$3.50 Flashlight + 20 Cent Upgrade = 500 Hour Run Time




Introduction: $3.50 Flashlight + 20 Cent Upgrade = 500 Hour Run Time

About: Hello

This flashlight will be perfect for your emergency preps. It will also make a great present for any kid. Kids love flashlights and with this one, you will not need to change the batteries every time they forget to turn it off and it runs all night long. This instructable is easy and inexpensive. We will look at a one LED version of the upgrade as well as data from the 2 LED test set up than ran continuously for over 10 days and is still running. I started with a Rayovac “floating lantern” that I bought from Walmart for $3.50 (battery included). It is a very bright flashlight but it only has 4-½ hour run time and some emergencies last a little longer. The 1 led upgrade that I made has a run time of about 500 hours. It is not nearly as bright but you can easily see your way around in the dark and do other things like read the cooking instructions on a can of soup or play checkers during a power outage etc. (No light = No fun). 

Step 1: Resistor Selection and Upgrade

All you will need is the flashlight, a LED and a resistor or 2 LEDS and 2 resistors for the 2 LED version.

Here is a list of resistors and the currents I measured in the series circuit with a new 6 volt lantern battery, a resistor and a LED. The 6 volt battery measured 6.28 volts with no load:

150 ohm = 20.3 milliamps

168 ohm = 18.5 milliamps

220 ohm = 14.5 milliamps

300 ohm = 9.8 milliamps

390 ohm = 8.5 milliamps

The 168 ohm resistor is actually a 150 ohm and 18 ohm in series. This is the resistance I used for each of the two LEDS I used in the test setup for the 11 day test. The currents were calculated by measuring the voltage across the resistor and dividing this by the resistor value (ohms not cents).

There are different ways to do the actual modification. For mine, I removed the light bulb and broke the glass with a pair of pliers (do this out side over a trash can. Wear safety goggles. Yada yada, safety first , yada yada, adult supervision if under 18, bla bla). Then I cleaned the inside of the metal can with a dremel.

I have shown two versions re-using the original bulb. The tall one was used in my flashlight because it was not necessary to utilize the parabolic reflector due to the fact that I used the 15 - 30 degree angle type LED and most of the light can never hit the reflector. You can also make the stubby type to get the full benefit of the reflector but it is a little more work to get the resistor wired up down in the can. A two led version would be a real challenge. By the way, to make the connection to the base of the lamp you need to remove the solder from the base. Solder wick works well for this. With the solder removed you will have a hole in the base.

You can add wires to the LED and resistor and solder directly to the bulb contacts instead of upgrading the actual bulb. If you choose to do this you will need to remove the spring that is used to make contact with the bulb.

Step 2: Locator Mode

Most flashlight have 2 modes. On and off. You can make another upgrade to this flashlight (5 - 8 cents more) to make the modes “on” mode and “locator” mode (locator mode also known as “Dude, where’s my flashlight?” mode [WMF MODE]). On my flashlight, I soldered a 100K ohm resistor across the back of the switch contacts as shown. So when the flashlight is off it is really on just a little bit so that the led glows dimly but bright enough to be easily seen in the dark. This feature can better be utilized by extending the LED or LEDS with wires and hot gluing it to a hole in the clear plastic disk on the front of the flashlight. This will increase the viewing angle of the led to a full 180 degrees and will also void the lifetime warranty on the $3.50 flashlight.

Below is a list of several resistors and the currents that I measured when powering up a single LED with the same 6 volt (6.28 volts) battery. I went the extra mile and included some resistors that I would not use for the locator mode or for the LED upgrade. These may be useful for some other project in the future.

300k ohm = 0.0127 milliamps

180k ohm = 0.021 milliamps

100k ohm = 0.037 milliamps (the one I used, 37 micro amps)

47.5k ohm = 0.079 milliamps

12k ohm = 0.31 milliamps

2.5k ohm = 1.4 milliamps

1.6k ohms = 2.2 milliamps

788 ohms = 4.3 milliamps (390 ohm x 2 measured 788 ohms)

The second picture shows the flashlight in WMF mode.

