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A Joule Thief is a minimalist, self-oscillating voltage booster circuit that converts a low DC voltage—such as a nearly depleted 1.5V AA battery—into a higher voltage capable of lighting a white LED requiring 3.0V to 3.2V. When connected, current flows through one winding of the toroidal ferrite transformer and a $1\text{ k}\Omega$ resistor into the base of an NPN transistor, switching it on. As the transistor turns on, a larger current surges through the collector and the second winding, expanding a magnetic field in the transformer core that pushes even more current into the base, rapidly forcing the transistor into full saturation. Once the core saturates and the magnetic field stops expanding, the induced base voltage collapses, suddenly switching the transistor off. According to Faraday's law of induction, this abrupt collapse causes the stored magnetic energy to discharge as a high-voltage inductive spike that combines with the battery voltage to overcome the LED's forward threshold. As soon as the stored energy dissipates through the LED, the transistor switches back on, repeating this cycle tens of thousands of times per second to create the visual illusion of a steady light.
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