white LED flashlight



A old flashlight from my father recently burned out its lightbulb, he also didnt like that was somewhat dimmer it used lots of batteries. I had a pile of very high efficiency white leds (20.000MCD at 40% efficiency) that where waiting for a good purpose. So after some thinking and figuring out the flashlight was designed for 1.5V batteries, not 1.2V NiMH batteries giving a total of 3.6V instead of the needed 4.5V. So i set out using a stepup converter to convert the 3.6V to 5V for the leds, and then using series pass resistors to limit the current trough the leds around 15mA. I didnt need max current, as the DCDC converter had a max current of 200mA.

This the PCB i constructed, basically the power comes in at the side screw connections from the batteries. I directly soldered 2 wires to the bulb terminals for easy access. The power is lead to a DCDC converter that uses a inductor to step up the voltage to 5V and power the leds. The leds have a 120Ohm series resistor each.

This is a close up of the DCDC converter for the leds. It consists of the following components:
The MAX756 has a integrated switch for stepping up the voltage to the needed 5V, it can also output 3.3V or a variable voltage with a external resistor network. The circuit for the stepup converter is taken from the Datasheet of the MAX756. It also contains many examples how to use this versatile chip, too bad its quite expensive at farnell (about 5 euro's).

The whole setup gets moderatly warm. Most of the heat is produced by the LED's and resistors, the DCDC converter is very efficient, the high load and low ESR components aid the efficiency from the converter.
Below are some action shots

Here you can see the flashlight in operation.

Here you can see the front of the flashlight. The image is quite dark becouse of the automatic adjustment from the digital camera. The leds really shine bright and illuminate at day and night.

Here you can see the flashligh illuminating my old 80286 PC, this photo was taken in bright room.

Conclusion:
The 20.000MCD leds where bought from ebay, and are quite good. The color composition is not that great, with a yellow center and blue ring. But combined in arrays they give a nice good bluish white bundle of light and are very bright. This conversion/hack was a total success, my dad was satisfied with the better light and higher focusing of the leds.

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Last update at: 15 May 2005