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| TIMER IC 555 |
555 Timer
TIME ON YOUR
HANDS ! With the monolithic integrated
circuit 555 you
can get accurate timing ranges of micro seconds
to hours,
independent of supply voltage variations. This versatile
device has a
large number of interesting practical applications,
especially for
electronic hobbyists.
Basically, the
555 timer is a highly stable integrated circuit
capable of
functioning as an accurate time-delay generator and
asa free
running multivibrator. When used as an oscillator ,the
frequency and
duty cycle are accurately controlled by only two
external
resistors and a capacitors. The circuit may be triggered
and reset on
falling wave forms. Its prominent features are
summarized
below :
* Timing from
micro seconds through hours.
* Monostable
and astable operation
* Adjustable
duty cycle .
* Ability to
operate from a wide range of supply voltages.
* Output
compatible with CMOS, DTL and TTL (when used
with a 5 volt
supply)
* High current
output can sink or source 200 mA
> Trigger
and reset.inputs are logic compatible
* Output can be
operated normal on and normal off
* High
temperature stability
Let us sce the
make-up and operation of the 555 IC and see
how the various
features can ‘be developed into practical circuits.
The 555 is
available in 8-pin and 14-pin dual-in-line packages
or in a
circular TO-99 metal can with eight leads. Pin connections for vrious packages are shown in "Fig # 1.
The SE and NE versions are similar except for maximum temperature ratings. The precision type SE maintains its essential characteristics over a temperature range of —55° C to +125° C while the general purpose type NE operates reliably only over a range of 0° C to 70°C. Some manufactures use the suffix C to indicate the commercial version for general purpose applications. Both types have a maximum rating of 18 volts and can handle power dissi-pation of upto 600 mW.
The 556 is a dual timer which is basically two 555's in a single package.
Comprising of 23 transistors, 2 diodes and 16 resistors, (fig. 2) the 555 has built-in compensation for component tolerance and temperature drift resulting in a temperature coefiencient of only 25 parts per degree centigrade.
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