Step 3: Testing

Test Data for 2 LED Test Setup

The test set up is 2 LEDS with their own current limiting resistors which is a 150 ohm and 18 ohm resistor in series for each led. I wanted to start the test with the current as close as possible to 20 milliamps per led without going over 20 milliamps. The goal of the test was not to find out how many hours a flashlight would run before the amount of light it put out was totally useless. This would be a matter of opinion. I wanted to run the test until the current per led was about half of the starting current or a little less. This is so we would all know that:

1. With a 2 LED upgrade you can run it for about 250 hours and the output will degrade from two LEDS at full brightness to one LED at full brightness because the current for each LED will be about 1/2 what it was after 250 hours. There probably would be some point of agreement That one LED at full brightness is useful in an emergency situation and it sure beats the alternative which is no LED at any brightness.

2. You can extrapolate the data to estimate that it will take twice as long for a single LED upgrade to go from full brightness down to 8 or 9 milliamps which in my opinion still provides a useable amount of light. But don’t take my word for it. Just take a lantern battery and some clip leads and connect a LED and a 390 ohm resistor in series to see what 8 - 9 milliamps looks like. While you are at it, try a 1.6k ohm resistor to see what 2 milliamps looks like.


The current measurement is for one LED so the actual current drain on the battery is about double.

Day 0 Evening (start): Battery voltage = 6.38 volts (no load). 18.6 milliamps.

Day 1 Morning: 16.8 milliamps x 2

Day 1 Evening: 16.2 milliamps x 2

Day 2 Morning: 15.4 milliamps x 2

Day 2 Evening: 14.7 milliamps x 2

Day 3 Morning: 14.7 milliamps x 2

Day 3 Evening: 14.5 milliamps x 2

Day 4 Morning: 14.1 milliamps x 2

Day 4 Evening: 13.8 milliamps x 2

Day 5 Morning: 13.1 milliamps x 2

Day 5 Evening: 12.8 milliamps x 2

Day 6 Morning: 12.4 milliamps x 2

Day 6 Evening: 12 milliamps x 2

Day 7 Morning: no data

Day 7 Evening: 11.3 milliamps x 2

Day 8 Morning: 10.9 milliamps x 2

Day 8 Evening: 10.5 milliamps x 2

Day 9 Morning: 9.9 milliamps x 2

Day 9 Evening: 9.5 milliamps x 2

Day 10 Morning: 9.4 milliamps x 2

Day 10 Evening: 9.3 milliamps x 2

Day 11 Morning: 9 milliamps x 2

If I was running the test circuit for only 6 hours a day, the test would have run about 41 days.

In my opinion, running only 1 of the LEDS at the end of the test at 8.5 milliamps, the LED provided plenty of light. I unplugged one of the LEDS in the test set up and went for a little walk around the house in the dark. I read the labels on the backs of a few products. One LED worked fine.

How did I come up with 20 cents for the upgrade?

This was based on the LEDS I bought from Deal Extreme a few years ago which can still be purchased for 10.5 cents each if you buy 75 at a time (10.5 cents includes the shipping). The resistor costs under 10 cents each for the single piece price but drops to about 5 cents each for 10 pieces and about 2 cents each for 100 pieces. You easily could do the 1 led upgrade for under a dollar with all Digikey parts (plus shipping). Here are the links:




Here are some links to shop for the Rayovac flashlight:


For the 2 LED upgrade you double the cost.

Run Times:

2 LED upgrade: 250+ hours

1 LED upgrade: 500+ hours

Pictures:  1. The test set up.  2.  Reading a book on 11th day.

Step 4: Quickie Flashlight

What if you need one of these flashlight right now (emergency). Well, in a pinch you can make the MacGyver version if you have the parts to make it. See the picture of the no solder / no flashlight, one LED emergency light. By day 3 or 4 of this disaster you will be the envy of all the neighbors. That is why it is good to have extra batteries, LEDS and resistors on hand.

So there you have it. We are all done. Or are we? Nope. I will be running this battery into the dust. I will update the list of days and currents per LED when I hit about 2 Milliamps per LED.

So you can mark emergency lighting off your list. Have you given any thought to water?

Step 5: The Rayovac Controversy (added 10-17-12)

Here is the link to the page on the Rayovac web site with the flashlight I used for this step by step instructble. At least I think it is.


