Many electronic oscillator circuits, especially radio frequency circuits, are based on the LC resonant circuit or 'tank' circuit. When a charged capacitor is connected across an inductor current can oscillate to and fro through the circuit formed. The oscillations are a consequence of resonance. The frequency of these oscillations (known as the resonant frequency) is dependent on the values of the inductor and capacitor. This type of circuit is known as a parallel LC circuit. If an A.C. voltage of the same frequency as the resonant frequency is applied to the circuit, then the circuit behaves like an open-circuit.
In contrast, if an inductor and capacitor are connected in series, a series LC circuit is formed which also has a resonant frequency. If an A.C. voltage of the same frequency as the resonant frequency is applied to the series LC circuit, then the circuit behaves like a short-circuit.
The 555 is an integrated circuit first introduced by Signetics, but now produced by many semiconductor manufacturers. The usual package is an 8-pin DIL. Internally the 555 is DC coupled. It can be used for monostable and astable oscillators. The internal construction of the 555 is as follows:
The 555 IC may be used to form an astable oscillator:
The 555 IC may also be used as a monostable:
