IC 555 Timer Working: Pin Diagram & Specifications
November 27, 2018
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The 555 timer IC is an integral part of electronics projects. Be it a simple project
involving a single 8-bit micro-controller and some peripherals or a complex one
involving system on chips (SoCs), 555 timer working is involved. These provide time
delays, as an oscillator and as a flip-flop element among other applications.
Introduced in 1971 by the American company Signetics, the 555 is still in widespread
use due to its low price, ease of use and stability. It is made by many companies in the
original bipolar and low-power CMOS types. According to an estimate, a billion
units were manufactured back in the year 2003 alone.
Depending on the manufacturer, the standard 555 timer package includes 25
transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-
in-line package (DIP-8). Variants consist of combining multiple chips on one board.
However, 555 is still the most popular. Let us look at the pin diagram to have an idea
about the timer IC before we talk about 555 timer working.
555 timer circuitry
1 of 2
555 Timer IC: block diagram
8 pin DIP configuration
Pin diagram and description
Pin Name Purpose
Ground reference voltage, low level (0
1 GND
V)
The OUT pin goes high and a
timing interval starts when this
input falls below 1/2 of CTRL
voltage (which is typically
1/3 Vcc, CTRL being 2/3 Vcc by
default if CTRL is left open). In
2 TRIG
other words, OUT is high as long
as the trigger low. Output of the
timer totally depends upon the
amplitude of the external
trigger voltage applied to this
pin.
This output is driven to approximately
3 OUT
1.7 V below +Vcc, or to GND.
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A timing interval may be reset
4 RESET
by driving this input to GND, but
the timing does not begin again
until RESET rises above
approximately 0.7 volts.
Overrides TRIG which overrides
threshold.
Provides “control” access to the
5 CTRL internal voltage divider (by
default, 2/3 Vcc).
The timing (OUT high) interval
ends when the voltage at
6 THR
threshold is greater than that at
CTRL (2/3 Vcc if CTRL is open).
Open collector output which
may discharge a capacitor
7 DIS
between intervals. In phase
with output.
Positive supply voltage, which is
8 Vcc usually between 3 and 15 V
depending on the variation.
Some important features of the 555 timer:
555 timer is used in almost every electronic circuit today. For a 555 timer working as
a flip flop or as a multi-vibrator, it has a particular set of configurations. Some of the
major features of the 555 timer would be,
It operates from a wide range of power ranging from +5 Volts to +18 Volts supply
voltage.
Sinking or sourcing 200 mA of load current.
The external components should be selected properly so that the timing intervals can
be made into several minutes along with the frequencies exceeding several hundred
kilohertz.
The output of a 555 timer can drive a transistor-transistor logic (TTL) due to its high
current output.
It has a temperature stability of 50 parts per million (ppm) per degree Celsius change
in temperature which is equivalent to 0.005 %/ °C.
The duty cycle of the timer is adjustable.
Also, the maximum power dissipation per package is 600 mW and its trigger and reset
inputs has logic compatibility.
555 timer working
The 555 generally operates in 3 modes:
1. A-stable
2. Mono-stable
3. Bi-stable modes.
Astable mode
This means there will be no stable level at the output. So the output will be swinging
between high and low. This character of unstable output is used as a clock or square
wave output for many applications.
Mono-stable mode
This configuration consists of one stable and one unstable state. The stable state can
be chosen either high or low by the user. If the stable output is set at high (1), the
output of the timer is high (1). At the application of an interrupt, the timer output
turns low (0). Since the low state is unstable it goes to high (1) automatically after the
interrupt passes. Similar is the case for a low stable monostable mode.
Bi-stable mode
In bistable mode, both the output states are stable. At each interrupt, the output
changes from low (0) to high (1) and vice versa, and stays there. For example, if we
have a high (1) output, it will go low(0) once it receives an interrupt and stays low (0)
till the next interrupt changes the status.
