Solar Powered Lamp Having a Circuit for Providing Turn-On at Low Light Levels

Europaisches Patentamt European Patent Office © Publication number: 0 527 347 A2 Office europeen des brevets

EUROPEAN PATENT APPLICATION

@ Date of filing: 15.07.92

® Priority: 17.07.91 US 731672 © Applicant: SIEMENS SOLAR INDUSTRIES LP. 4650 Adohr Lane P.O.Box 6032 @ Date of publication of application: Camarillo, California 9301 0(US) 17.02.93 Bulletin 93/07 @ Inventor: Moore, Fred A. © Designated Contracting States: 3907 Calle Valle Vista DE ES FR GB IT NL Newbury Park, California 91320(US)

© A solar powered lamp (10) having a circuit (11) CR1 r. ~i for providing positive turn-on at low light levels causes a light source (16) thereof to undergo a |R2 I R4 14 t discrete transition to an "on" condition and prevents J " drain on a battery thereof. The circuit + unnecessary R3 (11) comprises a positive feedback loop which amplifies current flow rapidly to a predetermined level C at which the light source (16) is illuminated. In doing cos SENSOR so, it causes a plurality of solar powered lamps (10) to illuminate at substantially the same time.

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FIELD OF THE INVENTION siderably shortens the run time of the lamp. Moreover, in situations where a plurality of This invention relates generally to solar power- such solar powered lamps are used, erratic illu- ed lamps. More specifically, the invention relates to mination often occurs. The lamps illuminate at dif- a solar powered lamp having a circuit for providing 5 ferent times, for example, sometimes at intervals as positive turn-on at low light levels. much as 30 minutes apart, because manufacturing tolerances inherent in individual components of BACKGROUND OF THE INVENTION these prior devices affect their operation. Such irregular illumination is inefficient, unattractive and Electrically powered outdoor lighting systems io consequently undesirable. are used to illuminate pathways, yards, parks and other like outdoor areas for security purposes. SUMMARY OF THE INVENTION Commonly, such lights are connected to public utility systems or similar sources of electrical pow- The present invention is directed to a solar er and are controlled by preset timing devices to 75 powered lamp having a circuit for providing positive illuminate desired areas at nightfall and automati- turn-on at a preselected low level which causes a cally turn off at a predetermined time, for example, light source thereof to undergo a discrete transition prior to daybreak. to an "on" condition and prevents unnecessary Many conventional lighting devices require ex- drain on a battery thereof. tensive cabling, suitable timing mechanisms and 20 In one aspect of the invention, the solar power- the like, and are thus relatively expensive to install ed lamp, comprises: a photovoltaic cell for receiv- and maintain. In addition, such lighting devices ing sunlight and generating electrical energy; an utilize electric power generated in a conventional electrical storage device coupled to the photovol- manner such as by burning fuel. Burning fuel con- taic cell for receiving and storing electrical energy; tributes to contamination of the environment and 25 light sensing means for generating a control signal depletion of existing fuel resources. in response to ambient light; and means for provid- More recent lighting devices include self-con- ing positive turn-on at a preselected low light level, tained solar powered devices which utilize the positive turn-on means having a feedback loop photovoltaic cells to charge batteries which, in turn, which amplifies the control signal to apply the activate a light source contained therein, in the 30 electrical energy to the light source. absence of sunlight. Such self-contained devices In another aspect of the invention, control cir- are desirable because they are relatively inexpen- cuitry of the solar powered lamp is configured to sive and maintenance-free. cause the lamp to turn on when ambient light A plurality of such solar powered lamps can be decreases to a level of approximately 5 LUX. disposed in any predetermined arrangement out- 35 In still another aspect of the invention, the doors to illuminate or delineate desired areas. For control circuitry of the solar powered lamp draws example, a particular area, such as a pathway, may only .1 mA of current. be easily delineated so that a person, even in These as well as other features of the invention complete darkness, can follow the pathway without will become apparent from the detailed description the necessity of producing overhead illumination. 40 which follows, considered together with the appen- Although prior solar powered illumination de- ded drawings. vices are known to serve their purpose, they have not proven entirely satisfactory. During a certain BRIEF DESCRIPTION OF THE DRAWINGS period of time approaching sundown, when the level of ambient light is such that illumination is not 45 A preferred embodiment of the present inven- required, for example when the ambient light de- tion is illustrated in and by the following drawings creases to a level between 600-1500 LUX, the solar in which like reference numerals indicate like parts powered lamps are known to turn on partially and and in which: slowly. This is undesirable because current is Figure 1 is a schematic diagram of a solar drawn from the batteries for a substantial period of 50 powered lamp having a circuit for providing time during which illumination is not necessary, positive turn-on at low light levels in accordance thereby significantly shortening the run time for the with a preferred embodiment of the present in- lamp. vention, configured for use with incandescent In addition, the control circuitry of the solar lamps; and powered lamps typically causes a significant drain 55 Figure 2 is a schematic diagram of the solar on the battery, for example between 8-30 mA, even powered lamp shown in Figure 1 , configured for during times when the solar powered lamp is op- use with fluorescent lamps. erating and illumination is required. Again, this con-

