The 555 is a common CMOS multivibrator used in timing applications. This IC is manufactured as L555, XR-L555, ICM7555, NE555 etc, (both linear and CMOS versions are available). It is also packaged in dual (556) and quad versions (558). This timer can be used as an astable multivibrator or as a monostable multivibrator

The 555 timer functions as an astable multivibrator, square wave generator, or signal source.

The standard 555, 556 and 558 timers are compatible with their CMOS versions, and can be used directly to clock CMOS circuits without an interface. However, the CMOS version consumes les than 5% of the power required by the standard 555.