I go through a lot of AA batteries when I use the flash on my digital camera. AA batteries are 1.5V, and my camera considers them ‘dead’ at 1.2V. 1.2V is 80% of 1.5V; so it looks like consuming 20% of the battery’s potential is all you get (I would call that 20% efficient). What a waste! That is like taking just one bite out of 10 apples because the first bite always tastes best! There has to be a way to make use of the ‘wasted’ 80% of a battery instead of throwing them into a landfill.
Around 1999 a hobbyist was trying to solve a problem with the LED flashlight he was trying to design. Look at the circuit below:
This circuit does not work. The LED requires a little more than 2V, and a single alkaline battery provides 1.5V. 2 batteries in series will provide 3V, which will light the LED; but you will need a series dropping resistor to limit its current to a safe value. A fresh pair of batteries might need a 220 ohm resistor while later on a 100 ohm resistor would provide many more house of use as the voltage declines. He solved his problem by modifying a WWII vacuum tube oscillator circuit to use a transistor instead and used a coil’s inductive kick to provide high frequency pulses to drive the LED. Inductive kick is a neat thing: Lenz’s Law states that an inductor resists a change in current and when that current is removed an ‘kick’ of opposite polarity is produced. This kind of circuit is used in cattle prods: they are battery operated yet provide short pulses of thousands of volts. With the higher voltage and frequency pulses, the series limiting resistor for the LED is not so critical anymore. His circuit maintained a bright LED light and it functioned over a much longer battery drain curve: all the way down to 0.5V; bring battery usage to a 66% efficiency as I recon it). His name was Z. Kaparnik. when the article was published, another Hobbyist, Clive Mitchell coined the name “Jewel Thief” for its energy-harvesting capabilities.
“A joule thief is a minimalist Armstrong[1] self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving light loads.
It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged (or “dead”); hence the name, which suggests the notion that the circuit is stealing energy or “joules” from the source. The term is a pun on the expression “jewel thief”: one who steals jewelry or gemstones.
The circuit is a variant of the blocking oscillator that forms an unregulated voltage boost converter. The output voltage is increased at the expense of higher current draw on the input, but the integrated (average) current of the output is lowered and brightness of a luminescence decreased.”
Wikipedia article on Jewel Thief
Circuit Diagram
There are many variations of the basic Jewel Thief circuit; I have used this one to build about 6 night lights that I have used for years. They run off of my “dead” batteries, so when I do throw them away, they are truly DEAD! (My night lights run for a week or more on a ‘dead’ battery).
Bifilar Winding on a Ferrite Toroid
There is nothing very hard about building this circuit: the most challenging part is the bifilar winding on the donut toroid to make the transformer. To simplify the process, I supplied twin cable used for wiring speakers (it will cut your labor in half, and will keep which wire is which straight). Take a close look at the speaker wire: the insulation one wire of the pair has a white stripe. I have laid them out on the coil pictured above in this way: Pair one white wire, other wire; pair 2 white wire, other wire. This is how they need to lay for the circuit you bread board.
Note the 2 big dots on the transformer windings: they are not across from each other, they are opposite. This signifies “opposing coils”. See the junction at the top of the transformer where the 2 coils join? It boils down to the white wire of one pair connecting to the non-white wire of the other pair. This junction also connects to the positive battery terminal.
Hands On!
Wind the coil (you should get about 11 turns) and lay the wires out like I described above. Construct the circuit and double-check your work. Install the battery and see if it lights. If not, try reversing the LED or swapping coils. Inside the LED is a larger hook-shared appendage, that is the cathode which goes to ground/negative.
When you are through with this project, unwind your coil so the next person gets to experience bifilar coil winding.
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