other side switch from a high voltage (nearly equal to V1) to a low voltage (nearly equal to G). This hysteresis effect is useful in preventing the operational amplifier 35 from switching back on in the event the signal voltage Vs decreases slightly in response to an increase in the light level, such as may occur with changes in the cloud cover. The reference voltage, and thus the operating level, of operational amplifier 35 can be accurately established, as can the desired amount of hysteresis, by properly selecting the values of resistors 41, 42, and 43 relative to each other. 
     As the light level declines further and near-darkness is reached, the resistance of photo-resistor 11 continues to increase as does the signal voltage Vs. Eventually, this voltage will equal the reference voltage at the non-inverting input (+) of operational amplifier 34 which will change its output from the high state to the low state. In a manner analogous to that described in the paragraph above, operational amplifier 34, by means of resistor 39 in conjunction with resistors 37 and 38, will lower its reference voltage thus providing hysteresis to insure its stable operation. 
     In addition, when the output of operational amplifier 34 switches to the low state, the voltage V, and hence the signal voltage Vs, will be lowered also because resistor 40, which has one side connected to the node marked V, will have the other side switched from the high state to the low state. The amount of change is determined by properly selecting the value of resistor 40 in relation to the value of resistor 36 and to the resistance versus light characteristics of photo-resistor 11. 
     The amount of change must not be so large as to cause operational amplifier 35 to switch on at that instant. Rather, Vs should be reduced enough so that when the light increases in the morning to the desired level (which is lower than the light level when operational amplifier 35 switched off the evening before) the decrease in the resistance of photoresistor 11 will cause Vs to decrease to the value of the reference voltage at the non-inverting input. The output of amplifier 35 will switch to the high state and energize load control relay 32 which will turn off the load 22. The hysteresis provided by feedback resistor 43 will again serve to insure the stability of amplifier 35 by increasing the voltage at its non-inverting input (+). 
     As the light level increases further in the morning, the signal voltage Vs will continue to decrease until it will equal the reference voltage at the non-inverting input (+) of operational amplifier 34 causing the amplifier to switch from the low state to the high state. This will cause the signal voltage Vs to increase. But the time of this reset operation and the values of the appropriate components must be chosen so that the amount of increase will not cause the output of operational amplifier 35 to switch to the low state again. This change of state will also increase the reference voltage of operational amplifier 34 thus insuring its stability. 
     The daily cycle of operation has now been completed for this control, and the various components are set to repeat it again. Thus the operation of this control is completely analogous to that of the control depicted in FIG. 1. The advantage of this embodiment is that the various operating levels, including the amounts of hystereses desired, can be selected with great accuracy and repeatability. 
     FIG. 12 depicts, schematically, another embodiment utilizing electronic components. In this embodiment the switch 54 utilizes a load control relay 51 which controls the load 22 by means of normally open contacts 52. This is accomplished by energizing the operating coil 53 of the load control relay 51 during the hours of darkness. 
     The same result could have been accomplished using the circuit shown in FIG. 11 by modifying the relay driver circuit 33 to include an inverter that would allow it to drive the load control relay when the output of operational amplifier 35 is low and not drive the relay when said output is high. However, the circuit of FIG. 12 was chosen to show that the same result can be achieved by inverting the voltage divider circuit consisting of photo-resistor 11 and resistor 36 in FIG. 11 and as represented by photo-resistor 11 and resistor 54 in FIG. 12. 
     Resistor 54 in FIG. 12 has been given a different designation from resistor 36 in FIG. 11 to illustrate that a different value resistor must now be chosen to achieve the proper operation of the control 54 with the inverted circuit. The same applies to the other components where resistors 55, 56, and 57 correspond to resistors 37, 38, and 39, respectively. Resistors 58 and 59 and capacitor 60 correspond to resistors 40 and 47 and capacitor 48, respectively. An resistors 61, 62, and 63 correspond to resistors 41, 42, and 43, respectively. 
     The operation of the embodiment of the invention shown in FIG. 12 is completely analogous to the operation of the FIG. 11 embodiment and will therefore not be considered in detail. 
     Further embodiments and modifications of the invention are contemplated, including element 11 being responsive to other conditions such as heat, pressure, or sound. 
     Also, it will be understood that the values of the components of the circuits shown in FIGS. 11 and 12 may be variable to permit to permit varying the operating characteristics of the circuits. Thus for example, as shown, the resistors 36, 39, and 43 in the circuit of FIG. 11 and the resistors 54, 57, and 63 in the circuit of FIG. 12 may be variable. However, it is not contemplated that the components of the circuit be variable in preferred commercial embodiments of the invention to prevent undesired setting of the circuits operating conditions by inexperienced circuit users. 
     It is the intention to include all embodiments and modifications of the invention as are defined by the appended claims within the scope of the invention.