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IC555 Datasheet

LMC555 / Timer IC

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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
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.
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A 555 Timer IC Tutorial
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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
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
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A 555 Timer IC Tutorial
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calculated with the simple expression:
intervals of the output pulses. As you
recall from your study of basic electronics,
it takes a finite period of time for a
capacitor (C) to charge or discharge
through a resistor (R). Those times are
clearly defined and can be calculated given
the values of resistance and capacitance.
The basic RC charging circuit is shown in
fig. 4. Assume that the capacitor is initially
discharged. When the switch is closed, the
capacitor begins to charge through the
resistor. The voltage across the capacitor
rises from zero up to the value of the
applied DC voltage. The charge curve for
the circuit is shown in fig. 6. The time that
it takes for the capacitor to charge to
63.7% of the applied voltage is known as
the time constant (t). That time can be
Assume a resistor value of 1 MegaOhm and a capacitor value of 1uF (micro-Farad). The time
constant in that case is:
t = 1,000,000 X 0.000001 = 1 second
Assume further that the applied voltage is 6 volts. That means that it will take one time
constant for the voltage across the capacitor to reach 63.2% of the applied voltage. Therefore, the
capacitor charges to approximately 3.8 volts in one second.
Fig. 4-1, Change in the input pulse frequency
allows completion of the timing cycle. As a general
rule, the monostable 'ON' time is set approximately
1/3 longer than the expected time between
triggering pulses. Such a circuit is also known as a
'Missing Pulse Detector'.
Looking at the curve in fig. 6. you can see that it takes approximately 5 complete time
constants for the capacitor to charge to amost the applied voltage. It would take about 5 seconds
for the voltage on the capacitor to rise to approximately the full 6-volts.
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A 555 Timer IC Tutorial
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Definition of Pin Functions:
Refer to the internal 555 schematic of Fig. 4-2
Pin 1 (Ground): The ground (or common) pin is the most-negative supply potential of the
device, which is normally connected to circuit common when operated from positive supply
Pin 2 (Trigger): This pin is the input to the lower comparator and is used to set the latch, which
in turn causes the output to go high. This is the beginning of the timing sequence in monostable
operation. Triggering is accomplished by taking the pin from above to below a voltage level of 1/3
V+ (or, in general, one-half the voltage appearing at pin 5). The action of the trigger input is level-
sensitive, allowing slow rate-of-change waveforms, as well as pulses, to be used as trigger
One precaution that should be observed with the trigger input signal is that it must not remain
lower than 1/3 V+ for a period of time longer than the timing cycle. If this is allowed to happen,
the timer will retrigger itself upon termination of the first output pulse. Thus, when the timer is
driven in the monostable mode with input pulses longer than the desired output pulse width, the
input trigger should effectively be shortened by differentiation.
The minimum-allowable pulse width for triggering is somewhat dependent upon pulse level, but
in general if it is greater than the 1uS (micro-Second), triggering will be reliable.
A second precaution with respect to the trigger input concerns storage time in the lower
comparator. This portion of the circuit can exhibit normal turn-off delays of several microseconds
after triggering; that is, the latch can still have a trigger input for this period of time after the
trigger pulse. In practice, this means the minimum monostable output pulse width should be in the
order of 10uS to prevent possible double triggering due to this effect.
Page 4
Part Number IC555
Manufactur ETC
Description LMC555 / Timer IC
Total Page 21 Pages
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