Easier Flashlight Mod Increases Run Time 3.6X





Introduction: Easier Flashlight Mod Increases Run Time 3.6X

We are going to take a long run time flashlight (65 hours) and turn it into a longer run time flashlight (Update: The title of this Instructable says 3.6x run time but actual run time ended up being over 360 hours with a zinc carbon battery. That would be over 600 hours with an alkaline battery).

Yes it is another long run time flashlight hack. This one is easier than my other one. All you need a resistor, a soldering iron and this flashlight.

This Eveready flashlight model number 5109LSH15 (old model number 5109LSH7 or 5109LS) costs under $5 if you shop around. This is a great long run time flashlight to have even if you don’t do the modification.

Where to buy:


I like this flashlight because as is it has a long run time before the battery needs to be replaced. It is bright. It is cheap and it is easy to hack. You can do this hack in 10 or 15 minutes and most of that time will be waiting for your soldering iron to heat up.

The only thing about this flashlight that is less than optimum is that the 3 LEDS (original design. the new design has one high power LED) have a little too much current going through them. The LEDS appear to be “straw hat” type (20 milliamp) LEDS. Each LED has about 60 milliamps running through it.

Note: See the last step for the modification for the current version of this flashlight.

Here is a data sheet for a straw hat LED:


The LEDS in this flashlight may be specially made to handle more current but in case they are not, this modification will bring the current down to a level that will insure tens of thousands of hours of LED life.

I chose to add a 56 ohm resistor to the circuit so each LED runs at about 16.5 milliamps. This will increase the run time by 3.6 times.

The other step by step instructable to make a long run time flashlight is here:


Either one will be good to have in an extended power outage.

Step 1: Un-solder

You will need to unscrew the black ring that hold the clear plastic cover on the front of the flashlight. Then remove the assembly shown in the picture above. Install the battery and re-assemble the flashlight. Test the flashlight to make sure it works. Remove the rind and assembly shown in the picture above. Un-solder one of the two red wires from it’s metal contact. I chose the one on the left.

Step 2: Solder the Resistor

Now lets select a resistor. I chose a 56 ohm resistor which reduced the current in the circuit going to the 3 LEDS to 50 milliamps (it was 180 milliamps before the modification). Here are some other resistor values and the currents I measured:

44 ohms (two 22 ohm in series) = 56 milliamps

You may want to try out some other resistors for longer run time or higher brightness. I was going for 200+ hours of run time which is about a month of use if used 6 hours a day.

100 ohms = 31 milliamps

Solder one end of the resistor to the metal contact and the other end of the resistor to the wire that was originally attached to the metal contact.

Step 3: Testing

The first picture shows the light from an unmodified flashlight on the left and a modified flashlight on the right. Note that the light on the right is produced using 3.6 times less power or about 28% of the power to produce the light on the left. The picture of the warning sign was taken using only the light from the modified flashlight at a distance of 18 feet (camera on zoom).

Long run time flashlights make great gifts. Make several for the people on your gift list. They are also great for emergencies. This one is good for a 3.6 x bigger emergency.

Update 11-10-12.

I started a test to see how long the batteries would last running 24 hours a day. The test started on the morning of 10-26-12. I was expecting a 200 hour run time. After 15 days (360 hours). The flashlight still puts out a usable amount of light. I can still read with it without any problem. I also got the opinion of a fellow flashlight enthusiast who thinks the flashlight is still useful for its intended purpose which is for the user to be able to easily see their way around the house in the dark and be able to easily read. The flashlight is noticeably dimmer than it was at the beginning of the test so I would conclude that this is a 360 hour flashlight. It has exceeded my expectations because the run time has been increased 5.5x. If I was using this flashlight 6 hours a day the test would have taken 60 days to get to this point. If this had been a real emergency I would gladly use this flashlight several more days.

Please share. Here is the link to this instructable with all the steps on one page:


UPDATE: 3-31-13:

If you want a long run time flashlight that is a little brighter try this one:


It’s another easy 10 minute modification.


