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White LED Flashlight

White LED Flashlight

I took my car to a mechanic recently and while inspecting the seal around the oil pan, he pulled out a small penlight that put out what appeared to be "white" light. When he showed it to me I realized the light was coming from a single, white LED.

Later that afternoon, I opened my latest copy of Electronic Design Magazine and turned to the Pease Porridge column. Lo and behold, Bob had written an article about a low dropout current source for a white LED flashlight. I have taken his hand-drawn schematic and reproduced it below.

D1 is the white LED from Chicago Miniature. They can be ordered from DigiKey for $3.00 each. U1, the LM334 current source provides negative feedback to Q1 to maintain a constant current of 40mA through the diode. If you want less light but longer battery life, you can increase R1 to 3.3 ohms. An LM334 in a TO92 package can be acquired from DigiKey for about $2.10.

When operating at 40mA, the AAA batteries should provide about 28 hours of operation. If the current is reduces to 20mA, 57 hours of operation should be possible.

TAGS: Components
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