Solar Powered Lamp and Engine

Introduction


The goal of this project was to produce a self contained reading lamp that could be used by students in rural areas for reading at night. The circuit can be used for a wide variety of lighting applications. The scope of the circuit was further extended by use of IC555 to tap the solar energy to drive a simple motor.

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 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. The other part of the circuit, i.e. a solar engine can be accessed by a switch. The motor is switched on after the capacitor is reasonably charged to trigger the IC


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 can be a string of 5-10 NiCd rechargeable batteries or 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.

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

Use the switch provided to switch between the lamp and motor.

Project By:
" Srinath G Iyer
" Akshay Kakar
" Pratik Maheshwari
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At the heart of most solar-powered robots is a circuit called the solar engine The purpose of a solar engine is to act like a power "savings account" -- a small trickle of incoming energy is saved up until a useable amount is stored. This stored energy is then released in a burst, in order to drive some useful (if only sporadic and incremental) work. The trigger level is about 3 times the photovoltaic output of the photodiode. You can add a series Si diode to raise the trigger level by 300mV (3x 100mV) . Since the photodiode voltage rises with the log of the light level, the photodiode voltage changes little over a large light range, making the trigger level and the "pop" relatively constant over light level. The 555 with the lower photodiode output voltage will of course trigger first. The cap should be sized to cause the motor to turn the bot no more than 45 degrees each pop -- any larger cap and the bot does "twirls.
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