The subject of this paper is analysis of theremin's working principle as well as designing and making one. The first chapter explains the history of theremin and it's influence on musicians and scientists. The second chapter describes the main components used to make a theremin, as well as the main principles behind it such as heterodyning, the change of capacitance and frequency caused by changing the position of our hand around the antenna and the conditions for Barkhausen criterion that are necessary for the circuit to oscillate. Other than that, the working principle of operational amplifiers and oscillators is explained with formulas and equations as well as graphical and schematic representations of the system made in MATLAB and LTspice programs. Schmitt trigger relaxations oscillators, which are used in making the theremins oscillation circuit, are explained in detail. The design of a theremin circuit as well as its connecting to a breadboard is shown in the third chapter. For better understanding the circuit is split into five parts; pitch oscillators, bypass capacitors, differential amplifier, filter and audio amplifier. Every part and its function in the circuit is explained in detail using the values used in making the theremin.