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DETAILED DESCRIPTION OF THE PREFERRED causes a resistor R1, having an exemplary resis- EMBODIMENT tance value of 39 kn, to be shunted to ground. This causes current to flow through the resistor R1 and Figure 1 illustrates generally a schematic dia- prevents current from flowing into the base of a gram for a solar powered lamp 10 having a circuit 5 transistor Q1, preferably a NPN transistor having 11 for providing a positive turn-on at low light an exemplary part number 2N3904. This turns off levels which is predictable and controlled in con- the transistor Q1, preventing current flow through trast with prior solar powered lamps. The circuit 1 1 the remainder of the circuit 11 and thereby pre- as shown in Figure 1 is configured for use with an venting the light source 16 from turning on. incandescent light source 16. The circuit 11 ad- io In the absence of light, the light sensor 18 vantageously causes the solar powered lamp 10 to provides a high resistance, which causes a portion undergo a discrete transition to an "on" condition of the current flowing through the resistor R1 to at a preselected level of ambient light, thereby flow into the base of the transistor Q1. This turns preventing an unnecessary drain on a battery 12. on the transistor Q1, which in turn causes current The circuit 11 also advantageously reduces the is to flow through resistors R2 and R3, having exem- dependence of this preselected light level on the plary resistance values of 10 kn and 1 kn, respec- manufacturing tolerances of individual components tively. The resulting voltage drop across the resis- by producing a high gain. In situations where a tors R2 and R3 biases the base of a transistor Q2, plurality of solar powered lamps are used, this preferably a PNP transistor having an exemplary results in each one turning on at substantially the 20 part number 2N3906, causing it to turn on. When same time. the transistor Q2 turns on, current flows through The solar powered lamp can typically include resistors R4 and R5, having exemplary resistance one or more batteries 12 of conventional design values of 10 kn and 2 kn, thereby turning on a (preferably nickel-cadmium or the like) which is transistor Q3, preferably a PNP transistor having an maintained in a charged condition by a photovoltaic 25 exemplary part number 2N3906. cell 14 which controls the application of electrical When the transistor Q3 turns on, current flows power to the light source 16. The light source 16 is through a resistor R7, having an exemplary resis- an incandescent lamp, of conventional design typi- tance value of 2 kn, and creates a voltage drop cally used in solar powered lamps. As is well across the resistor R7. The voltage drop across the known to those skilled in the art, the photovoltaic 30 resistor R7 creates a bias, through a resistor R8, cell 14, when generating electrical power as a having an exemplary resistance value of 100 n, at result of some light striking the same, is used to the base of a transistor Q4, thereby turning it on. charge the battery 12. During such period of time, The transistor Q4 is preferably a NPN transistor there is no need for the light source 16 to provide having exemplary part number 2N4401. When the light. Thus, the light source 16 is disconnected 35 transistor Q4 turns on, it allows current to flow from from the power source, whether it be the photovol- the battery 12 through the light source 16, thereby taic cell 14 or the battery 12, during such time. causing it to illuminate. The transistors Q1 , Q2, Q3 However, when the voltage generated by photovol- and resistors R3, R5 and R6 are configured in taic cell 14 drops below a predetermined level, accordance with a Schmitt trigger circuit. which represents the level of ambient light, then 40 A feedback resistor R6, having an exemplary the power source consisting of the battery 12 is resistance value of 220 kn, is connected between automatically connected so as to illuminate the the resistor R7 and the base of the transistor Q1. light source 16. When the transistor Q3 is just beginning to turn on, The photovoltaic cell 14 is interconnected with the voltage drop across the resistor R7 begins to the battery 12. Connected between the positive 45 build. As this voltage drop builds, the feedback terminal of the photovoltaic cell 14 and the positive resistor R6 provides an increasing amount of cur- terminal of the battery 12 is a current steering rent to the base of the transistor Q1 . This increases diode CR1, having an exemplary part number the rate at which the transistor Q1 turns on, thereby 1N5817, which prevents the battery 12 from dis- increasing the rate at which the transistor Q3 turns charging through the photovoltaic cell 14 when it is 50 on and the rate at which the voltage drop across not producing a significant amount of electricity, the resistor R7 builds. As the voltage drop across such as during nighttime. the resistor R7 builds more rapidly (toward a A switch S1, of conventional design, connects steady state value of approximately 2.5 volts which the battery 12 to the light source 16 to provide is the battery voltage), the amount of current pro- power thereto when a light sensor 18 does not 55 vided through the feedback resistor R6 also in- detect light. The light sensor 18 is preferably a creases until the circuit 11 reaches a "fully-on" cadmium sulfide photo cell. Upon sensing light, the steady state. Thus, the feedback resistor R6 light sensor 18 provides a low resistance which causes the circuit 11 to turn on rapidly and thus

