I want to build a simple flashlight with an LED and one or two AAA batteries. This is for a Cub Scout project so I need to keep it very inexpensive. I am hoping someone might have suggestions about what type of LED to use and if I need a resistor. Any suggestions who else can answer these questions or where to get the LED?


Thanks in advance!

 

Hi 41fool

I spent some time in the Cub Scouts, myself - a few years back

This is a good project.

First, you will need to decide upon what color you wish to use. Red is generally the least expensive, and it doesn't affect night vision. White is more convenient, because that's what you're used to seeing in daylight.

Then you will need to decide upon how bright you wish for the flashlight to be. Cheap LED's don't give off much light. The more expensive LED's DO give off a lot of light, but can draw a lot of current.

Do you know Ohm's Law? If you don't yet, please search for Ohm's Law on the Internet.

Do you have a physical size in mind (or in constraints) for the flashlight? As in length, diameter?

Usually, the power requirements for LED's is not expressed in voltage, but in current (milliamperes, or 1/1000 of an Ampere)

In a perfect world, the source of the current used to light the LED will always supply the precise voltage (or pressure) necessary to keep the current through the LED constant.

But in reality, what usually happens is that instead of a constant current source, an LED gets a resistor instead - so that with fresh batteries, it's bright, and as the batteries wear, they grow dim.

Only a constant-current source can keep the amount of light emitted relatively constant until the battery (or batteries) are nearly exhausted.

So, what kind of project are you looking for? One that simply works? Or one that is somewhat more elegant?

 

I am going to try to fit it into a small Altoids gum tin which will fit a couple of AA, AAA, or a 9V batteries. I want it to be a white light that is bright enough that it is useful as a flashlight but need to keep the cost per LED down to a few dollars each since it is for Cub Scouts.

 

You will find most cheap white LED torches use 3 AAA batterys, a resistor, and LED/LEDS. This is because a White LED has to have a forward voltage of around 3.4 - 3.7 volts across it before it draws sufficient current to light.
If you plan on using just 2 AA batteries, you may find the light from the LED's not that bright, and battery life dreadfull, as only a small amount of drain will drop the voltage below the threshold of what is required.
A 9 volt battery should run 2 white LEDs and an appropriate resistor in series with great results, and would be the simplist alternative. Try a 56 ohm resistor as the series resistor for this option.

 

I am going to try to fit it into a small Altoids gum tin which will fit a couple of AA, AAA, or a 9V batteries. I want it to be a white light that is bright enough that it is useful as a flashlight but need to keep the cost per LED down to a few dollars each since it is for Cub Scouts.

This is probably more complicated and more expensive than you wanted but it
is 2 AA cells and a boost converter in an Altoids tin. If you changed the MAXIM chip to a
MAX757 you could adjust the output voltage to match the LED Vf.

http://www.ladyada.net/make/mintyboost/process.html

(* jcl *)