Important Note* If you start to read this and wonder what is so cool about turning a flashlight into a different flashlight Google "Joule Thief". This is an instructable for turning a flashlight into a low source voltage more complicated flashlight!
The basic idea is that you are taking a voltage (dead battery or single battery) that wouldn't normally be able to drive an LED and using a simple boost circuit make it able to drive one or more LEDs (*efficiency is pretty low).
One day I was in need of a variety of white leds for a project. My local shack is a a rip off and the nearest eletronic component shop is pretty far away from my house. There is a dollar store up the road and I knew that they had a few different led flashlights I could scavange parts from in a pinch. One of the led flashlghts I bought was a dual purpose model that would terrify potential attackers with an annoying piezo if you were the victim of muggers with sensitive ears. This instructable is based on that particular flashlight/attack prevention tool thingy.
Step 1: What You Will Need...
1) Either one of the flashlight sirens below will work just fine for this. The components inside are identical.
2) Soldering iron
3) Small phillips screwdriver
5) Wire cutter/stripper
6)About 20 or 30 minutes of quiet time
Step 2: What's Insde the Thingy?
Take the light/siren apart using the smal phillips screwdriver. Carefully desolder all of the components and save everything.
Inside you will find:
1) a transformer (the biggest thing attachd to the board)
2) a capacitor (small and orange on back side of board with transistor)
3) a resistor
4) a briht white LED
6) a switch
7) a transistor
If you've ever built a Joule thief you will immediately recognize this parts list.
Step 3: The Circuit.
Here is a pic of the circuit I drew from memory. I apologize if it's a hack job...
R1 is the only resistor you removed.
C1 is the only capacitor you removed.
Sorry I forgot to label the transformer. It's the component you pulled out that looked kind of like a capacitor with 4 leads.
You will notice the transformer windings are labeled A and B on my drawing.
The A pair, if metered will have around 6 ohms resistance and will have longer leads (you will see this when you desolder it fomr the board).
The B pair, if metered will have around 100 ohms resistance and will have the shorter leads.
There are two ways to tell which leg of the LED is the + and whih is the -.
The - side of the LED will usually be flat where the lead comes out. If not the + side of LEDs have the longer lead.
The transistor has three legs.
Hold it with the flat side facing you and the three legs hanging down.
The left leg (collector) will be the leg that connects to the top or + side of the circuit.
The middle leg (base) will be the leg that ties to the resistor.
The right leg (emmitter) will be the leg that ties to the bottom or - side of the circuit.
From here you have two options....
Step 4: Breadboard or Quick and Dirty Version. Choose Your Own Adventure.
These two pics represent two different options. If you are confident in your cmponent ID and soldering skills you may just take a peek at the schematic and solder up the Joule thief shown below. This was made with only parts I found inside the flashlight (wire and all).
If you take your time this can be built out as a pretty small circuit. I wouldn't recomment soldering anything too heaily or cutting the leads down until you see it work. This will make disassembly and reassembly much easier.
If you aren't as confident I'd recommed adding a a little bit of solid wire to all of the component leads so that you can breadboard this bad boy up and tinker with different resistor values and add some capacitors in random places and see what happens!
Step 5: Final Result
In the pic I addded the following scraps to the circuit to finish it up.
1) Old hard drive magnet as base...
2) I swapped the LED for a blue one that came in another dollar store puchase.
3) Two paperclips for stability.
4) Rubber shock mount from dead cd rom drive..
5) Heat shrink tubing.
6) One rare earth magnet I had stuck to the bench. Came from Harbor reight tools 10 pack for $1.49 I think.
You'll notice it's running on a dead AA battery but will also run on any one of the button cells that came with the flashlight. I like using magnets to attach the batteries so that you can use several different size batteries as you pull them out of other devices that claim they are dead.
THANS FOR LOOKING!