Philip Kane @ jameco.com has published an article about the famous 555 timer IC and how to configure in monostable and astable modes.
The 555 timer was introduced over 40 years ago. Due to its relative simplicity, ease of use and low cost it has been used in literally thousands of applications and is still widely available. Here we describe how to configure a standard 555 IC to perform two of its most common functions – as a timer in monostable mode and as a square wave oscillator in astable mode.
How To Configure a 555 Timer IC – [Link]