Simplest LED light from a mobile battery, using just a 1 kilo-ohm resistor!
Step 1: Parts & Assembling:
1. One LED
2. One resistor - one kilo-ohm resistor
3. One mobile battery
Connect the 1K resistor to the positive of the LED. (Or to negative; resistor to any side)
Connect this positive of LED to (outside) positive of the mobile battery.
Just bend and manually connect the LED leg to the mobile slots.
NEVER solder directly to a battery.
Step 2: How Long ?
if you have a 1000 mAh battery and a normal white led of 20 mA, it should last 1000 / 20 = 50 hrs.