Problem Description

Problem Description

Considering the availability of two separate doors for Entry and Exit, two separate Proximity Switches can be
used to detect entry and exit of students.

One proximity switch is mounted at the Entry door and the other at the Exit door.

Both the switches will generate two different outputs which can be then fed to PLC to operate the lights
according to the Ladder Logic Program written in its memory.

Counter must be used to count the number of students entering and exiting.

Comparator must also be used to compare the count value with the given maximum capacity of 120.
PLC Program
Here is PLC program to Maintain the Capacity of a Particular Classroom, along with program explanation
and run time test cases.
List of Inputs and Outputs
I:1/0 = Proximity Switch to detect Entry of a student.
(Input)
I:1/1 = Proximity Switch to detect Exit of a student.
(Input)
O:1/1 = Red Light to indicate availability in the classroom
(Output)
O:1/0 = Light to indicate classroom’s maximum capacity.
(Output)
C5:0 = Counter to count the number of students entering. (Counter)
C5:0 = Counter to count the number of students exiting. (Counter)
LES = Comparator to compare the counter value
NOTE: Note here that the address of Counter Up and Counter Down is kept same. This must be kept
same in order to decrement and increment the same value in the accumulator of CTU and CTD.

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Ladder
 
Diagram
 
to
 
solve
 
this
 
problem
Program Description

For this problem maximum limit is set to 120.

RUNG000- Whenever a student enters in the class, Proximity Switch 1 generates a pulse which increments
counter by 1.

RUNG001- Whenever a student exits from the class, Proximity Switch 2 generates a pulse which
decrements counter by 1.

RUNG002- When the number of students in the class is less than 120, Green Light goes ON.

RUNG003- When the number of students in the class is 120 which is the maximum capacity of the class,
Red Light goes ON.

RUNG004- It terminates the program and the scan cycle is repeated again.