In this instructable, I'll show how I made this compact but powerful LED flashlight. All parts were purchased with nothing being pre-assembled. The overall cost will probably vary seeings how I got some things in a one-time sale.

Step 1: Parts Acquisition

For this instructable you will need:

-1 Dual Printed Circuit Board
-4 10mm High Brightness LEDs
-4 150 Ohm resistors
-1 PC board terminal

-Soldering iron
-Solder
-Either AA or AAA batteries

As you can see, I got most of these things from Radio Shack, but the LEDs I picked up during a 90% off closing sale. The screw terminals were on sale for a dollar and the circuit boards were only a few dollars at full price.

Step 2: Figure Out Your Power Supply

Since I'll be using this during a night-time tour of a large building that no longer has electricity, you can imagine I'd want the brightest possible light with the lowest power draw. I settled on a square of four of the High Brightness white LEDs. Each one has a power draw of 3.3-4.5VDC at 20mA. I wired them in parallel, meaning I have a demand of 3.3-4.5VDC at 80mA.

The easiest way to power this would be with 4 AA batteries, and a resistor to drop the supply.
I used this lovely site to help figure out what rating to look for without having to sit and do unwanted, tedious math.

http://led.linear1.org/led.wiz

According to the wizard, I need a 150 Ohm resistor on the negative lead of each LED.

Step 3: Assembly

Now comes the fun part!

We need to solder our pieces onto the board.

The alteration I made to the diagram was to put the PC terminal on so I could switch between my AAA battery pack and a 9v connector, instead of just committing to the 9v connector all together.

Each LED needs a 150 Ohm resistor soldered to its negative lead. Then I soldered each positive lead to the positive terminal on the screw connector.

Step 4: It Looks Like You're Done

Once you have everything soldered together, give it some power to test it out. If it doesn't work, check for short circuits and if that's not the problem, check your batteries.

If you don't care for a bare circuit board(I sure don't) then go ahead and prepare to mount it in your case. This is another reason I suggest you use a screw terminal as you can disconnect your power supply and make a small hole in your case as compared to an oversized hole or trying to solder around things.

Step 5: Final Considerations

The two things to consider on your power supply are how long do you want to have light, and how compact do you need it to be.

AA batteries can have twice the milliamp hours of AAA, but are obviously larger.
For this it doesn't really matter since overall the entire board and batteries fit comfortably in the palm of my hand. If you're looking to add more LEDs, keep in mind that each one will add a draw of 20mA.

More lights=Greater draw

Greater draw=Shorter runtime

With the Double A pack, I should be able to get several hours out of this simple circuit.

Thanks for reading, this was my first instructable and I tried to be as detailed as possible. If you have any questions feel free to leave them in the comments section.
Happy building!