A 555 Timer IC Tutorial
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© by Tony van Roon
The 555 timer IC was first introduced arround 1971 by the Signetics Corporation as the
SE555/NE555 and was called "The IC Time Machine" and was also the very first and only
commercial timer ic available. It provided circuit designers and hobby tinkerers with a relatively
cheap, stable, and user-friendly integrated circuit for both monostable and astable applications.
Since this device was first made commercially available, a myrad of novel and unique circuits
have been developed and presented in several trade, professional, and hobby publications. The
past ten years some manufacturers stopped making these timers because of competition or other
reasons. Yet other companies, like NTE (a subdivision of Philips) picked up where some left off.
This primer is about this fantastic timer which is after 30 years still very popular and used in
many schematics. Although these days the CMOS version of this IC, like the Motorola MC1455,
is mostly used, the regular type is still available, however there have been many improvements
and variations in the circuitry. But all types are pin-for-pin plug compatible. Myself, every time I
see this 555 timer used in advanced and high-tech electronic circuits, I'm amazed. It is just
In this tutorial I will show you what exactly the 555 timer is and how to properly use it by
itself or in combination with other solid state devices without the requirement of an engineering
degree. This timer uses a maze of transistors, diodes and resistors and for this complex reason I
will use a more simplified (but accurate) block diagram to explain the internal organizations of
the 555. So, lets start slowly and build it up from there.
The first type-number, in Table 1 on the left, represents the
type which was/is preferred for military applications which
have somewhat improved electrical and thermal
characteristics over their commercial counterparts, but also
a bit more expensive, and usually metal-can or ceramic
casing. This is analogous to the 5400/7400 series
convention for TTL integrated circuits.