The white LEDs in this circuit are quite bright and produce a tightly focused beam. The LED array can provide enough light to illuminate a small work or reading area at night. The red, orange, yellow and green LEDs broaden the lamp’s color spectrum to produce a slighly warmer color temperature. This project would make an excellent science fair project.

solar-charged-led
The solar panel puts out about 60ma of current, this is enough to float-charge the 12V gell cell without overcharging. With this limited amount of charging current, a charge controller is not required. Other batteries may be used such as smaller lead acid batteries or NiCd cells (use 10 in series), but overcharging and leaking battery goo may result if you exceed the battery’s charge current ratings. A smaller and lighter 3.5 Amp Hour 12V lead acid battery has been successfully used in a different version of this circuit.

There are 3 series strings of LEDs in this circuit. The resistors limit the current through each string of LEDs to prevent them from burning up. The white LEDs have a higher voltage drop and only 3 can be used in series with a 12V (11-13V) source. The red/orange/yellow LEDs have a lower voltage drop per diode, so 4 can be wired in series. The colored LEDs are optional, they are used to increase the red side of the color spectrum. When this project was first created, white LEDs were fairly expensive, they are rapidly coming down in price.

It is possible to adapt this basic circuit to work with a lower battery voltage, such as 6V. At 6V, the lamp circuit would need to be changed to one white LED and one resistor per string, and a set of red/yellow LEDs and one resistor per string. The resistor values would need to be changed to maintain a 20-25ma current through each LED at the full battery voltage. A lower voltage solar panel could be used to charge the 6V battery, a 7.2V (open circuit voltage) panel would be a good fit.

Turn off the LED switch and point the solar panel at the sun in the morning, adjust the panel towards the sun a few times a day, or simply point the panel in the direction of the sun at noon. The battery will charge. Turn the light on at night, it should be able to produce light through the night. The box will maintain a charge if it is left in a sunny spot indoors. One full day of charging should be enough for several nights of light.

The pin jacks can be used to occasionally check the battery voltage. Under sunlight the voltage may rise to over 13V and at night, it should not be allowed to go below 11V or the battery life will be shortened. A new, fully charged battery will run the light for many hours, so if you put the box in the sun during the day, total discharge should not be a big problem.
Operating Voltage: 11-13.8V DC
Solar Charging Current: 60ma approx.
LED Lamp Operating Current: 60ma approx.

source : www.solorb.com

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