Timer. 555 Datasheet
The 555 Timer IC
(Adapted from http://www.electronics.dit.ie/staff/mtully/555%20folder/555%20timer.htm)
The 555 timer IC was first introduced around 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 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 myriad 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.
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.
In this tutorial the 555 timer is examined in detail along with its uses, either by itself or in combination with other
solid state devices. This timer uses a maze of transistors, diodes and resistors and for this complex reason a more
simplified (but accurate) block diagram is used to
explain the internal organizations of the 555.
The 555, in fig. 1 and fig. 2 above, comes 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.
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 maximum total resistance for R (Ra +Rb) is 20
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 negligible (0.1%/V) drift with supply voltage.
Thus long-term 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 intervals of the output pulses. 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 calculated with the simple expression:
Assume a resistor value of 1 MΩ and a capacitor value of 1uF.
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
time is set approximately 1/3 longer than the expected time between
triggering pulses. Such a circuit is also known as a 'Missing Pulse
Looking at the curve in fig. 6. you can see that it takes
approximately 5 complete time constants for the capacitor to charge
to almost the applied voltage. It would take about 5 seconds for the
voltage on the capacitor to rise to approximately the full 6-volts.
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 (ground) when operated from positive supply voltages.
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 sources. The trigger pulse must be of shorter duration than the time interval determined by the