Solar Powered Reading Lamp


Introduction:

The goal of this project was to produce a self contained reading
lamp that could be used by students in developing countries for reading
at night. The circuit can be used for a wide variety of lighting
pplications.The reading lamp consists of a small solar panel, a standard
UPS style lead acid battery, and an LED circuit board. The circuit board contains a low power solar charge controller (regulator), a set of 8 white LEDs, a switch, an LED current regulator, and a low voltage disconnect circuit.

The circuitry will insure a long battery life by preventing over
charging and excessive discharging. The circuit was designed to work
with lead acid batteries, it should also work with a string of 10 NiCd
cells. Both the charge controller and LED regulator circuits can be used independently for other applications.

Specifications:

Solar charging current: 150 ma – 1 Amp
Voltage drop through charge controller: 0.5V typical
Nominal battery voltage: 12 Volts
Battery rating: 3-7 amp hours
LED lamp current: 100ma regulated, 25ma per LED pair (1.2W nominal)
LED regulation range: constant light level from 11V to >15V
Low voltage disconnect: gradual current drop from 10.8V to 9.8V
Night time battery current drain with LED off: almost zero
Light duration: approximately 70 hours with a 7AH battery
Charge time: approximately 40 hours max, several hours typical

Theory:

Charge Controller: The charge controller section consists of an
LM2941CT low dropout voltage regulator and a 1N5817 schottky diode. The
regulator determines the battery full voltage, this set-point is
adjusted by the 5K 20 turn trimmer potentiometer. The 1N5817 schottky
diode prevents the battery from discharging through the voltage
regulator during the night. It also protects the circuitry against
reverse battery connection. The V727 part is a transzorb, it absorbs
lightning induced voltage spikes above 27V. The fuse prevents short
circuits from burning up the battery wiring.

Circuit

Circuit diagram

LED Circuitry: The 8 white LEDs
are connected in series with an LM317L IC that is wired as a constant
current regulator. The 13 ohm resistor sets the regulated current to
100ma. This current is split evenly through the four pairs of LEDs. The 33 ohm resistors help to keep the current through the four LED pairs balanced evenly. The 2N3904 transistor, 1N5239 zener diode, and 470 ohm resistor form the Low Voltage Disconnect (LVD) circuit. Current through the LED
starts to drop when the battery voltage drops below 10.8V, the circuit
shuts off almost all of the current when the battery drops below 10V.
The 1N5817 schottky diode blocks current flow in the event of a reverse
battery connection.

Construction:

The lamp consists of a small wood battery box and a vertical board for holding the LED assembly and solar panel. A small carrying handle protrudes from the top of the vertical board. The LED
assembly consists of a small printed circuit board and the various
parts. It is sandwiched between a piece of hard-board and a piece of
clear plexiglass to protect the circuitry from physical damage and
short-circuiting. The battery used for this device is a standard 12V-7
Amp-hour gell cell UPS battery. The solar
panel is home-made, two or three parallel-wired GM-684 12V 60ma solar
panels (p/n 08SLC09 from Elecronix Express), would be a good substitute.

Alignment:

Connect the board’s BAT – terminal to the battery – terminal.
Connect the board’s BAT + terminal to the battery + terminal.
Connect the board’s PV – terminal to the solar panel – terminal.
Connect the board’s PV + terminal to the solar panel + terminal.
Point the solar panel toward the sun.
Measure the connected solar panel’s voltage with the meter.
Measure the battery voltage with the meter.
The solar panel must be at a higher voltage than the battery for the
circuit to charge the battery.
Turn the potentiometer, VR1 25 turns clockwise.
Monitor the battery voltage, as the battery charges, the voltage should
rise above 13V.
When the battery voltage has reached 13.8V or the desired full point,
turn VR1 counter-clockwise until the battery voltage stabilizes.
Let the battery charge for several minutes at the full setting, then
re-adjust VR1 for the final desired full voltage setting.

Use:

Daytime:

Place the solar panel in a location that gets at least a few hours of direct sunlight each day. Turn the LED switch off. If the battery is extremely discharged, it may take several days in the sun to fully recharge.

Night:

Use the lamp as you would use any other reading lamp. If the lamp
starts to dim, the battery is almost completely empty, shut the light
off. If the lamp is recharged daily, the battery should rarely reach the
Low Voltage Disconnect (LVD) point.


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