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

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

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). 


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Step 1: Resistor Selection and Upgrade

Picture of Resistor Selection and Upgrade
UG ON.jpg

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.

More people should know ohms law & how to solder.
luxstar (author)  dreiseratops1 year ago
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.


Kevanf1 luxstar3 months ago
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 :)
luxstar (author) 1 year ago
The 1000 hour test was a success. See step 5.

poofrabbit1 year ago
Congratulations on being a finalist in the be prepare contest!
fretted1 year ago
Great ible wish i could understand this stuff the mind locks up when i start one of these projects
luxstar (author)  fretted1 year ago
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.


luxstar (author)  luxstar1 year ago
I did a new instructable with the easier flashlight mod here:

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
luxstar (author)  pandyaketan1 year ago
Such as this one:

I use the same type terminal blocks for making voltage regulator circuits.
zomfibame1 year ago
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
luxstar (author)  zomfibame1 year ago

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.
luxstar (author)  luxstar1 year ago
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.

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