I've had this dead flashlight lying around in my shop for a few months, and I decided that it was time for it to live again!

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

The rubber handle was glued in place, but since this flashlight was a few years old, the glue was brittle and the handle peeled right off. After that, it was just a case of prying the light open.

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

Ah, math! Isn't math great! without it we wouldn't have spaceflight,  medical advances, or grilled cheese sandwiches! (2:1 bread to cheese ratio. See? Math!)

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!)

Part one:

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


VS= 4.5V,
VF= 3.6V
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!

Part Two

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

Just a quick note about LEDs.

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

Insert the resistor/ LED assembly into the socket, so that the reisistor lead pokes out of the bottom of the base. Solder the resistor lead and then solder each of the remaining leads to the top of the socket.

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.

Put the new and improved bulb back in the light, and reassemble. I used Methylene Chloride (AKA liquid glue) to glue the rubber grip back onto the flashlight body, and I put fresh batteries in while I was at it.

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...


So, there's the Uber light, sitting on the workbench, when along comes one of my coworkers. Having seen the light sitting on the counter for weeks before hand, he picks it up and pointing it at his face, he turns it on. (He wasn't aware of my jiggery-pokery)

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!
Dude your math boggled my brain.<br><br>Could you please make it a little dumber??
<p>Just as a medical doctor thinks &amp; performs his/her duty within a certain paradigm,</p><p>there are others that do the same thing because that is how they were educated....</p><p>Be it high school, college, or some other educational institution, these people work, think, &amp; live, within a certain paradigm...</p><p>I do not know anything about bat159, and I hope that he/she doesn't think I'm trying to disrespect him/her, but I think that may be the case here. bat159 is so used to thinking within this paradigm, that he/she often forgets that the rest of us do not think that way &amp; hence, do not understand a lot about LED's, or in my case, I do not understand a lot about electronics in general....but thanks to him/her taking the time, I think I understand now about LED's.</p><p>http://www.merriam-webster.com/dictionary/paradigm</p>paradigm<p><em>noun</em>\ˈper-ə-ˌdīm, ˈpa-rə- <em>also</em> -ˌdim\ <br></p><p>: a model or pattern for something that may be copied</p><p>: a theory or a group of ideas about how something should be done, made, or thought about</p>
<p>You sir are correct! I've been playing with/ working with electricity/electronics for about 40 years now. Sometimes I have to step back and think about an alternate explanation when I'm talking about it to someone who isn't familiar with the little invisible things running around in the wires.</p><p>Bat</p><p>By the way, are you familiar with the theory that all things electronic run on SMOKE. It says that if you let the smoke out, the electronics quit working.!!</p><p>(Geek humor)</p>
<p>Thank You. I thought I was the only one who got lost.... 8(</p>
<p>This sounds cool as all get out, but you lost me after step #3.</p><p>the first 2 steps I understood completely, but, I'm not an educated man, so it's easy for me to get lost when trying to calculate anything higher than 6th grade math...&amp; sometimes even that gets me lost....</p><p>Please keep in mind, I'm not saying the instructable was badly written, it's the math &amp; electronics that I don't understand. : (</p>
With the room you have in the case, could a voltage/current regulating circuit (maybe a zener diode based unit) be installed? My understanding is that batteries are useful down to .8 volts per cell. That would allow us to put 4 cells with a start voltage of 6 volts and it would drop to 3.2 volts. That would allow keep LEDs lit from full charge down to useless. I realize you have 3 batteries in this one but the regulator would work for a variety of input voltages.
Remember that the regulator is essentially just wasting power: the current I through it is the same as through the LED, so the wasted power is I*(VS-VF) (battery voltage minus the output voltage of the circuit, equal to the LED forward voltage). The same goes for the resistor in the Instructables circuit---essentially it regulates the required current, and so the only way to minimize the wasted power is to select VF to be as close to VF as possible
Perfect, I have a few dead lights kicking around, was going to replace bulbs anyway! Good project to do with my 11yr old Daughter. How much longer will batteries last over incandescent?
Hem, I can't say for sure how much longer it will last, but it will be considerably longer with the LEDs, as they have a very low current draw. I'm glad that you find that this will make a good project for you and your daughter, make sure to document it and post it here afterwards!
Electrical or other tape around the bulb works better than paper towel. If you heat the bulb base the epoxy/glue will melt and you can free the bulb in one piece in many cases.
nice tip, thanks
Most excellent work. Good use of stuff on-hand... like cheese &amp; bread.
NICE PROJECT!!!!!!! I really like the fact that you didn't modify the flashlight itself, only the bulb. You can use this techneque to convert almost any flashlight to LED. This is a perfect project for someone with limited skills (like myself). I've got a couple expensive camping lights at home that I never use because the batteries are always dead. I woud hate to end up destroying an expensive flashlight but have no fear if the only thing i'm destroying is a bulb. Thanks
Nicely done. I went the other way around when I upgraded my xenon headlamps. It just had the 2 AA batteries so I did a joule-thief as a driver circuit for it to work.
Thanks Best part is the batteries work for ages now :)
This is what recycling is all about. Instead of adding to the garbage of the world, this project gives us an idea what we might be able to revive and make it a useful thing at home. This way, we are saving the earth as well as saving some hard earned dollars when we have to buy a new one. Great idea!
Co-worker: Hmm, isn't this like, dead?<br> You: I dunno. It's just been sitting here for weeks. I haven't tested it out yet. *Co-worker examines flashlight* Co-worker: hmm..... *turns on flashlight* You: HOLY (lol, considering it <strong>is</strong> bright)<br> Co-worker: I THOUGHT THIS IS SUPPOSED TO BE DEAD!<br> You: Well, I may have tweaked it a little...
So timely,I was about to toss out and replace a flashlight but this is such a simple fix. Thanks.
Very good instructable indeed! I actually did this when my trusty 1$ novelty flashlight broke. Installed one 10mm superbright led inside &gt;: D. Oh, the performance was astonishing!
Awesome instructable. Very well explained and I prefer a 2:2 ratio for my GC..

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