Astable Mode with IC 555
In this mode the 555 is used as an oscillator. The oscillator frequency and mark / space ratio is set using one capacitor and two resistors. The reset pin may be used to stop the oscillator.
The oscillator frequency is given by :
f = 1.44/((R1 + 2R2)C)
The time period is given by : Output High th = 0.693(R1+R2)C Output Low tl = 0.693R2C
Decoupling
The power supply is usually decoupled by a capacitor of 10 - 100uF. The control voltage on pin 5 is decoupled by a smaller capacitor of about 10nF. Some variants of the 555 do not require a cap on this pin. A general rule of thumb is that if your circuit suddenly starts behaving unpredictably, put a cap on pin 5.
