Simple Logic Indicator

Simple Logic Indicator

       The circuit described is an electronic logic-indicator
light using a single 74LS221 nonretriggerable monostable
multivibrator IC.  The indicator glows when the input is logic high,
remains dark when the input is logic low, and flashes when the input
is time-varying.  The circuit can be used to indicate, among other
things, the status of a handshake line.  The one-shot divides down
the input frequency -- which can be several megahertz -- to a visible
rate.

      The circuit is shown in the figure.  When the input is time-
varying, operation is as follows: A rising input edge causes + QA to
go high for a fixed period T-on.  This period is determined in the
usual way by an external resistor and capacitor.

      After T-on, +QA falls and -QA rises.  This causes one of two
events that will trigger the second one-shot:
o    -QA will rise while the input is low, or
o    the input will fall while -QA is high.

      -QB will go low for a period T-off, again determined by an
external resistor and capacitor, forcing a reset of the first
one-shot.  During the reset interval, the LED cannot light.

      After T-off, -QB goes high and the first one-shot is free to
retrigger on the next rising input edge.  The result is that the
light flashes at a rate that can be much slower than the pulse rate
of the input.

      When the input is steadily high, a constant-appearing output is
provided; thus, the first one-shot goe...