3 5 EP 0 527 347 A2 6 avoids the problem of slow turn-on of solar power- other. If transistor Q4 is conducting, the feedback ed lamps. winding 64 electrically connected thereto more When the light sensor 18 detects ambient light, positively biases transistor Q4 with respect to tran- it provides a low resistance. The low resistance sistor Q5, causing transistor Q4 to turn on fully and shunts the current from the resistor R1 and the 5 transistor Q5 to turn off. When transistor Q4 is feedback resistor R6 to ground, thereby turning the conducting, current flows from the battery 12 circuit 11 off. This causes the light source 16 to be through an inductor L1 , having an exemplary induc- turned off. tance value of 130 M.H, to a center tap 66 of the The transistor Q3 serves to enhance the cur- primary winding 60 and through an upper half 68 of rent drive capability of the circuit 11 such that io the primary winding 60. The current flows through variations in the values of the various components the transistor Q4 from the collector to the emitter from one device to another are relatively insignifi- and returns to the negative terminal 56 of the cant. Thus, the manufacturing tolerances in the battery 12. values of the circuit elements have very little im- The flow of current along this path continues pact on operation of the circuit 1 1 and particularly is until the transformer T1 begins to saturate and the on the level of ambient light at which the solar polarity of the feedback winding 64 between the powered lamp 10 turns on. This causes the solar bases of transistors Q4 and Q5 is reversed. Tran- powered lamp 10 to turn on in a predictable and sistor Q4 is then turned off and transistor Q5 starts controlled manner, as a result of which a plurality conducting, thus creating flow in the opposite di- of such solar powered lamps 10 exposed to similar 20 rection through transistor Q5. lighting conditions turn on substantially together. In When transistor Q5 is conducting, current flows a preferred embodiment, this occurs when the level from the battery 12 through the inductor L1 to the of ambient light decreases to approximately 5 LUX. center tap 66 of the primary winding 60 and Referring now to Figure 2, a circuit 50 which through a lower half 70 of the primary winding 60. provides power to a solar powered lamp having a 25 The current flows through the transistor Q5 from cold cathode fluorescent lamp 52 is shown. The the collector to the emitter and returns to the circuit 50 incorporates the positive turn on circuit negative terminal 56 of the battery 12. 11 of Figure 1. In a situation where the solar This switching continues in the manner de- powered lamp 10 has a shadow fall thereon which scribed above to convert the low voltage of ap- decreases the level of ambient light and causes the 30 proximately 2.5 volts DC provided by the battery to lamp 10 to turn on, resistor R8 drains the energy of approximately 170 to 180 volts alternating current. the circuit 11 to quickly turn off the lamp 10 once A capacitor C1, having an exemplary capacitance the shadow has gone. The circuit 50 includes cir- value of 0.047 u.F, connected in parallel with the cuitry 53 for converting the low voltage of approxi- primary winding 60 of the transformer T1 , produces mately 2.5 volts DC provided by the battery 12 into 35 a parallel resonant LC circuit which helps control an alternating current approximately in the range of the frequency of oscillation which is approximately 170-180 volts AC for operating the cold cathode 30,000 Hz. fluorescent lamp 52. A square wave used commonly with hot cath- A transformer T1 having a primary winding 60, ode fluorescent lamps, would destroy the char- a secondary winding 62 and a tertiary or feedback 40 acteristics of the cold cathode lamp and degen- winding 64 is electrically connected to transistors erate its lifetime considerably. Accordingly, the in- Q3 and Q4, preferably NPN transistors having ex- ductor L1 together with the transformer T1 creates emplary part number 2N4401. Resistors R8 and a resonant invertor circuit which provides a sine R9, having exemplary resistance values of 680 0, wave output voltage. The inductor L1 builds charge are connected between the collector of the transis- 45 when the current flows through it in a given direc- tor Q3 and the bases of transistors Q4 and Q5, tion, and when flow reverses, discharges back respectively. The feedback winding 64 is connect- through the transformer T1 to aid in generating a ed between the bases of transistors Q4 and Q5. sine wave. The inductor L1, of conventional design, Transistors Q4 and Q5 act as switches alternately preferably has 84 turns. The transformer T1, also connecting the low voltage of approximately 2.5 50 of a type known to those skilled in the art, has 12 volts DC across the primary winding 60. turns in its primary winding 60, 6 turns in its The feedback winding 64 is arranged in such a feedback winding 64 and 638 turns in its secondary way that the base of the non-conducting transistor winding 62. The saturation characteristics of the is negative whereas the base of the conducting transformer T1 cause the switching to occur. transistor is positive. The feedback winding 64 is 55 A capacitor C2 electrically connected between electrically connected between the bases of tran- the secondary winding 62 of the transformer T1 sistors Q4 and Q5, as a result of which one of the and a switch S1 B connected across the cold cath- transistors Q4 and Q5 conducts more than the ode lamp 52 is the series output capacitor, having

