Solar powered lamp having a circuit for providing positive turn-on at low light levels

A solar powered lamp having a circuit for providing positive turn-on at low light levels causes a light source thereof to undergo a discrete transition to an "on" condition and prevents unnecessary drain on a battery thereof. The circuit comprises a positive feedback loop which amplifies current flow rapidly to a predetermined level at which the light source is illuminated. In doing so, it causes a plurality of solar powered lamps to illuminate at substantially the same time.

FIELD OF THE INVENTION 
This invention relates generally to solar powered lamps. More specifically, 
the invention relates to a solar powered lamp having a circuit for 
providing positive turn-on at low light levels. 
BACKGROUND OF THE INVENTION 
Electrically powered outdoor lighting systems are used to illuminate 
pathways, yards, parks and other like outdoor areas for security purposes. 
Commonly, such lights are connected to public utility systems or similar 
sources of electrical power and are controlled by preset timing devices to 
illuminate desired areas at nightfall and automatically turn off at a 
predetermined time, for example, prior to daybreak. 
Many conventional lighting devices require extensive cabling, suitable 
timing mechanisms and the like, and are thus relatively expensive to 
install and maintain. In addition, such lighting devices utilize electric 
power generated in a conventional manner such as by burning fuel. Burning 
fuel contributes to contamination of the environment and depletion of 
existing fuel resources. 
More recent lighting devices include self-contained solar powered devices 
which utilize photovoltaic cells to charge batteries which, in turn, 
activate a light source contained therein, in the absence of sunlight. 
Such self-contained devices are desirable because they are relatively 
inexpensive and maintenance-free. 
A plurality of such solar powered lamps can be disposed in any 
predetermined arrangement outdoors to illuminate or delineate desired 
areas. For example, a particular area, such as a pathway, may be easily 
delineated so that a person, even in complete darkness, can follow the 
pathway without the necessity of producing overhead illumination. 
Although prior solar powered illumination devices are known to serve their 
purpose, they have not proven entirely satisfactory. During a certain 
period of time approaching sundown, when the level of ambient light is 
such that illumination is not required, for example when the ambient light 
decreases to a level between 600-1500 LUX, the solar powered lamps are 
known to turn on partially and slowly. This is undesirable because current 
is drawn from the batteries for a substantial period of time during which 
illumination is not necessary, thereby significantly shortening the run 
time for the lamp. 
In addition, the control circuitry of the solar powered lamps typically 
causes a significant drain on the battery, for example between 8-30 mA, 
even during times when the solar powered lamp is operating and 
illumination is required. Again, this considerably shortens the run time 
of the lamp. 
Moreover, in situations where a plurality of such solar powered lamps are 
used, erratic illumination often occurs. The lamps illuminate at different 
times, for example, sometimes at intervals as much as 30 minutes apart, 
because manufacturing tolerances inherent in individual components of 
these prior devices affect their operation. Such irregular illumination is 
inefficient, unattractive and consequently undesirable. 
SUMMARY OF THE INVENTION 
The present invention is directed to a solar powered lamp having a circuit 
for providing positive turn-on at a preselected low level which causes a 
light source thereof to undergo a discrete transition to an "on" condition 
and prevents unnecessary drain on a battery thereof. 
In one aspect of the invention, the solar powered lamp, comprises: a 
photovoltaic cell for receiving sunlight and generating electrical energy; 
an electrical storage device coupled to the photovoltaic cell for 
receiving and storing electrical energy; light sensing means for 
generating a control signal in response to ambient light; and means for 
providing positive turn-on at a preselected low light level, the positive 
turn-on means having a feedback loop which amplifies the control signal to 
apply the electrical energy to the light source. 
In another aspect of the invention, control circuitry of the solar powered 
lamp is configured to cause the lamp to turn on when ambient light 
decreases to a level of approximately 5 LUX. 
In still another aspect of the invention, the control circuitry of the 
solar powered lamp draws only 0.1 mA of current. 
These as well as other features of the invention will become apparent from 
the detailed description which follows, considered together with the 
appended drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates generally a schematic diagram for a solar powered lamp 
10 having a circuit 11 for providing a positive turn-on at low light 
levels which is predictable and controlled in contrast with prior solar 
powered lamps. The circuit 11 as shown in FIG. 1 is configured for use 
with an incandescent light source 16. The circuit 11 advantageously causes 
the solar powered lamp 10 to undergo a discrete transition to an "on" 
condition at a preselected level of ambient light, thereby preventing an 
unnecessary drain on a battery 12. The circuit 11 also advantageously 
reduces the dependence of this preselected light level on the 
manufacturing tolerances of individual components by producing a high 
gain. In situations where a plurality of solar powered lamps are used, 
(shown in FIG. 3) this results in each one turning on at substantially the 
same time. 
The solar powered lamp can typically include one or more batteries 12 of 
conventional design (preferably nickel-cadmium or the like) which is 
maintained in a charged condition by a photovoltaic cell 14 which controls 
the application of electrical power to the light source 16. The light 
source 16 is an incandescent lamp, of conventional design typically used 
in solar powered lamps. As is well known to those skilled in the art, the 
photovoltaic cell 14, when generating electrical power as a result of some 
light striking the same, is used to charge the battery 12. During such 
period of time, there is no need for the light source 16 to provide light. 
