This is a mod/trick I recently used to make 8 LED (retrofitted from standard) flashlights for $8.00 or 16.00 if you count the $8.00 for the broken L.E.D flashlight I salvaged LED's from. The new flashlights were obtained from Family Dollar for $1.00 each and even included some generic batteries "Dorcy" brand.
This is intended to be a general Instructable not a in depth guide, so I wont get into voltages ect.. If you have question ask around the instructables community, there are many people here with vast knowledge pertaining to L.E.D's.
1 Cheap flashlight with standard bulb, I used a "Dorcy" brand which is available at Dollar General stores with included batteries at the time of the publishing of this instructable for $1.00.
Pliers, preferably with wire snips
Solder gun & solder & flux unless the solder has flux in it.
Hot glue gun
Sandpaper or steel wool or abrasive pad
Helping hands these will assist greatly, can be substituted with a second set of pliers if pliers are forced to grip by means of a rubber band places tight around the handle.
Small screwdriver or nail to help you manipulate the bulbs leads while soldering.
Step 1: Obtain Standard Bulb and Prep
Take apart the flashlight and remove the bulb. Now carefully with safety glasses on, over a garbage can outside use a pair of pliers (preferably nut pliers with a nut holding gap) to gently squeeze the regular styled bulb until it pops taking special care not to break the leads (aka wires) inside of the bulb, don't worry about the filament connecting the two leads.
Now carefully go around the outer edge of the remaining bulb base trying to gently crush the remaining sharp points of glass protruding from the bulb base once again proceeding with care, until there are no sharp edges left protruding from the base of the bulb. With protective gloves on some sticky shards can be removed by placing your small nail or screwdriver to pry them out, Once again proceeding with caution to not break leads, they can be re-fabricated with bread ties, but this is not fun to do!
Once you have all the sharp points and shards of glass removed below the lip of the base you may need to use the pliers to re flatten the lip of the base, go ahead and do that now if needed.
Now go ahead and remove the filament connecting the two leads as best as you can, then trim the leads to a even length.
Next you need to clean the leads and prep them, I first hold them in the pliers and put a flame to the leads for about 15 seconds to burn of any insulated coatings that are usually on the wires, we will be replacing this later so no harm.
Next clean the leads gently with sandpaper or steel wool *ONLY pulling in one direction away from the base so you don't jam them up or shear them.
Now that their cleaned up, put the base in you helping hands or makeshift pliers holder, having the bulb pointing sideways for the whole soldering process will make your life easier! Then apply a little flux and solder to "tin" up the leads, this is not super easy just get a little on there and try not to connect the two leads with the solder drip.
Now we have the base prepped for soldering.
Step 2: Identify Polarity
Mark one side of the base near a lead with a permanent marker.
Now reassemble the whole flashlight into working condition, including the now broken & marked base *EXCEPT* for the lens that protects the bulb.
Now turn the flashlight to the on position, using tweezers or long nose pliers place the led into the small housing for the bulb, touch the L.E.D's leads to the leads on the bulb base, if it does not light up, flip the L.E.D's leads the other direction, now it should light up.
Mark the side of the L.E.D that lines up with the side of the blub base you have marked earlier.
Now we have polarity identified to the degree of the electricity is flowing through the circuitry in a working manner.
Step 3: Solder L.E.D to Bulb Base
You may want to trim the length of, and tin the L.E.D leads before you go ahead with the soldering
With the bulb base pointing sideways match up the polarity marks you made earlier and solder the leads of the L.E.D to the corresponding leads of the bulb base . After you are done with the soldering before you proceed to the next step you need to check to make sure everything is in working order.
Reassemble the whole flashlight and switch it on, if the bulb lights up without flickering, proceed to next step, if your light is flickering recheck all your solder connections for integrity do necessary repairs, then proceed forward.
Step 4: Reinforce the Unit for Insulation and Durability
Place you bulb pointing upward in a pair of pliers with the handles held closed by a rubber band. Now use hot glue to start filling from the bottom of the bulb base up to the bottom of the L.E.D.
This is more easily done in 2 parts.
1 : Fill from the bottom of the bulb base to the top of the bulb base, let cool.
2 : Fill from the top of the bulb base to the bottom of L.E.D. let cool.
Turning the bulb while applying hot glue or circling the hot glue around the base from the top of the base to the bottom of the L.E.D
as opposed to applying to one side at a time, will result in a more even distribution of the hot glue.
You may want to hold the bulb upside down while spinning and gently blow on the hot glue to ensure you obtain/retain a even shape, that the hot glue doesn't try to fall back down to the bulb base area.
Clean up any slag over drip or messes with a razor or something similar, you can usually peel the hot glue right off of the L.E.D bulb itself.
Also pay attention that you do not have slag on the lip of the bulb base, this could result in malfunction or non functioning or worse yet, breaking of the bulb housing when you reassemble it!
Next step >
Step 5: Reassemble and Use Your New Improved Flashlight!
Now just put it all back together, including the protective lens, your ready to go!
You now have a L.E.D flashlight that is more durable and efficient than when it was factory issued with a incandescent bulb , for relatively cheap!