The box also doubles as a 12V power source and can run other small loads such as a 12V transistor radio. This project was built with the Simpler is Better approach, the materials are common and many substitutions are possible.
Operating Voltage: 11-13.8V DC Solar Charging Current: 60ma approx. LED Lamp Operating Current: 60ma approx.
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.
The LED mounting system is somewhat fragile but allows the light to be rotated up and down. The rotating mount cylinder is made from an unthreaded aluminum spacer with a 4-40 screw hole drilled and tapped into the side as a mounting point for the LED circuit board. The cylinder is held in place with a C-shaped aluminum holder, the holder was fashioned from flat metal that was bent in a vise, hammered into shape, trimmed with a pair of tin snips, and filed smooth. The LED circuit board is mounted behind a white plastic water bottle cap which shields the light from the side of the LEDs.
The LED circuit board was custom built with press-n-peel film, it can easily be built with a piece of perforated board or prototype PCB material using point-to-point wiring. The resistors are mounted on the circuit board, along with the LEDs. Be careful not to overheat the LEDs when soldering, LEDs can easily be destroyed by overheating. A clip-on heat sink is recommended during soldering.
All wiring was done with stranded #22 gauge wire and wire nuts. Note that the specified solar panel comes with its own built in 1N4001 diode, that will work, but a 1N5818 schottky diode in parallel with the existing diode will produce a smaller voltage drop and thus increase the charging current.
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.
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. Other 12V loads can be powered by this system, just plug them into the pin jacks or better, add a polarized output connector.
I have been using this device for four years. It sits in a sunny window and continues to provide a reliable source of light at night.
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
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/