One of the most popular integrated circuits (ICs) of all time, the 555 timer was introduced in the 1970s and is still made by the billions today. It's easy to use, inexpensive and suited to a wide range of applications.
The 555 timer IC is an 8-pin electronic pulse generator. Its general-purpose design lets you configure it for different modes of operation, such as astable (free-running oscillator) or monostable (one-shot). It has a frequency range of less than one pulse per minute up to about 500 kilohertz.
Many digital circuits need a centralized pulse source, called a clock, to synchronize signals and events. The 555 IC's astable mode produces a steady pulse waveform suitable for clocking digital circuits.
The same astable mode that generates pulses can be used in the audio range to create tones in a speaker. You can use a 555 as a beeper, buzzer or siren.
The 555 can be configured in monostable, or one-shot, mode. When it receives a trigger pulse, it produces a single, precisely timed pulse of its own. You can, for example, use this to open an automatic door for ten seconds, then close it.
You can use a 555 to directly drive a light-emitting diode (LED). Running as a pulse generator, it will flash the LED at any speed you wish. Several 555s flashing LEDs can create a colorful, twinkling display.