Alarm System_ Educational Example Using the Omron PLC C28K

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Alarm system: educational example using

the Omron PLC C28K

By arag61

Created 10/06/2008 - 13:52

Programming alarm system c28k electric circuit ladder ladder logic Omron plc programmable logic controller

Alarm system - an educational example using the Omron PLC (programmable logic controller)C28K

Therefore a PLC (programmable logic controller)is a device, equipped with a microprocessor, destined to control electric devices, but with the advantage that the program modification is easy and speedy compared to the one of the electric circuit that should be wired again. The PLC, after elaborating the input signals by the program, can control engines, electro-valves, warning lights etc.

So there are :

input circuits that " adjust " the electric signals, coming from devices or systems, to the PLC voltages and currents

output circuits that accomplish the inverse operation

Below, as educational exercise only, an alarm system using a PLC will be illustrated. A C28K Omron PLC and a ladder diagram will be used ( of course a PLC is wasted for this job only !!!).

Before starting, let’s spend some words on ladder diagram.

The name is based on the observation that programs in this language resemble ladders, with two vertical "rails" and a series of horizontal "rungs" between them.

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Alarm system: educational example using the Omron PLC C28K http://dev.emcelettronica.com/print/51972

Now the lamp will come on if either contact A or contact B is actuated, because all it takes for the lamp to be energized is to have at least one path for current from wire L1 to wire 1. What we have is a simple OR logic function, implemented with nothing more than two contacts and a lamp. We can mimic the AND logic function by wiring the two contacts in series instead of parallel:

Now the lamp energizes only if contact A and contact B are simultaneously actuated. A path exists for current from wire L1 to the lamp ( wire 2 ) if and only if both switch contacts are closed. The logical inversion function, or NOT, can be performed on a contact input simply by using a normally closed contact instead of a normally open contact:

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Now the lamp energizes if the contact is not actuated and de-energizes when the contact is actuated.

Ladder logic has "contacts" that "make" or "break" "circuits" to control "coils." Each coil or contact corresponds to the status of a single bit in the programmable controller's memory. Unlike electromechanical relays, a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with an indefinitely large number of contacts.

So-called "contacts" may refer to physical ("hard") inputs to the programmable controller from physical devices such as pushbuttons and limit switches via an integrated or external input module, or may represent the status of internal storage bits which may be generated elsewhere in the program.

Each rung of ladder language typically has one coil at the far right. Some manufacturers may allow more than one output coil on a rung.

--( )-- a regular coil, true when its rung is true --(\)-- a "not" coil, false when its rung is true

--[ ]-- A regular contact, true when its coil is true (normally false) --[\]-- A "not" contact, false when its coil is true (normally true)

The "coil" (output of a rung) may represent a physical output which operates some device connected to the programmable controller, or may represent an internal storage bit for use elsewhere in the program.

Coming back to our alarm system, we suppose to have the following components: a power supply: exit 24 Volts (DC)

a PLC interface a PLC

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1 siren: acoustic alarm 1 flashing light: visual alarm

1 telephonic combiner: to call the police dept. or other people The working conditions are:

alarm connection: the push button must be pressed - there are 15 seconds to exit or to disconnect the alarm system again

door opening: there are 15 seconds to disconnect the alarm system window opening: there are 15 seconds to disconnect the alarm system

when the alarm system is working (intrusion under way !!), it will be ON for 5 minutes. To stop it the push button must be pressed (of course the push button is not in a visible location)

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