PLC Program for Burglar Alarm Security System


This is a PLC Program for Burglar Alarm Security System.

Problem Description

Consider the design of a Burglar Alarm for a house. This alarm will be activated if an unauthorized person is detected by a Window Sensor or a Motion Detector. Implement this Alarm System in PLC using Ladder Diagram programming language.

Problem Solution
  • Basically two sensors are used, one is Motion Detector and other one Window Sensor.
    Window sensor is nothing but a loop of wire that is a piece of thin metal foil which encircles the window.
  • The motion detector is designed such that when a person is detected, the output of sensor goes true.
  • Important thing to note here is that in Window Sensor, current is always passing until there is a breakage in glass of a window. Hence output is always true. When alarm system is active and someone tries to break the window, current does not flow through the metal foil causing output to go false.
  • Karnaugh-Map can be used here to solve the equation and then to implement its logic in Ladder Diagram.

Solved K-Map and its equation is shown belowplc-program-burglar-alarm-security-system-01

Note: This security system is normally not implemented using PLC since the hardware cost of the entire system will be effectively high. Purpose of this problem is just to learn how to implement such logics in PLC.

PLC Program

Here is PLC program for Burglar Alarm Security System, along with program explanation and run time test cases.

List of Inputs and Outputs
 I:1/0	= Master Switch				(Input)
 I:1/1	= Deactivate system			(Input)
 I:1/2	= Motion Detector			(Input)
 I:1/3	= Window Sensor				(Input)
 I:1/4	= Button to Stop Alarm (not the system)	(Input)
 O:2/0	= Master Coil				(Output)
 O:2/1	= Alarm Coil				(Output)
 O:2/2	= Alarm					(Output)

Ladder Diagram for Burglar Alarm Security System

Problem Description
  • RUNG000 simply shows a latching of a coil O:2/0 to activate the entire security system.
  • When Activate button I:1/0 is pressed momentarily, the security system is activated.
  • If the system is not activated, alarm does not indicate anything since sensors will have no effects on the Alarm Coil O:2/1.
  • When system is active and Motion detector detects a person, the alarm coil will momentarily go high activating the Alarm O:2/2 which stays ON until I:1/4 is pressed manually.
  • As we can see in RUNG001 that XIO (Normally Closed) contact is used for Window Sensor input I:1/3 because it is normally in true state when not activated. So when the breakage of a window is detected, it goes false from its true condition allowing Alarm Coil O:2/1 to go high for a moment which in turn activates Alarm O:2/2.
  • In RUNG002, latching has to be provided in order to keep the alarm ringing even if the detection by the sensors are momentary or to be accurate, pulsating.
Runtime Test Cases
Master  Window	Motion	Alarm
0	x	x	0
0	x       x       0
0	x       x       0
0	x     	x       0
1	0       0       1
1	0      	1       1
1	1       0	0
1	1       1	1

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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