Joule thief circuit is popular among electronics enthusiasts. It has many implementations but probably the most common is a 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 the LTSpice simulator. You may find some difficulties when adding a transformer to the circuit, there is an excellent video on how to do this.