Solar Charged LED Flashlight

INTRODUCTION

Tired of always spending money on flashlight batteries only to have them fail when you need them? Try this simple circuit out. It would make an excellent science fair project. The white LEDs are quite bright, they can provide enough light to illuminate a small room at night and produce a flashlight quality focused beam. You can read by the light of this device.

The box also doubles as a 12V power source and can run other small loads such as a small radio. This project was built with the Simpler is Better approach, the materials are common and many substitutions are possible.

SPECIFICATIONS

Operating Voltage: 11-12.8V DC
Solar Charging Current: 60ma approx.
LED Operating Current: 60ma approx.

THEORY

The solar panel puts out about 60ma of current, this is enough to keep the 12V gell cell charged, but won't overcharge the battery. Due to this limited current, a charge controller is not necessary. 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 charge ratings. A smaller, lighter 3.5 Amp Hour lead acid battery would probably work perfectly, but the price is the same as a 7 Amp Hour battery so I chose the latter.

There are 3 series strings of LEDs in this circuit, the resistors limit the current through the LEDs and 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 but tend to add a warmer color to the light. White LEDs are expensive, if you are on a budget, try building the light with just one of the three LED strings shown.

One could certainly adapt this basic circuit to work on a lower voltage such as 6V, the LED resistors would need to be changed to maintain a 20ma current through each string and a lower voltage solar panel could be used.

CONSTRUCTON

Most of the effort went into building the box, any shape will do, but I built this unit so that the battery just fit inside and the solar panel is mounted at approximately 45 degrees from horizontal. I used particle board and wood screws to hold the box together, other materials will be more rain proof. White gloss paint was applied to give the particle board some protection from rain and keep the box cool in the sun.

The LED mounting system is somewhat fragile but allows the light to be rotated up and down. The cylinder is made from an unthreaded spacer with a 4-40 screw hole tapped in the middle for connecting the LED circuit board. The C mount is just a piece of flat aluminum that I bent in a vise, hammered into shape, trimmed with a pair of tin snips, and filed smooth. The white diffuser around the LEDs is a cap from a juice bottle that shields the intense white light from the side.

The LED circuit board was custom built with press-n-peel film but can easily be built with a piece of perforated board or prototype PCB material. Don't overheat the LED wires when soldering, white LEDs are expensive and are very easy to burn up. Use a heat sink.

All wiring was done with stranded #22 gauge wire and wire nuts. Note that the specified solar panel comes with its own built in diode, that will work fine, but a 1N5818 schottky diode in parallel with the existing diode will give slightly more charging current due to a lower voltage drop.

USE

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 run for many hours. The box will maintain a charge if it is left in a sunny spot inside.

The pin jacks can be used to occasionally check the battery voltage, under sunlight the voltage rises up to about 12.8V 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 around 100 hours, so if you put the box in the sun during the day, total discharge should not be a big problem.

An Old 12V UPS battery can be used, just be sure that it has enough capacity left to put out 12 Volts or more when the LEDs are turned on.

Experimenters will want to measure the current in and out of the battery, this can be done by inserting an ammeter in series with the fuse and the battery.

PARTS

1x 12V 7AH sealed lead acid battery, Panasonic LC-R127R2P or Yuasa NP7-12, from DigiKey
1x GM-684 12V 60ma solar panel, p/n 08SLC09 from Elecronix Express
1x red LED, T1-3/4 size
1x orange LED, T1-3/4 size
1x yellow LED, T1-3/4 size
1x green LED, T1-3/4 size
6x white LEDs, T1-3/4 size from Jameco
2x 120 ohm 1/4 W resistors
1x 220 ohm 1/4 W resistor
1x 1/2 Amp 28VDC fuse
1x fuse holder
2x crimp on battery clips
3x wire nuts
1x 1N5818 diode
1x spst toggle switch
2x pin jacks
1x box large enough to fit the battery and solar panel.
   misc hardware

PARTS SOURCES

Jameco 1-800-831-4242 http://www.jameco.com/ 
Digi-Key 1-800-DIGIKEY http://www.digikey.com/ 
Electronix Express 1-800-972-2225 http://www.elexp.com/

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