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Simple Joule Thief

Wikipedia says :
A joule thief is a minimalist self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads. This circuit is also known by other names such as blocking oscillator, joule ringer, vampire torch. 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 “jewel thief”. 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.
I say it’s a fun little weekend project that can be done with very few parts (even salvaged) and discharged/dead batteries.

- 1 Red LED (or any other color)
- 1 2N3904 Transistor (or equivalent)
- 1 1k Resistor
- 1 Toroid
- 2 Thin wires, different colors
- 1 Battery holder

- Soldering
- Iron Solder
- Helping Hands

TIP: You can salvage the toroid and transistor from a dead CFL bulb.”

Link to article