A Solar Powered Reading Light

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.

Step 1: The Parts List

The  components required are:

BC327 PNP transistor
BC337 NPN transistor
1N914 or 1N4148 Low power signal diode
10K resistor
5.6-ohm resistor
100uF 6v capacitor
330pF 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.

Comments

[calebjc]

Love it! If I wanted to increase the size of solar panels and increase the battery life to say 3 hours with 4 hours of charging, what do you suggest? Simply adding another solar panel or maybe four? And two more batteries in series? I'm trying to make one for use in Vermont. Realistically, even if the panels are directly in a south window, the charge time is low on any given day. So I want to make really sustainable.

[qs]

The most direct way is to start by calculating the power (V * I) of the LEDs, then match that to the capacity of the batteries and, finally to the solar batteries.

For example, if you're using a white 1A LED, then its power requirement is 3.3W.
- To run it for 3 hours, you'll need 10WHr.
- So the batteries will need to be at least that capacity, while the capacity of solar cells will be 3/4 of that.

Obviously, you will need to double if not triple the capacity of the batteries and solar power for rainy days.

[calebjc]

Also...I want to try low-blue light for supposedly less circadian rytham disruption. I'd love to here what folks think if these LED bulbs and how to hook them up best?

4 in. - Yellow - LED - Strip Light - Dimmable - 12 Volt 3M Taped Backing - Constant Voltage LED: https://www.1000bulbs.com/product/177036/FLX-00033...

Bulletproof Upgraded Low-Blue Nightlighthttp://a.co/9UmW6fZ

[douglas marcelo]

desculpa e que eu ñ prestei atenção mas se for 50% da iluminaçao

[douglas marcelo]

sou novato, tenho um carregador universal quebrado pretendo juntar ele com esse circuito para carregar baterias de 500, 900 e 1020ma cera que dar certo?

[douglas marcelo]

a iluminação dura quanto tempo?

[qs]

2 hours.