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).
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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).