Dead Battery Lights White LED with Faraday Induction: A Simple Toroid Circuit

Overview

This video explains how a dead battery can light a white LED using a simple toroid based circuit that relies on Faraday's induction to boost voltage. By routing current through two coils, a resistor, and a transistor, a small base current is amplified to drive a larger current through the primary coil, generating a changing magnetic field. That changing field induces voltage in the second coil, which helps sustain LED conduction. The LED appears continuously lit due to rapid pulsing at scales visible to the eye. The host also covers winding the toroid and building a DIY kit with all components included, offering hands on learning of the underlying physics.

Introduction

The video presents a clever circuit that lets a dead battery illuminate a white LED. It centers on a toroid core with two windings, a resistor, and a transistor. The key idea is that a changing magnetic field can transfer energy between coils through mutual inductance, producing voltages higher than the original supply.

Core Principles

The explanation anchors on Faraday's law: a changing magnetic field induces voltage in nearby coils. When current changes in path B, the cobalt ferrite toroid stores energy in its magnetic field. This stored energy and the changing field induce voltage into coil B, which feeds the transistor base and boosts current in the main circuit. The chain of events creates a feedback loop that continues until the core saturates.

Circuit Layout and Operation

In the depicted circuit, current from the battery enters via the switch and splits into two paths. Path B through the toroid drives the transistor into partial conduction, allowing current through path A to increase. The growing magnetic field in the core induces voltage into coil B, which adds to the base drive, further increasing current through coil A. This continues until the core saturates. When saturation occurs, the induced voltage collapses, causing the base current to drop and the transistor to switch off. The collapsing field then generates a voltage spike, which the LED conducts, releasing light. This cycle repeats hundreds of thousands of times per second, so the LED looks steadily lit to the human eye.

Key Components and Phenomena

The resistor limits current to protect the transistor, while the transistor acts as a switch whose conduction depends on base current. The toroid concentrates the magnetic flux with a ferrite core, increasing coupling between windings. Mutual inductance is the mechanism by which energy is transferred from the primary coil to the base of the transistor and ultimately to the LED. Observing the circuit with an oscilloscope reveals large voltage spikes that appear as brief surges before the LED lights permanently.

DIY Build and Learning

The host offers a DIY kit containing all components and demonstrates winding the toroid and assembling on a breadboard before soldering to a PCB. A breadboard prototype helps validate functionality prior to final assembly. Viewers are encouraged to try building the circuit themselves, with guidance on component placement and winding technique to avoid miswiring and saturation problems.

Audience Engagement

The video ends with a playful North Pole polarity puzzle, inviting viewers to comment their answer before the solution is revealed, and closes with a call for viewer interaction on related experiments.