This is just an interesting circuit I heard about and can be helpful for some battery-powered applications. Here are some various schematics with some details about the specific ones. Note: the various pictures are uploaded from Wikipedia Commons and our done by "Rowland - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31834817."
The coil consists of a standard ferrite toroid core with two windings of 20 turns each using 0.15 mm (0.006 inch) diameter wire (38 swg) (34-35 AWG). The circuit can utilize an input voltage down to about 0.35 V and can run for weeks using a 1.5 V LR6/AA. The battery voltage is usually 1.5 V. The resistor is ~1 kΩ, 1/4 W. The transistor could be a 2N3904, BC547B, 2SC2500, BC337, 2N2222, 2N4401 or other NPN. Vceo= 30 V, P= 0.625 W. A white light-emitting diode with Vf= 3.2 V might be used. The waveform of an operating joule thief, showing a 30% duty cycle at approximately 40 kHz
This is one with regulated output voltage.
This the final one and can be used to run something like a microcontroller off of a single AA battery.
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