Blocking oscillator
A blocking oscillator (sometimes called a pulse oscillator) is a simple configuration of discrete electronic components which can produce a free-running signal, requiring only a resistor, a transformer, and one amplifying element such as a transistor or vacuum tube (although typically a capacitor is present as well). The name is derived from the fact that the amplifying element is cut-off or "blocked" for most of the duty cycle, producing periodic pulses on the principle of a relaxation oscillator.