How I turned an ordinary flashlight into a "night vision friendly" one 

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I created this page so you could see how I made my "night vision friendly" flashlight. I call it "night vision friendly" because I replaced the incandescent bulb with a red LED. Red is chosen because red light does not interact with the rods in your eye, which are responsible for vision in low light levels.


I started with a cheap rechargeable flashlight:

 

I took the front off and measured the voltage at the connections, marked by the red arrows, after removing the light bulb and holder. Note: LED already installed in this pic.

Turned out to be about 2.5v DC. The specs on this LED are: 20mA and 2.6v max. You don't need a resistor in this case because the voltage supplied is less then the max voltage the LED can take. If the battery puts out more then the LED can handle, then follow this equation to find the resistor needed.

Source voltage - max LED voltage= x

x/LED current (in mA)= resistor needed in ohms

Example: 

Source voltage(flashlight batteries)=6 volts DC

max LED voltage=2.6v DC

6v-2.6v=3.4v

LED current in mA=.020 

3.4v/.020mA=170ohms

a 170ohm resistor would be needed in this case.

Then just solder the LED to the contacts (with resistor if needed) and reassemble. 

Results:

I applied this same rule when modifying a head light I bought at Wal-Mart for 5 bucks. Turned it into a roughly $30 LED light with maybe $4 in parts.

I added a 9v battery to the two 1.5v AA batteries it had, giving it 12v, and then wired in three 4v LEDs in series. Works great.