A Modified Version of a Solar Flashlight

by Le Magicien

A few years ago I bought a nice solar powered waterproof flashlight with an incandescent bulb on one side and 3 bright red blinking leds on the other. This flashlight is also known as Solar Multi-Purpose Flashlight and Bicycle light

It uses 2 AA size rechargeable NiMH batteries (1.2V 600mAH) being recharged by the solar panel and also has a conventional battery holder for 2 more AA size cells in parallel with the rechargeable ones. Thus, if the conventional cells are low you may always recharge the other battery holder. Suggested uses for this torch are: bike, car and at home.

I like this torch a lot but... the incandescent bulb eats the NiMH cells pretty fast and the blinking leds... who needs them? So the idea was.. why do not replace the rear red leds with white ones? There's only one problem - white leds do not work with less than 3.2 volts - therefore some kind of voltage booster was needed.

The circuit was originally taken from LED TORCH but this link is no longer active (Feb 2007). Anyway HERE is a zipped file of this page in mht format (Web archive, single file or Multipurpose Internet Mail Extension HTML)

There are however some active useful links like ELM - White LED Head Lamp which uses basically the same circuit as mentioned above.

In order to wind the special transformer needed here... this link, Make a Joule Thief, is great!
Also check the LED Torch Project.

Last two projects use a simple version of the circuit I used, meaning no current control is given. The circuit used in this page has a second transistor (Q2) which controls the main oscillating transistor Q1. If current gets too high, then Q1 is shut down by Q2. This is a nice feature.

There's a bonus with this modification: incandescent bulbs need almost full charged cells and eat a lot of current so, when AA batteries are no more able to run an incandescent bulb, you'll still be able for many hours to use them with white leds. In fact, if you store the torch in some place where sunrays reach the solar panel, although no charge will be available to run the bulb, you will always have the white led option in working condition! Not bad for a use and forget torch!!! Never more need to worry about low voltage cells !

Fig 1 Shows the modified waterproof solar torch showing the conventional incandescent bulb (right screen side- yellow light) versus 3 white leds (left screen side - white light).

The new circuit board replaces the original one which has with 3 bright blinking red leds. (see Fig 3 & 4)

This solar waterproof torch has a transparent ABS plastic case protecting the main torch body. It also floats on water.

Switches in this plastic case are rubber buttons that actually press the real switches located on the torch body.

Here is the original board with 3 blinking leds. The black dot behind this board is the built in oscillator.

Also shown is the new board with 3 white leds and the driver circuit.
A driving circuit is needed because this torch uses only 2 AA cells to drive an incandescent bulb. Thus there's only 3V DC available and white leds need 3.2 volt at least!

The circuit is a voltage booster.

Same dimensions and led position.

This torch uses 2 AA rechargeable NiMH 1.2 600mAH batteries. On the other side of the torch there's an array of ten solar cells to recharge the pack.

Also, as shown in this figure there is a 2 AA battery holder for normal type batteries (non-rechargeable). The solar panel does not supply charge to these batteries.

This torch thus has two AA packs in parallel - one rechargeable and the other conventional.

There's no place to put the C1 electrolytic capacitor on the rear side of the torch, thus it was placed on the opposite side.

Just point to point soldering, no special pc board needed here!!!!

This is the circuit, no special parts here.

T1 is a hand made step up transformer built on a ferrite toroid 1cm diameter (anything like this will work the same). As you can see primary has 35 turns and secondary 20 turns (for example #28AWG or smaller enameled wire.

Please note that transformer windings are opposed (black dots on opposite sides) - thus bifilar winding is required here!

Detailed view: switches side.

Detailed view: lateral.

Detailed view: solar panel side.

Detailed view: lateral.

Comments? Contact me! Le Magicien

last actualization: Feb 2007

Le Magicien

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