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an exemplary capacitance value of 68 pF. The second transistor. switch S1B is any three way switch indicating a "High," a "Low" and an "off" position. The capaci- 3. A solar powered lamp as defined in Claim 2, tor C2 controls the output impedance of the circuit wherein said feedback loop further comprises: and limits the amount of current flow through the 5 a first resistive means electrically connect- cold cathode lamp 52. ed between said base of said first transistor A capacitor C3, having an exemplary capaci- and said collector of said third transistor. tance value of 68 pF, electrically connected in parallel with the capacitor C2, increases the lamp 4. A solar powered lamp as defined in Claim 3, current and decreases the output impedance when io further comprising: the switch S1B is in the "High" position. When the a second resistive means electrically con- switch S1B is in the "Low" position, the lamp nected between said collector of said first tran- current decreases by shunting the output winding sistor and said base of said second transistor; and returning output power to the circuit. and The 170 to 180 volts alternating current gen- is a third resistive means electrically con- erated supplies power for heating the electrodes of nected to said collector of said second transis- the cold cathode fluorescent lamp 52 and creating tor, said first, second and third transistors and a discharge within the cold cathode florescent lamp said first, second and third resistive means 52. This sine wave enhances and extends the life connected in a Schmitt trigger configuration. of the cold cathode fluorescent lamp 52 in contrast 20 with a low voltage square wave which would de- 5. A solar powered lamp as defined in Claim 1, generate the characteristics of the cold cathode wherein said light source is an incandescent fluorescent lamp 52. lamp. Although the invention has been described in terms of a preferred embodiment thereof, other 25 6. A solar powered lamp as defined in Claim 1, embodiments that are apparent to those of ordinary wherein said light source is a cold cathode skill in the art are also within the scope of the fluorescent lamp. invention. Accordingly, the scope of the invention is intended to be defined only by reference to the 7. A solar powered lamp as defined in Claim 1, appended claims. 30 wherein said preselected low light level is substantially 5 LUX. Claims

1. A solar powered lamp, comprising: a photovoltaic cell for receiving sunlight 35 and generating electrical energy; an electrical storage device coupled to said photovoltaic cell for receiving and storing electrical energy; light sensing means for generating a con- 40 trol signal in response to ambient light; and means for providing positive turn-on at a preselected low light level, said positive turn- on means having a feedback loop which amplifies said control signal to apply said elec- 45 trical energy to the light source.

2. A solar powered lamp as defined in Claim 1, wherein said positive turn-on means further comprises: 50 a first transistor having a collector, base and emitter; a second transistor having a collector base and emitter, said collector of said first transistor electrically connected to said base of said 55 second transistor; and a third transistor having a base thereof electrically connected to said emitter of said

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FIG. 1

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