Timer circuits are used to control one or more sequences of events within an electronic system. Timer circuits work by changing the circuit from its current state to another in advance following a set parameter.
You can use timer circuits for a wide range of basic timing functions, like turning a light on for a certain amount of time or on and off repeatedly within a set time to create a flashing light or lighting sequence. Timer circuits can also be used to create musical notes of a particular frequency.
The most widespread type of timer circuit is the 555 timer, which is made up of an eight-pin dual in-line package containing 25 transistors, two diodes and 16 resistors. They can be used as either astable or monostable timers: