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Timer. IC555 Datasheet |
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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
incredible.
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.
http://www.uoguelph.ca/~antoon/gadgets/555.htm
18/11/00
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A 555 Timer IC Tutorial
Página 2 de 21
The 555, in fig. 1 and fig. 2 above, come in two packages, either the round metal-can called
the 'T' package or the more familiar 8-pin DIP 'V' package. About 20-years ago the metal-can type
was pretty much the standard (SE/NE types). The 556 timer is a dual 555 version and comes in a
14-pin DIP package, the 558 is a quad version with four 555's also in a 14 pin DIP case.
maximum total resistance for R (Ra +Rb) is 20 Mega-ohm.
Inside the 555 timer, at
fig. 3, are the equivalent
of over 20 transistors, 15
resistors, and 2 diodes,
depending of the
manufacturer. The
equivalent circuit, in block
diagram, providing the
functions of control,
triggering, level sensing
or comparison, discharge,
and power output. Some
of the more attractive
features of the 555 timer
are: Supply voltage
between 4.5 and 18 volt,
supply current 3 to 6 mA,
and a Rise/Fall time of
100 nSec. It can also
withstand quite a bit of
abuse.
The Threshold current
determine the maximum
value of Ra + Rb. For 15
volt operation the
The supply current, when the output is 'high', is typically 1 milli-amp (mA) or less. The initial
monostable timing accuracy is typically within 1% of its calculated value, and exhibits negligble
(0.1%/V) drift with supply voltage. Thus longterm supply variations can be ignored, and the
temperature variation is only 50ppm/°C (0.005%/°C).
All IC timers rely upon an external
capacitor to determine the off-on time
http://www.uoguelph.ca/~antoon/gadgets/555.htm
18/11/00
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