One of my project ideas which I have been wanting to do for quite some time now is to make my own solar powered lantern. It basically works something like a solar garden light but it doesn’t automatically turn on when its dark.

Parts list:

• solar cell
• Nicad/Nimh rechargeable battery
• terminal block
• resistor
• diode
• double sided tape
• a switch
• wires
• LEDs
• transparent plastic canister

Tools needed:

• soldering iron
• multi meter (to measure voltage/current)
• wire cutter
• precision screwdriver
• drill
• pliers

Basic Theory of Operation

The solar lantern is composed of 2 main modules. The charging module, and the lighting module. The lighting system would be an LED or a number of LEDs connected to a power source. The charging system would be a way to charge the battery via small solar cell.

Lighting module

This diagram shows how the lighting circuit works. The LED is passed through a resistor to limit the voltage/current that will pass through the LED. It is then connected to the battery.

How to compute for the value of the resistor.

The value of the resistor will depend on what LED you will be using and how much voltage is your power source. A typical LED needs about 15mA of current (0.015A).

In my case, I have a 3.6v Nicad battery, however once fully charged it goes to about 3.9v. Now a typical white LED has a voltage drop of about 3.4v so if I were to compute for the value of the resistor, it would be like this.

3.9v - 3.4v = 0.5v

Using V=IR

R=V/I = 0.5/0.015 = 33 ohms.

Watts = V * I = 0.0075 watts

Find a resistor with the next higher value available in your local electronics store. A 1/4 watt resistor will do.

Series or Parallel LEDs?

In my case, since my battery has only enough voltage for 1 LED, I don’t have a choice but to make a parallel connection. In my research on the internet, most advise not to wire LEDs in parallel. According to some of the articles I read, it is because parallel LEDs causes unstable current distribution. Some LEDs will get more current than it can take. Parallel LEDs can be done only if each LED has is own resistor.
As for now, I don’t have enough resistors so I will kinda violate this for a moment. Worst case is I’ll bust some of the LEDs which would be ok for now since it is not that expensive.

What I’ll be doing is to use 6 LEDs and I would make a parallel connection. Each LED requires about 15mA so the total current needed is 0.015A * 6 so that would be 0.09A or 90mA of current.

Using the V=IR formula again this is the resistor value I need

3.9v-3.4v= 0.5 volts needed to be dropped by the resistor

R=V/I = 0.5v/0.09A = 5.5 or 6 ohms.

Now to give a little allowance, I would use a resistor higher than 6 ohms but lower than the 33 ohms I computed if it were just 1 LED. I happen to have a 27ohm resistor so I’ll use that one.

To be continued…..

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