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. 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. 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, LEDs, Elwire, or small neon indicators. If the output is used as an audio signal, the simple tones are also 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. 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. Circuit operation The circuit works due to positive feedback through the transformer and involves two times—the time Tclosed when the switch is closed, and the time Topen when the switch is open. The following abbreviations are used in the analysis: t, time, a variable Tclosed: instant at the end of the closed cycle, beginning of open cycle. Also a measure of the time duration when the switch is closed. Topen: instant at the end of the open cycle, beginning of closed cycle. Same as T=0. Also a measure of the time duration when the switch is open. Vb, source voltage e.g. Vbattery Vp, voltage across the primary winding. An ideal switch will present supply voltage Vb across the primary, so in the ideal case Vp = Vb. Vs, voltage across the secondary winding Vz, fixed load voltage caused by e.g. by the reverse voltage of a Zener diode or the forward voltage of a light-emitting diode (LED). Im, magnetizing current in the primary Ipeak,m, maximum or "peak" magnetizing current in the primary. Occurs