Eveready recently re-designed the 5109LS flashlight. The original flashlight had three round low power LEDS. The new version has one square high power LED. There are no exposed wires so one of the metal strips that connects the switch needs to be cut. The resistor of your choice is soldered across the cut. The current of the unmodified flashlight was in excess of 160 milliamps. I tested 3 resistors:

50 ohms = 50 milliamps

68 ohms = 44 milliamps

100 ohms = 32 milliamps

I like this version better than the original. It is an easier modification.


To make it easy to find your modified flashlight in the dark you can add a second resistor. This resistor is electrically across the switch so that when the switch is open it is really a little bit closed. Pictured is a 100,000 (100K) ohm resistor. The current in the circuit with the flashlight "off" is 0.04 milliamps or 4 hundredths of a milliamp. This produces enough light to put a dim but visible spot on the ceiling in total darkness. This will run the battery down eventually but at a rate of less than 1000th the current when on (used with a 68 ohm resistor), The locator light should run for several years. I also tried a 200,000 ohm resistor which uses half the power.



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    18 Discussions

    Modern LED flashlights already have an IC that can let you select between low/high power setting or low/med/high and even extra options like SOS.

    I made this modification out of Ron Brown's e-book (in which you are mentioned, luxstar), but unknowingly picked up one of each kind of flashlight when I was at Lowe's. Thanks for putting up a version of the mod for the new style!

    1 reply

    Ok, what did I do wrong. I did this with a 47ohm resistor and I can barely see the light. New battery and new flashlight, but it's really dim.

    1 reply

    I am not sure what went wrong. I have modified over 10 of them and clubmike and mkia did it too without problems. I have two thoughts. Did you tests your flashlight before you did the modification and it worked fine then? Maybe you actually are using a 470 ohm or a 4.7k ohm or a 47k ohm by mistake. You may want to check it with an ohm meter.

    Good luck

    Day 131 still on but not enough useful light. 3144 hours on time. Bummer, I feel lost not carrying my light with me. Time for new battery.

    1 reply

    131 days is quite impressive and useful.

    I am re-charging my original 6 volt alkaline with 3 of the cheap walmart solar lawn lights in series. I took out the circuit board, batteries and LEDS and wired them in series and put them on the roof and ran cat5 cable down to the garage. Also I have a blocking diode in the circuit. I am up to 5.82 volts. If I can get up to about 6.2 volts I will run one of the tests over again.

    Another update on your forever flashlight. Test started on 3/1/13 at 3:10 pm.
    First update on 5/27/13 on 87 days{2088hrs}. 7/7/13 at 3:10 pm flashlight will have been on for 128 days{3072hrs}!!!
    Still used for a nightlight, finding car door, house key locks etc.
    First major drop in brightness was after being on for 14 days{336hrs}.
    Since then it has been dropping gradually. Thank you.

    1 reply

    Ok i'm going to give this a shot i have to get some batteries but i have some of these flashlights laying about can't hurt to give it a shot thanks for this great idea you made it easy for me to understand and teach to my kids too .....

    1 reply

    Cool. I like those flashlights. The old kind with the maglite bulbs eat batteries, and the Nite Ize maglite LED bulbs don't work in them because of polarity. The ones you show (come with 3 LED) are pretty good, but I recommend gorilla taping the ring cap if you put it on a bike handlebar or something. I used one to mow my yard at dusk when it's cooler.

    Thanks for showing me the light!! All I could get was a 42 ohm resistor. It has worked fantastic. Turned mine on on 3/1/2013. Today 5/27/2013 at 3:10pm it will have been on for 87 days straight{2088 hours}. Still has a usable light, unlocking doors, reading if you want to.

    1 reply

    Thanks for the feedback MKIA. It is good to hear that people are actually trying this.

    ok, I did this, it worked great in fact it is still working. I soldered a 100 ohm resistor in the flashlight on Feb 5 and turned on the flashlight. It is still on and it is March 30. I can still read my calender with it and see down my back stairs to let my dog out at night. In an emergency situation this is exactly what I need. The question is how long will it last?????

    1 reply

    Thank You Clubmike.

    When your battery finally does run down you can more than double the run time by replacing the junky carbon battery that came with the flashlight with an alkaline.