A blocking 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. The name is derived from the fact that the transistor (or tube) is cut-off or "blocked" for most of the duty-cycle, producing periodic pulses. The non-sinusoidal output is not suitable for use as a radio-frequency local oscillator, but it can serve as a timing generator, to power lights or LEDs, and the simple tones are sufficient for applications such as alarms or a morse-code practice device. Some cameras use a blocking oscillator to strobe the flash prior to a shot to reduce the red-eye effect.
When it comes to the components involved in this circuit, specific types of each component are needed to have it work to its full potential. The transformer is a vital component. For example, a pulse transformer creates rectangular pulses, which are characterized by fast rise and fall times with a flat top. There are a seemingly endless amount of combinations of voltages, transformers, capacitors, transistors and resistors that can be used to vary and model the circuit.
Due to the circuit's simplicity, it forms the basis for many of the learning projects in commercial electronic kits. The secondary winding of the transformer can be fed to a speaker, a lamp, or the windings of a relay. Instead of a resistor, a potentiometer placed in parallel with the timing capacitor permits the frequency to be adjusted freely, but at low resistances the transistor can be overdriven, and possibly damaged. The output signal will jump in amplitude and be greatly distorted.
Read more about Blocking Oscillator: Circuit Operation
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