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PDF IC555 Data sheet ( Hoja de datos )
| Número de pieza | IC555 | |
| Descripción | LMC555 / Timer IC | |
| Fabricantes | ETC | |
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A 555 Timer IC Tutorial
Página 1 de 21
© 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.
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A 555 Timer IC Tutorial
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The voltage range that can safely be applied to the trigger pin is between V+ and ground. A dc
current, termed the trigger current, must also flow from this terminal into the external circuit.
This current is typically 500nA (nano-amp) and will define the upper limit of resistance allowable
from pin 2 to ground. For an astable configuration operating at V+ = 5 volts, this resistance is 3
Mega-ohm; it can be greater for higher V+ levels.
Pin 3 (Output): The output of the 555 comes from a high-current totem-pole stage made up of
transistors Q20 - Q24. Transistors Q21 and Q22 provide drive for source-type loads, and their
Darlington connection provides a high-state output voltage about 1.7 volts less than the V+ supply
level used. Transistor Q24 provides current-sinking capability for low-state loads referred to V+
(such as typical TTL inputs). Transistor Q24 has a low saturation voltage, which allows it to
interface directly, with good noise margin, when driving current-sinking logic. Exact output
saturation levels vary markedly with supply voltage, however, for both high and low states. At a
V+ of 5 volts, for instance, the low state Vce(sat) is typically 0.25 volts at 5 mA. Operating at 15
volts, however, it can sink 100mA if an output-low voltage level of 2 volts is allowable (power
dissipation should be considered in such a case, of course). High-state level is typically 3.3 volts
at V+ = 5 volts; 13.3 volts at V+ = 15 volts. Both the rise and fall times of the output waveform
are quite fast, typical switching times being 100nS.
The state of the output pin will always reflect the inverse of the logic state of the latch, and this
fact may be seen by examining Fig. 3. Since the latch itself is not directly accessible, this
relationship may be best explained in terms of latch-input trigger conditions. To trigger the output
to a high condition, the trigger input is momentarily taken from a higher to a lower level. [see "Pin
2 - Trigger"]. This causes the latch to be set and the output to go high. Actuation of the lower
comparator is the only manner in which the output can be placed in the high state. The output can
be returned to a low state by causing the threshold to go from a lower to a higher level [see "Pin 6
- Threshold"], which resets the latch. The output can also be made to go low by taking the reset to
a low state near ground [see "Pin 4 - Reset"].
Pin 4 (Reset): This pin is also used to reset the latch and return the ouput to a low state. The reset
voltage threshold level is 0.7 volt, and a sink current of 0.1mA from this pin is required to reset
the device. These levels are relatively independent of operating V+ level; thus the reset input is
TTL compatible for any supply voltage.
The reset input is an overriding function; that is, it will force the output to a low state regardless
of the state of either of the other inputs. It may thus be used to terminate an output pulse
prematurely, to gate oscillations from "on" to "off", etc. Delay time from reset to output is
typically on the order of 0.5 uS, and the minumum reset pulse width is 0.5 uS. Neither of these
figures is guaranteed, however, and may vary from one manufacturer to another. When not used,
it is recommended that the reset input be tied to V+ to avoid any possibility of false resetting.
Pin 5 (Control Voltage): This pin allows direct access to the 2/3 V+ voltage-divider point, the
reference level for the upper comparator. It also allows indirect access to the lower comparator, as
there is a 2:1 divider (R8 - R9) from this point to the lower-comparator reference input, Q13. Use
of this terminal is the option of the user, but it does allow extreme flexibility by permitting
modification of the timing period, resetting of the comparator, etc.
When the 555 timer is used in a voltage-controlled mode, its voltage-controlled operation ranges
from about 1 volt less than V+ down to within 2 volts of ground (although this is not guaranteed).
Voltages can be safely applied outside these limits, but they should be confined within the limits
of V+ and ground for reliability.
In the event the control-voltage pin is not used, it is recommended that it be bypassed with a
capacitor of about 0.01uF (10nF) for immunity to noise, since it is a comparator input.
Pin 6 (Threshold): Pin 6 is one input to the upper comparator (the other being pin 5) and is used
to reset the latch, which causes the output to go low.
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A 555 Timer IC Tutorial
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comparators cause the flip-flop to be
repeatedly set and reset. The resulting
output is a continuous stream of rectangular pulses.
The frequency of operation of the astable circuit is dependent upon the values of R1, R2, and C.
The frequency can be calculated with the formula:
f = 1/(.693 x C x (R1 + 2 x R2))
The Frequency f is in Hz, R1 and R2 are in ohms, and C is in farads.
The time duration between pulses is known as the 'period', and usually designated with a 't'. The
pulse is on for t1 seconds, then off for t2 seconds. The total period (t) is t1 + t2 (see fig. 10).
That time interval is related to the frequency by the familiar relationship:
f = 1/t
or
t = 1/f
The time intervals for the on and off portions of the ouput depend upon the values of R1 and R2.
The ratio of the time duration when the ouput pulse is high to the total period is known as the
duty-cycle. The duty-cycle can be calculated with the formula:
D = t1/t = (R1 + R2) / (R1 + 2R2)
You can calculate t1 and t2 times with the formulas below:
t1 = .693(R1+R2)C
t2 = .693 x R2 x C
The 555, when connected as shown in Fig. 9b, can produce duty-cycles in the range of
approximately 55 to 95%. A duty-cycle of 80% means that the ouput pulse is on or high for 80%
of the total period. The duty-cycle can be adjusted by varying the values of R1 and R2.
Applications:
There are literally thousands of different ways that the 555 can be used in electronic circuits. In
almost every case, however, the basic circuit is either a one-shot or an astable.
The application usually requires a specific pulse time duration, operation frequency, and duty-
cycle. Additional components may have to be connected to the 555 to interface the device to
external circuits or devices.
In the remainder of this experiment, you will build both the one-shot and astable circuits and learn
about some of the different kinds of applications that can be implemented. Furthermore, the last
page of this document contains 555 examples which you can build and experiment with.
Required Parts:
In addition to a breadboard and a DC powersupply with a voltage in the 5 to 12 volt range, you
will need the following components: 555 timer, LED, 2-inch /8 ohm loudspeaker, 150-ohm 1/4
watt resistor, two 10K ohm 1/4 resistors, two 1-Mega ohm 1/2 watt resistors, 10 Mega ohm 1/4
watt resistor, 0.1 uF capacitor, and a 0.68uF capacitor. All parts are available from Radio Shack or
Tandy.
http://www.uoguelph.ca/~antoon/gadgets/555.htm
18/11/00
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