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.