Joule Thief

A joule thief is a minimalist Armstrong 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; 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.”