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I currently have an ongoing supply of "Dead" (actually half used) alkaline D cells from a local manufacturing company. They use D cells in a couple of pieces of equipment as well as the automatic paper towel dispensers in all the restrooms. I understand from my friend who works there that they are charged a fee to dispose of them properly. So we are being green by completely depleting these batteries instead of buying new ones and also saving this company money. I believe the towel dispensers are the reason why so many of the batteries I get test at 1.1 to 1.4 volts. I assume the motor that turns the great big roll of towels draws a lot of current.

I bought a two pack of Eveready single D cell flashlights (model: EVEL152S). You can get these for under $5 if you shop around. They work well and I really like the click on / click off push button switch. I like mine (pictured above) better as a reading light and task light and emergency light around the house because of the following features:

Variable control for light intensity

RCA / Phono jack output for interchangeable LEDS.

The output jack also allows you to connect any length RCA cable to run the LED where needed.

Operates from 0.5 volts to over 3 volts input voltage under load.

Can be used to power other external 3 - 3.5 volt circuits (I have powered an LM386 amplifier circuit with it).

Step 1: The Circuit

This circuit uses a circuit board removed from my favorite flashlight to hack. The Rayovac BRSLEDPEN-BA single AAA LED pen light. It is a nice little light as is. I believe the specs say 5.5 hours with the supplied zinc carbon battery. You should be able to find this flashlight for $3.50 - $4.00.

My testing shows that the circuit puts out about 3.5 volt with an input voltage all the way down to 0.5 volts under load (which is not much of a load). So this circuit makes a nice replacement for a joule thief. I plan on testing the circuit driving two LEDS to see how many hours it will run but I have been dragging my feet since I have dozens of free, half used ones. Doing the math based on the specs on the Rayovac packing, I come up with 160 hours with a new alkaline battery. We shall see starting this weekend. I will update. The lit up LEDS shown in the pictures are "Piranha" type LEDS. I bought these from Lighthouseleds.com.

We have the results.

Run time test: 144 hours (equal to 6 hours a day for 24 days).

Testing was done done with a new Rayovac "High Energy" alkaline D cell ($7.97 for a six pack).

<p>Something is wrong in your schematics, you can't have a 5k pot in serie with the Led and this for 2 reasons :</p><p>1) a white Led has a trigger voltage of 3v min. which leaves 0.5v max. for the 5k, implying an intensity of 0.1mA, this means 0.3mW for the Led, I don't think such Leds exist appart from special domains.</p><p>2) when the pot value decreases close to 0, the max available intensity, delivered by the converter, passes thru the pot and it simply burns.</p>
<p>And yet is works. Try it. </p><p>1. The circuit works fine before being modified with the LED supplied with the flashlight, a new LED or multiple LEDS, So your first comment about knowing if this type of LED exists is invalid.</p><p>2. I am adding an additional resistor to the one in the original circuit so no problem there (and also no problem with anything burning).</p><p>I also run 4 LEDS in series with 12 volts and a resistor. Figure that one out.</p><p>Try it.</p>

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