Thus, the light source 16 is disconnected from the power source, whether 
it be the photovoltaic cell 14 or the battery 12, during such time. 
However, when the voltage generated by photovoltaic cell 14 drops below a 
predetermined level, which represents the level of ambient light, then the 
power source consisting of the battery 12 is automatically connected so as 
to illuminate the light source 16. 
The photovoltaic cell 14 is interconnected with the battery 12. Connected 
between the positive terminal of the photovoltaic cell 14 and the positive 
terminal of the battery 12 is a current steering diode CR1, having an 
exemplary part number 1N5817, which prevents the battery 12 from 
discharging through the photovoltaic cell 14 when it is not producing a 
significant amount of electricity, such as during nighttime. 
A switch S1, of conventional design, connects the battery 12 to the light 
source 16 to provide power thereto when a light sensor 18 does not detect 
light. The light sensor 18 is preferably a cadmium sulfide photo cell. 
Upon sensing light, the light sensor 18 provides a low resistance which 
causes a resistor R1, having an exemplary resistance value of 39 k.OMEGA., 
to be shunted to ground. This causes current to flow through the resistor 
R1 and prevents current from flowing into the base of a transistor Q1, 
preferably a NPN transistor having an exemplary part number 2N3904. This 
turns off the transistor Q1, preventing current flow through the remainder 
of the circuit 11 and thereby preventing the light source 16 from turning 
on. 
In the absence of light, the light sensor 18 provides a high resistance, 
which causes a portion of the current flowing through the resistor R1 to 
flow into the base of the transistor Q1. This turns on the transistor Q1, 
which in turn causes current to flow through resistors R2 and R3, having 
exemplary resistance values of 10 k.OMEGA. and 1 k.OMEGA., respectively. 
The resulting voltage drop across the resistors R2 and R3 biases the base 
of a transistor Q2, preferably a PNP transistor having an exemplary part 
number 2N3906, causing it to turn on. When the transistor Q2 turns on, 
current flows through resistors R4 and R5, having exemplary resistance 
values of 10 k.OMEGA. and 2 k.OMEGA., thereby turning on a transistor Q3, 
preferably a PNP transistor having an exemplary part number 2N3906. 
When the transistor Q3 turns on, current flows through a resistor R7, 
having an exemplary resistance value of 2 k.OMEGA., and creates a voltage 
drop across the resistor R7. The voltage drop across the resistor R7 
creates a bias, through a resistor R8, having an exemplary resistance 
value of 100 .OMEGA., at the base of a transistor Q4, thereby turning it 
on. The transistor Q4 is preferably a NPN transistor having exemplary part 
number 2N4401. When the transistor Q4 turns on, it allows current to flow 
from the battery 12 through the light source 16, thereby causing it to 
illuminate. The transistors Q1, Q2, Q3 and resistors R3, R5 and R6 are 
configured in accordance with a Schmitt trigger circuit. 
A feedback resistor R6, having an exemplary resistance value of 220 
k.OMEGA., is connected between the resistor R7 and the base of the 
transistor Q1. When the transistor Q3 is just beginning to turn on, the 
voltage drop across the resistor R7 begins to build. As this voltage drop 
builds, the feedback resistor R6 provides an increasing amount of current 
to the base of the transistor Q1. This increases the rate at which the 
transistor Q1 turns on, thereby increasing the rate at which the 
transistor Q3 turns on and the rate at which the voltage drop across the 
resistor R7 builds. As the voltage drop across the resistor R7 builds more 
rapidly (toward a steady state value of approximately 2.5 volts which is 
the battery voltage), the amount of current provided through the feedback 
resistor R6 also increases until the circuit 11 reaches a "fully-on" 
steady state. Thus, the feedback resistor R6 causes the circuit 11 to turn 
on rapidly and thus avoids the problem of slow turn-on of solar powered 
lamps. 
When the light sensor 18 detects ambient light, it provides a low 
resistance. The low resistance shunts the current from the resistor R1 and 
the feedback resistor R6 to ground, thereby turning the circuit 11 off. 
This causes the light source 16 to be turned off. 
The transistor Q3 serves to enhance the current drive capability of the 
circuit 11 such that variations in the values of the various components 
from one device to another are relatively insignificant. Thus, the 
manufacturing tolerances in the values of the circuit elements have very 
little impact on operation of the circuit 11 and particularly on the level 
of ambient light at which the solar powered lamp 10 turns on. This causes 
the solar powered lamp 10 to turn on in a predictable and controlled 
manner, as a result of which a plurality of such solar powered lamps 10 
exposed to similar lighting conditions turn on substantially together. In 
a preferred embodiment, this occurs when the level of ambient light 
decreases to approximately 5 LUX. 
