A 555 timer is a type of integrated circuit, also known as a chip, which is extremely popular for a variety of purposes. Its main use is as a timer, but it can also be used as a multivibrator.

The 555 timer takes its name from the fact that the original model had three resistors, each rated at five thousand ohms. The first model was formally known as the SE555/NE555 and marketed as the IC Time Machine. At the time of its 1971 release, the 555 timer was the only commercially available timer circuit. Today there are many different models from different companies, though nearly all manufacturers include 555 as part of the model number because it’s such a recognizable name.

There are two different types of 555 timer, both being eight-pin chips. The most common one is the rectangular ‘V’ package, which has four pins down each side. The other version, which used to be the most common but has now fallen from favor somewhat, is the circular ‘T’ package.

The eight chips carry out the following functions:

1. Ground, which acts as a safety measure as with electrical plugs
2. Trigger, which passes on voltage to start the timing operations
3. Output, which carries voltage to the device using the timer
4. Reset, which is used to end the timing operation
5. Control voltage, an optional pin used for controlling the timer from outside the main circuit set-up
6. Threshold, which determines how long the timer should output voltage in each on/off cycle – in other words, how long the timing interval should be
7. Discharge, connected to a capacitor which also influences the timing interval
8. V+, which is the voltage input

There are three different types of use for a 555 timer. Monostable operation is when the output signal simply switches between the default off position and a temporary on position at regular intervals, which is most commonly used for timers. Astable operation is when the output voltage rises and falls in a set pattern, making it an oscillator. As the pattern can be varied, it can be used for any purpose which requires a particular tone pattern. Bistable operation is when the signal can be held in one of two positions, meaning the 555 timer can act as the smallest possible unit of computer memory.

There are several products which are derived from the 555 timer. The 556 combines two 555 timers on a single chip, while the 558 combines four 555 timers on a single chip, albeit it with some modification. The 7555 is an extremely low-powered version of the 555 timer.