As a young individual working as a Junior Technician at Mac Ltd – a company that specializes in fixing
domestic electronic equipment. I have been assigned by my owner to this role in order to develop my
skills and knowledge surrounding electronic devices and circuits. As part of this opportunity to develop
my knowledge in these two areas, I have to investigate commonly used digital electronic devices and the
type of circuits in which these devices are used. Also I have to keep accurate records of every action I do,
in order to refer back to these in the latter in order to build upon my skills and knowledge within these
two areas.
Combinational Logic Circuits:
What is a combinational logic circuit, its characteristics and example?
A combinational logic circuit is defined as a circuit that is made up of several logic gates like NOT, AND,
OR, NAND etc. Its output is always the same for the same combination of inputs it has as it is a logical
combination of the inputs. It doesn’t use any memory as the previous state of input doesn’t have any
effect on the current state of the circuit. And a combinational logic circuit has an n number of inputs and
an m number of outputs.
An example of a combinational logic circuit is an encoder. An encoder is the reverse of a decoder, and it
is a sensing device that provides feedback. This circuit works by converting motion into an electrical
signal that is read by a type of control device within a motion control system like a counter or a PLC. An
encoder also sends a feedback signal that is used to regulate position, count, speed or direction.
Task:
A chemical process is controlled by a logic circuit. The circuit uses three sensors A,B and C, which switch
an alarm Q=1 , when certain conditions are reached.
Inputs Description Binary
A Flow rate ≥ 5 litre/second 1
Flow rate < 5 litre/second 0
B Temperature ≥ 30oC 1
Temperature < 30oC 0
C Pressure ≥ 1 atmosphere 1
Pressure < 1 atmosphere 0
You should:
Draw the truth table for the logic circuit if the conditions for the alarm Q=1 are:
Flow rate < 5 litre/second and Temperature ≥ 30oC
, Or
Flow rate ≥ 5 litre/second and Temperature ≥ 30oC and Pressure < 1 atmosphere
Boolean expression for truth table using AND,OR and NOT gates:
For when Q=1 if any of the following three conditions is true:
(NOT A AND B AND NOT C),(NOT A AND B AND C),(A AND B AND NOT C)
Q=(NOT A AND B AND NOT C) OR (NOT A AND B AND C) OR (A AND B AND NOT C)
Schematic Diagram and Simulation of logic circuit:
,
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