The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation and oscillator applications. The part is still in widespread use, thanks to its ease of use, low price and good stability. The 556 timer is simply two 555 timers in a single package.

The timer IC is a monolithic (chip) used in a variety of timer, pulse generation and oscillator applications. The part is still in widespread use, thanks to its ease of use, low price and good stability. As of 2003, it is estimated that 1 billion units are manufactured every year. - Wikipedia

It can be found in a variety of versions, both and , metal or plastic packaging. It even comes in a dual package as the 556 Timer, providing two independently programmable 555 timers. There is a less common 558 Quad timer, but it has some pins of the 4 internal 555 timers connected together internally.

Further reading:

555 circuit collections:

One could also call these "cookbooks".

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