I've broken this instructable (and the flashlilght) in to a few easy steps:
Step 1: Open up the flashlight,
Step 2: Clean out the dead bulb,
Step 3: Calculate the resistance needed,
Step 4: Solder the LEDs and the resistor together
Step 5: Solder the LEDs to the bulbs, and
Step 6: Reassemble the flashlight.
Step 1: Open Up the Flashlight
Step 2: Clean Out the Dead Bulb
I wanted to use the bulb socket, so I needed to be rid of the glass bulb. After wrapping some paper towel around the bulb, I crushed the glass with a trusty pair of pliers
Afterwards, I used a power drill to clean out the insulating material out of the base. Remember TO USE SAFETY GLASSES! There will still be bits of glass in the socket. I will use a non conducting epoxy later to replace what I've removed.
Step 3: Calculate the Resistance
In this instance however, we're just using using math to make sure we don't blow up the LEDs (which I've done in the past. I mean, like Pow!)
Since my source voltage (VS) is the three AA batteries in series, the total is 4.5 volts. I intend to connect the LEDs in parallel, so I'm going to take the easy route, which is to calculate the resistance for one LED, then using Ohm's law, calculate the resistance for three LED's in parallel. White LEDs require about 3.6 volts to light (known as forward voltage, or VF), and draw 20 miliamps (Ma) of current , known as I
I= 20 Ma= .02A
What does this all mean? Using the formula R= (VS-VF)/I we get this:
R= (4.5V-3.6V)/ .02A
R= 45 Ohms
Now then, since the next highest resistor is 54Ohms(Always go up in resistors, never down), we will use this value for the next step, Calculating the equivalent resistance for three LEDs in parallel!
This is by far the easiest part. When you have to calculate an equivalent resistance (Req)for three LEDs in parallel, all you need to do is take your resistance (R) from step one, and divide it by the number of LEDs (n LED for this instructable) you intend to use.
Req= R/n LED
Req= 54 Ohm/ 3
Req= 18 Ohms! Easy peasy!
Step 4: Solder the LEDs and the Resistor Together
LEDs have a polarity, The long lead is the positive, the short one is negative. If you've already snipped your LED leads, you can also tell which one is the short (negative)by looking at the base of the "bulb". One side of the base has a flat spot. The negative lead is the same side as the flat spot .
Here you can see that I have soldered the like polarities together, and I wrapped and soldered the 18 Ohm resistor (this is the negative end). The other three leads will be soldered to the bulb socket.
Step 5: Solder the LEDs to the Bulbs
After the soldering was done and the socket had cooled, I mixed a bit of Aves Apoxie Sculpt (Oh, that wonderful Aves!) and pressed it into the voids around the resistor. Aves is a non conductive two part epoxie that scale and figure modellers use for sculpting. You could just as easily use epoxy glue, I just had the Aves on hand.
Step 6: Reassemble the Flashlight.
To the casual observer, it looks like any other el-cheapo flshlight. The only clue to its true nature is found by looking right at the bulb.
...although some people miss that clue...
Step 7: EPILOGE
He lets out a girlish squeal (think nine years old, with pigtails), and laments,"Why is this thing so bright!"
Hopefully you've all found this instructable entertaining and educational. Happy hacking!