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.

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 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.

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