It's time to breathe new life into aging incandescent flashlights. Installing LEDs can make your flashlight brighter and improve its battery life by a factor of up to 50 times.
The difficulty with transforming conventional flashlights is their voltage. Typically 3V does not provide enough voltage to power the bright white LEDs. That's why most commercial LED products use three 1.5V batteries. You can still get a useful and efficient flashlight by using amber-colored LEDs.
If you have a flashlight that puts out more than 3V, spring for the brighter white LEDs.
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Step 1: Getting Access to the Cathode and Anode Leads
You could try removing the bulb and trying to solder some wires in place yourself, but the bulb's casing does it all for you in a very convenient way.
Don't hesitate to take a hammer to it since the bulb won't be useful to us anymore. Be sure to place the bulb inside a rag so that glass don't go flying.
Having access to the leads made it easy to solder an LED directly into place. Make sure that you don't solder with the wrong polarity. The positive terminal will always be adjacent to the notch in the bulb's casing.
Step 2: Simplicity Is Breathtaking
However it's not the best we can do. The flashlight may run for a millennium now but it isn't nearly as bright as the original bulb. And it isn't the ideal situation for LED health either - it would do much better with a current-limiting resistor.
Step 3: Upgrade Two to Five LED Bulbs
I connected five amber LEDs in parallel on a small piece of perfboard. I chose a resistor that would stabilize the current without drastically diminishing the brightness of the LEDs. 10 ohms did the trick.
They will be brighter if you overdrive them, but not much. They will also last much longer operating at their prescribed voltage and current. The five amber LEDs were about equivalent in brightness to the original bulb. The reflector housing was small to begin with, so the LEDs were really cramped.
Step 4: Getting Brighter
With a 6V flashlight was it possible to reap the benefits of super bright white LEDs. Rated at 4.5V, they could provide the necessary illumination to overshadow the original bulb's brightness. Because of the 1.5V difference some of the energy was wasted by the necessary current-limiting resistor. (If you own a 4.5V flashlight, these LEDs would be perfect.)
I went for a parallel array of 8 LEDs with a 50-ohm resistor in series. It definitely outperformed the original bulb in brightness.