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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. 

Step 1: 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

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.  

Step 3: Bypass the Battery Pack

In this step, I will modify the unit so it runs off of 2 AA vice 4 AA batteries.  To do so take a look at your lantern.  The lantern uses the batteries in series.  To tap into half of the pack, we will add a jumper on the base of the battery holder.  A small piece of stranded wire serves as the jumper.  For this unit I carefully soldiered the wire on the two springs.  This is not as easy as it looks, I find that wrapping the wire around the spring in a loop then covering the bare copper wire with solder makes a pretty good joint.

While we are on this step, take a look at the inside of the battery holder.  Two sockets are connected with a metal bar and two are not.  The two sockets that have a single tab next to the lap are the ones we will use for our batteries.  The other two will be left open.

Step 4: Test Your Battery Mod

Now lets pull out the voltmeter.  This step validates that your battery mod is working and shows you which is the positive terminal.  This lantern's positive terminal is the tab that is closest to the top of the lamp socket.  Yeah- 2.6 volts, a bit low, better get some fresh batteries.

Step 5: Add Your Circuit.

How much more simpler of a circuit can you get?  Take your 1K ohm resistor and soldier it to the lantern's negative terminal.  Then solder the diode's anode to the resistor and solder the cathode to the other terminal.  The anode will be the longer leg of the LED.  Now for the math.  I have 3 volts power source and the forward voltage for the blue LED is about 2.4 volts.  Use our E=IR formula and solve for R.  I'm going to use 5 ma for the current as I have a super bright LED and am just looking for something with extremely long battery life.  So 3.0-2.4/5ma = 120.  Round that down to an even 100 for a nice little lamp.  

Step 6: Enjoy!

That is it, now you have a cool BLUE tent lantern/nightlight.  

Step 7:

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.
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.
oh, right. i forgot about there discharge... im stuck in the li-ion world where its almost linear.

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