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
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
Step 3: Upgrade Two to Five LED Bulbs
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
I went for a parallel array of 8 LEDs with a 50-ohm resistor in series. It definitely outperformed the original bulb in brightness.