As of today (10-17-12) it says that the battery included with the flashlight is an alkaline (not zinc-carbon) battery. I did some shopping on line and I read different things on different web sites like this:

The flashlight will run 4.5 hours with an alkaline battery and the flashlight includes a battery.

The flashlight includes an alkaline battery.

The flashlight includes a carbon battery.

Well, if Rayovac says the battery I got with the flashlight is an Alkaline then by golly it is an alkaline. So after work today I bought an alkaline battery that says “alkaline” on it. I weighed the Rayovac alkaline and it weighed about 1-¼ pounds. I weighed the Energizer alkaline and it weighed 2-1/8 pounds. I figured the battery that I got with the flashlight must weigh less because I had been using it for a week and a half straight (ha ha) so I weighed a new one (still over 6 volts) and it was a light weight also. So for the rest of this instructable I will do away with any confusion and refer to the Rayovac alkaline as the lightweight alkaline. By the way, I like Rayovac. The flashlight I bought is a great deal at $3.50, even with the lightweight alkaline. I am sure I will buy more in the future.

I have some battery data from several years back from Panasonic. It shows that alkaline batteries last about twice as long as their zinc-carbon counterparts. I have started a test today. I will be running one led with a 168 ohm resistor (150 + 18) connected to the Energizer alkaline battery for 1000 hours. I will measure and record the current in the circuit once a day and update this step a couple of times a week. My expectation based on the testing I have done already and the Panasonic specifications is that the LED will provide a useful amount of light for most if not all of the 1000 hours. I hope the current at 1000 hours will be about half of the starting current. We will see in 42 days.


Day 0 = 20 milliamps

Day 1 = 18.6 milliamps

Day 2 = 17.9 milliamps

Day 3 = 17.4 milliamps

Day 4 = 17.2 milliamps

Day 5 = 16.8 milliamps

Day 6 = 16.5 milliamps

Day 7 = no data

Day 8 =16.25 milliamps

Day 9 = 16.2 milliamps

Day 10 = 16. 12 milliamps

Day 11 = 16.1 milliamps

Day 12 = 15.8 milliamps

Day 13 = 15.7 milliamps

Day 14 = 15.6 milliamps

Day 15 = 15.5 milliamps

Day 16 = 15.34 milliamps

Day 17 = 15.27 milliamps

Day 18 = 15.2 milliamps

Day 19 = 15.18 milliamps

Day 20 = 15.14 milliamps

More to follow in the days ahead..

The picture shows the one LED / alkaline test setup.


Day 47 = 12.7 milliamps (about 1100)

Day 71 = 9.7 milliamps (about 1700 hours)

Step 6: Updates

After 16-1/2 days the 2 led test setup is running at 3.4 milliamps per LED for a total of 6.8 milliamps (see picture).

Step 7: All New and Improved Easier Flashlight Hack Increases Run Time 6.5 X

Here it is:


You will need this flashlight and a resistor.


See all my instructables here:


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    16 Discussions


    5 years ago

    This is great


    6 years ago on Step 6

    Interesting instructable I connected a led onto a 3 volt silver battery that was already depleted and the computer showed that the battery needed replacement. After three weeks with a clothes peg to keep the wires connected the light was still usable as a light strong enough to keep from each other bumping into another or furniture!


    Reply 6 years ago on Step 6

    Thanks KROKKENOSTER, I always save my “dead batteries“ which are primarily AA, AAA, C, D and 9V. I can re-use or continue to use them by the following:

    I have used the circuit board out of the rayovac pen light type flashlight (part number is

    BRSLEDPEN-BA ) to make flashlights of my own that use a larger battery than the AAA supplied with it. It will light up until the battery voltage hits about 0.5 volts. So if you have a batteries that need to be replaced in a device that are above that, you can still use them. This is typical. A 6 volt device that uses 4 AAA batteries will probably stop working before the batteries put out 3 volts. But each battery is still at 0.7 volts and therefore still useable. By the way. This board works better than the typical joule thief circuit in my opinion.

    I also have the guts of a few solar lawn lights on the roof (the housing and solar cell only, no circuit board or LED). I have connected them to CAT5 cable (several pairs of insulated copper wire in one cable, Cheap) down to the house. One solar cell can recharge a non-rechargeable alkaline battery over the period of a few days. This is hit and miss. Some recharge and some do not. No regulator is needed, none have leaked or exploded since the charge current is about 10 -20 milliamps. Just check them once a day.


