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Larry Kahaner WB2 NEL
4259 Bedford Ave.
Brooklyn NY 11229
A cl oc k is a devi ce that
emits pul ses at designated
interval s . The pul ses,
either logi c 0 or 1, can be
used to sy nc h ro n ize
different parts of a circuit
so they function or turn
on at the same time . . .
22
A
ll right already: what's a clock? That's
what many readers have asked , and
this article not only answers the question
but shows how to build one and incor porate
it into a complete Ie experimenting unit.
And to satisfy those who have asked for
simple projects to show tC logic "even if
they don't do much," we have that, too.
First, a clock is a device that em its pulses
at designated intervals . The pulses, either
logic 0 or 1, can be used to synchronize
different parts of a circuit so they function
or turn o n at the same ti me. For instance,
we may have fi ve flip flops in one circu it and
want them to activate together. By connecti
ng them to the same clock we are
assu red that they operate simu ltaneously.
If we have a clock that pu lses at o ne
second intervals and it drives a seve n segmcnt
LED so that numbers are counted, we
have a clock, in the everyday sense.
555 rc Chip
The heart o f the clock is the 555 IC chip
shown in Fig. 1. The internal circuitry is
complex. The equivalent circuit made of
disc rete components takes over twenty transistors,
half a dozen resistors and a few
diodes. It is a linear device.
It has many uses, bu t the two most
common ones are astable operation, where it
continues to emit pulses, and mo nostable, or
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Clocks...
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Fig. 1. 555 timer. 1 - ground; 2 - trigger; 3
- output; 4 - reset; 5 - control voltage; 6 ­
threshold; 7 - discharge; 8 - Vee.
"one shot," where it acts as a timer and
pulses once at a predetermined time.
The circuit for the one shot timer is
shown in Fig. 2.
Pin 2 is the trigger and a pulse un der '{3
Vcc will set an internal flip flop . This flip
flop releases the short circui t on C, imposed
via pin 6. Th is in turn drives the output (pin
3) high . The capacitor now starts to charge
[remember T = RC) and when it reac hes 2/3
Vcc the fli p flo p is rese t by a comparator
and the output goes low, as the capacitor
discharges quickly. 2/3 vee is the threshold
voltage.
A very good feature is that once
tr iggered, th e clock wi ll not retrigger until
t he elapsed t ime is up. Th en it can be
manually rese t. However, if we do want to
ret ri gger it du ring the timing cycle, a ll we
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Fig. 2. One shot timer. Fig. 3. Astable operation.
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