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Solar Powered Reading Lamp

(C) G. Forrest Cook July 31, 2004
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 applications.

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.

Newer VW and Audi automobiles come with small solar panels for keeping the battery charged in the sales lot. These panels are available on eBay for around $15 and are a perfect fit for this project. An inexpensive 12V 7AH lead acid gell-cell battery that is typically found in a computer UPS is also a good fit for this circuit. Be sure to use a new battery.

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.

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 you forget to shut off the lamp, the LVD circuit
will shut the lamp off when the battery is nearly empty.
If the lamp is recharged daily, the battery should
rarely reach the Low Voltage Disconnect (LVD) point.