White LEDs Flashlight

Posted on Dec 11, 2012

Here is a schematic of a rechargeable 9-LED flashlight using two Gates SLA cells and nine LEDs. It is as bright as when the single bulb was used, and lasts 3 times longer per charge. That is about 10 hours of bright light, at a lower brightness, is available for about 10 hours more (at the risk of abusing the rechargeable cells). At that point, the regular bulb would have long ago gone dark. LED flashlights continue to produce usable light for so long because the resistance of the LED increases sharply as the voltage decreases below a certain point, yet the LED continues to produce light. At the time that flashlight was built in 2002, the brightest LEDs reasonably available were rated at 6000mCd. Things are only getting better.

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Note: when the supply voltage is close to the LED voltage, the resistance value is more critical and difficult to select. There are other ways to limit the current, using a PWM or linear curent regulator, and this will be left to the reader to examine. The LM350 linear IC regulator can be configured to limit the current precisely. If you are using such a regulator with several LEDs in parallel, each will have to have a small-value series resistor, to help equalize the current through the array (unless you want to use 9 IC regulators!). Also be aware that a simple regulator like the LM350 has a certain amount of voltage drop across it (about 1.2V) in order to operate. The next design in the works will use an array of 36 LEDs, mounted on a 2" diameter circuit board salvaged from an IR illuminator which was part of a security system. This will be placed inside the 5" reflector of a larger flashlight of the type which uses a 6 volt "lantern battery". The reason that a much larger reflector is needed, is because the resistors stick out the back of the improvised PCB, and it all has to fit inside the cone-shaped area of the reflector. The reflector is not really needed, since almost all the light fires straight out the fronts of the LEDs. With 6VDC, 125 ohms is a good resistance for each LED. When the battery is new, and putting out about 6.4VDC, current will be 24 mA, within acceptable limits. This would be a very good place to...

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