Joule thief circuit is popular among electronics enthusiasts. It has many implementations but probably most common is very minimalist voltage booster based on NPN transistor, coil, resistor, and LED. Its primary intent is to squeeze the remaining juice of dying batteries to light a LED. Since a pile of dying batteries keeps growing, I decided to give it a try. Instead of building the real circuit blindly, I decided to understand what is going on inside the circuit. So I made this circuit on LTSpice simulator. You may find some difficulties when adding a transformer to the circuit, there is an excellent video on how to do this (LTSpice Transformer).