A solar camping light

In the summer of 2001, I constructed a very useful camping light that may not produce as much light as a fluorescent lantern, but is much smaller, can operate for 55 hours before needing to be recharged, which can be done with nothing more than a solar panel and sunlight. I built it using parts from Radio Shack and the Electronics Goldmine for about $25. The solar panel I use to charge it cost an additional $15. Here's what it looks like:

The unit contains a solar controller board from the Electronics Goldmine that turns the light on at dusk and off at dawn. The batteries are two lithium 3.6 v cells in series, providing 7.2 v @ 1.1 amp hours. The two white LEDs are in series and draw 20ma. The only other parts used are a switch and power connector for the solar panel connection. I would have preferred a white box for the project since it is in the sun for charging. Now, let's take a look at what's inside the box.

Solar light interior

At the left is the solar controller circuit board. Although the board has a power switch on it, I chose to use a separate switch to simplify mounting of the circuit board. The board contains a 358 dual op amp. One op amp is used to turn the light on at dusk and off at dawn and the second is used to turn off the light when the battery drops to about 5.5 volts. The CDS cell pokes through a hole drilled in the box. There are various ways to mount the LEDs. Not having any mounting hardware at the time, I mounted them to a circuit board which I bolted to the box.

Below is a picture of the unit being charged. The meter measures the current. I use a rate of about 100ma for 14 hours.

Charging the unit

Below is a parts list I used to construct the unit

2 White LEDs - Radio Shack (276-320)
6x3x2" box - Radio Shack (270-1805)
connector for solar panel - 5.5 x 2.5mm jack and plug - Radio Shack (274-1576) and (274-1573)
miniature toggle switch - Goldmine G1944
7.2V 1100ma Lithium Ion Battery Pack - Goldmine G4580
solar controller circuit board - Goldmine G4601
Rugged Solar Panels 19V @ 100ma - Goldmine G2260
1/4 watt resistor (see text below to determine value)

The resistor is in series with the two LED's. It is selected to limit the current through the LEDs to 20 ma when the batteries are fully charged. Start with a value of 100 ohms. Measure the current and adjust the resistor value as needed to limit the current to 20 ma. Hooking up the circuit board is pretty straight forward. Unsolder the two wires that go to the incandescent lamp that comes with the board and replace with two wires to the two LEDs and series resistor. Two wires go to the battery and two to the connector to the solar panel.

The batteries come in a plastic module that fits into a cell phone. This has to be broken apart and the batteries removed. After unsoldering the batteries from a circuit board that contains charging circuitry, I mounted them onto a perf board and mounted that to the box.

I made a variation of this unit that feeds an output connector rather than the two LEDs, which I use to power an LED light chaser string, that will flash 40 LEDs. You can buy a kit to build the light chaser circuit from Electronic Rainbow. The transistor on the solar controller board can supply up to about a 200ma load. The light chaser circuit draws about 100ma at 7.2 volts, with a string of 40 LEDs.