Picture of Low power LED lantern mod
Do those battery hungry toys drive you crazy?  Are you worried about keeping 3 small children happy during the next family campout without running to Radio Shack every day for batteries?   If so prepare to be amazed on how you can greatly extend the life of a simple battery operated lantern with this simple mod. 
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Step 1: Gather the materials

Picture of Gather the materials
For this project, we will need a small battery operated lantern.  This one is a pretty nice design as it is small, lightweight and has a really nice on-off pushbutton.  It also has lots of room to add additional circuits.  Heck- you can probably fit an Arduino in this thing!
Materials required are: lantern, soldering iron, 1K ohm resistor, super bright LED, and a small piece of stranded wire. 

Step 2: Figure out how the lantern works

Picture of Figure out how the lantern works
Figure out how you are going to tap into the battery pack.  This lantern uses ...gasp... 4 AA batteries.  The bulb receives voltage from the two metal tabs seen when the bulb is removed.  I measured the current of the stock lantern as shown in the last picture.  Yes that is right, 500mA.  You would be lucky to get 3 hours of burn time out of the stock unit.  
-max-3 years ago
the forward voltage for galium nitride based leds is closer to 3.5V. you should use 3AA batterys with a resistor. you should use a buck converter to boost the voltage as the battery dies. otherwise the light will be too dime and the batteries still 75% charged.
jim_2000 (author)  -max-3 years ago
Save yourself some trouble and search for a LED with a low Vf. The discharge curve of alkaline cells is nearly flat for currents this low.
-max- jim_20003 years ago
oh, right. i forgot about there discharge... im stuck in the li-ion world where its almost linear.