Using the basic Solar charger developed here, we can build a very simple and effective reading light. Bright enough to light a small room, it is more than bright enough to work by, and a day's recharging is all it needs to provide over 2 hours of use.
A garden-variety solar cell (2-volt, 20mA) is all that's needed, and the entire circuit consists of ten basic components.
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Step 1: The Parts List
The components required are:
BC327 PNP transistor
BC337 NPN transistor
1N914 or 1N4148 Low power signal diode
100uF 6v capacitor
100uH inductor (A larger can-type is chosen for its lower internal resistance. Thanks to acmefixer for bringing this up!)
1 x Red LED 20mA
1 x White LED 100mA. A wide angle unit (90-degrees or more) is recommended here to avoid glare and hot-spots. I also chose the piranha style so I can get the 90-degree angle when I assembled it.
Connecting and Mounting hardware.
Step 2: Assembly
The simplicity of the circuit allows almost any method to put this together. I used a copper-padded perf board and I've included the wiring layout below. Remember the first view is from the component side and the second is a mirror image of the first.
Allow the space but do not solder the transistors till the very end to avoid any heat damage to them.
Step 3: Circuit Operation
Transistors Q1 and Q2 transfers all the charge accumulated in C1 by the Solar Cell into the inductor, L1, building up the magnetic field in it. When Q1 is switched off, this field collapses and generates a voltage which is used to charge a 3-volt Lithium cell.
LED1 and LED2 serves 3 purposes. Firstly, they limit the output of the coil to about 4.6V so that the Lithium cannot overcharge. Secondly, the Red LED starts to glow when the battery reaches about 3.3v, and becomes brighter as we approach full charge. And finally, when J2 is turned on, LED2, a higher powered White LED is then turned on as the main light.
The output of the battery is so close to the Vf of the LED that we can get 90% efficiency.
The second image shows the faint glow of the Red LED against the sun as the device recharges.
Step 4: Putting Together the Lamp
To increase the usefulness of the light, a flexible goose neck extension is used for the White LED. I used about 8-inches (20cm) of 24-awg (.5mm) solid speaker wire enclosed in heat-shrink tubing. The wires are soldered to the terminals of a Piranha LED and the other end is anchored by securing to both the 100uH inductor and the 100uF capacitor.
The battery is a 3-volt CR2 Litium rechargeable which I wrote about here.