Simple Logic Indicator
Original Publication Date: 1990-Aug-01
Included in the Prior Art Database: 2005-Mar-16
Publishing Venue
IBM
Related People
Abstract
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.
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...