    By the way. The board out of the rayovac flashlight is used as the LED driver in one of my super capacitor flashlights:


    In some cases you can use the board out of the solar lawn light as a joule thief circuit also.


    Reply 7 years ago on Introduction

    Electronics is not everyones cup of tea but for those who want to learn more, there are lots of Instructables about ohms law and soldering. I checked.

    That is what Instructables are all about. Learning new things.



    Reply 6 years ago on Introduction

    Soldering is really, really easy. As long as you follow the cardinal rule of leanliness. Always thoroughly clean whatever metal you are wishing to solder. Do not touch it with bare fingers or allow any grease to come into contact with it. Get some flux on straight after cleaning if it's needed (not usually with electrical/electronic soldering). Lastly, use an iron or other heat source that is up to the job. If the heat source is too low a temperature you are going to struggle to get a good electrically sound joint. If you only have a large soldering iron then there are tricks to using it with delicate electronic components such as wrapping a coil of copper wire around the tip. Again, it must be clean :) Nice 'ible', thank you. Oh, Ohm's law, now that one I am still learning but I am getting there :)


    7 years ago on Introduction

    Congratulations on being a finalist in the be prepare contest!


    7 years ago on Introduction

    Great ible wish i could understand this stuff the mind locks up when i start one of these projects


    Reply 7 years ago on Introduction

    Hello Fretted,

    You can buy an off the shelf, long run time flashlight if you do not want to make one:


    There are actually several out there. The above thread deals we three that I bought and have used and I think I did run times on two of them. You got me thinking. One of them is really easy to hack (I made a lightwave sound transmitter by adding only 2 parts). I could easily add a resistor and a switch to make a hi/lo feature for long run time and really long run time. The benefit would be that in “hi” mode the flashlight is very bright. So the user would have the benefit of a bright flashlight and a super long run time flashlight. I think I will add that step to this ible this weekend.




    7 years ago on Introduction

    Nice work! See my solder free (using wire connector) projects...

    "May the good belong to all the people in the world.
    May the rulers go by the path of justice.
    May the best of men and their source always prove to be a blessing.
    May all the world rejoice in happiness.
    May rain come in time and plentifulness be on Earth.
    May this world be free from suffering and the noble ones be free from fears"

    ---- Vedic blessing


    7 years ago on Introduction

    I know this might be a dumb'ish question... but with the one LED set up and the resistor; Since it is a 6 volt battery could you just use 2 LED's in parallel hooked directly to the battery


    Reply 7 years ago on Introduction

    If you put 2 leds in parallel (anode to anode and cathode to cathode ther will be 6 volts across each LED. So they will burn out quickly (probably right away). If you connect them in series (anode of one to cathode of the other one) you will get about 3.1 to 3.2 volts across each LED and they will light up but not at full brightness. Also the run time may be a little low because the battery will only need to degrade to a little under 6 volts and then the leds will be very dim.


    Reply 7 years ago on Introduction

    Hello Zomfibame

    I tried 2 LEDS in series with no resistor this morning and here are the results:

    2 LEDS in series with 6.23 volts: LEDS are bright.
    2 LEDS in series with 5.28 volts: LEDS are dim.
    2 LEDS in series with 4.13 volts: LEDS do not light up.

    The battery that test at 4.13 volts is the one in my test set up which is powering 2 LEDS each with a resistor in parallel. I am on day 13 (I think) and each LED currently has 7.5 milliamps going through it for a total of 15 milliamps which is plenty of light. I estimate that I will need to run the test for 5 more days before I get down to about 5 milliamps.

    The concept still has a use. I run 4 LEDS in series instead of 3 when powering them with 12 volts. Intuitively one would think this would not work well since each LED needs to be over 3 volts to be at full brightness but in fact it works fine and I still need a 33 ohm resistor in the circuit. I could get a longer run time with 3 in series with a larger resistance but it works out well to have a shorter run time when using lead acid batteries which is what I use for the 12 volts. The shorter run time before the LEDS start to get a little dim prevents me from draining the lead acid battery too far which shortens its life.