Referring now to FIG. 2, a circuit 50 which provides power to a solar 
powered lamp having a cold cathode fluorescent lamp 52 is shown. The 
circuit 50 incorporates the positive turn on circuit 11 of FIG. 1. In a 
situation where the solar powered lamp 10 has a shadow fall thereon which 
decreases the level of ambient light and causes the lamp 10 to turn on, 
resistor R8 drains the energy of the circuit 11 to quickly turn off the 
lamp 10 once the shadow has gone. The circuit 50 includes circuitry 53 for 
converting the low voltage of approximately 2.5 volts DC provided by the 
battery 12 into an alternating current approximately in the range of 
170-180 volts AC for operating the cold cathode fluorescent lamp 52. 
A transformer T1 having a primary winding 60, a secondary winding 62 and a 
tertiary or feedback winding 64 is electrically connected to transistors 
Q3 and Q4, preferably NPN transistors having exemplary part number 2N4401. 
Resistors R8 and R9, having exemplary resistance values of 680 .OMEGA., 
are connected between the collector of the transistor Q3 and the bases of 
transistors Q4 and Q5, respectively. The feedback winding 64 is connected 
between the bases of transistors Q4 and Q5. Transistors Q4 and Q5 act as 
switches alternately connecting the low voltage of approximately 2.5 volts 
DC across the primary winding 60. 
The feedback winding 64 is arranged in such a way that the base of the 
non-conducting transistor is negative whereas the base of the conducting 
transistor is positive. The feedback winding 64 is electrically connected 
between the bases of transistors Q4 and Q5, as a result of which one of 
the transistors Q4 and Q5 conducts more than the other. If transistor Q4 
is conducting, the feedback winding 64 electrically connected thereto more 
positively biases transistor Q4 with respect to transistor Q5, causing 
transistor Q4 to turn on fully and transistor Q5 to turn off. When 
transistor Q4 is conducting, current flows from the battery 12 through an 
inductor L1, having an exemplary inductance value of 130 .mu.H, to a 
center tap 66 of the primary winding 60 and through an upper half 68 of 
the primary winding 60. The current flows through the transistor Q4 from 
the collector to the emitter and returns to the negative terminal 56 of 
the battery 12. 
The flow of current along this path continues until the transformer T1 
begins to saturate and the polarity of the feedback winding 64 between the 
bases of transistors Q4 and Q5 is reversed. Transistor Q4 is then turned 
off and transistor Q5 starts conducting, thus creating flow in the 
opposite direction through transistor Q5. 
When transistor Q5 is conducting, current flows from the battery 12 through 
the inductor L1 to the center tap 66 of the primary winding 60 and through 
a lower half 70 of the primary winding 60. The current flows through the 
transistor Q5 from the collector to the emitter and returns to the 
negative terminal 56 of the battery 12. 
This switching continues in the manner described above to convert the low 
voltage of approximately 2.5 volts DC provided by the battery to 
approximately 170 to 180 volts alternating current. A capacitor C1, having 
an exemplary capacitance value of 0.047 .mu.F, connected in parallel with 
the primary winding 60 of the transformer T1, produces a parallel resonant 
LC circuit which helps control the frequency of oscillation which is 
approximately 30,000 Hz. 
A square wave used commonly with hot cathode fluorescent lamps, would 
destroy the characteristics of the cold cathode lamp and degenerate its 
lifetime considerably. Accordingly, the inductor L1 together with the 
transformer T1 creates a resonant invertor circuit which provides a sine 
wave output voltage. The inductor L1 builds charge when the current flows 
through it in a given direction, and when flow reverses, discharges back 
through the transformer T1 to aid in generating a sine wave. The inductor 
L1, of conventional design, preferably has 84 turns. The transformer T1, 
also of a type known to those skilled in the art, has 12 turns in its 
primary winding 60, 6 turns in its feedback winding 64 and 638 turns in 
its secondary winding 62. The saturation characteristics of the 
transformer T1 cause the switching to occur. 
A capacitor C2 electrically connected between the secondary winding 62 of 
the transformer T1 and a switch S1B connected across the cold cathode lamp 
52 is the series output capacitor, having an exemplary capacitance value 
of 68 pF. The switch S1B is any three way switch indicating a "High," a 
"Low" and an "off" position. The capacitor C2 controls the output 
impedance of the circuit and limits the amount of current flow through the 
cold cathode lamp 52. 
A capacitor C3, having an exemplary capacitance value of 68 pF, 
electrically connected in parallel with the capacitor C2, increases the 
lamp current and decreases the output impedance when the switch S1B is in 
the "High" position. When the switch S1B is in the "Low" position, the 
lamp current decreases by shunting the output winding and returning output 
power to the circuit. 
The 170 to 180 volts alternating current generated supplies power for 
heating the electrodes of the cold cathode fluorescent lamp 52 and 
creating a discharge within the cold cathode florescent lamp 52. This sine 
wave enhances and extends the life of the cold cathode fluorescent lamp 52 
in contrast with a low voltage square wave which would degenerate the 
characteristics of the cold cathode fluorescent lamp 52. 
Although the invention has been described in terms of a preferred 
embodiment thereof, other embodiments that are apparent to those of 
ordinary skill in the art are also within the scope of the invention. 
Accordingly, the scope of the invention is intended to be defined only by 
reference to the appended claims.