Traffic Light Control using PLC Ladder Logic

We most often come across a three-way traffic jam in our city. This PLC program gives the solution to control heavy traffic jams using programmable logic control.

Traffic Light Control using PLC

traffic light plc program

Problem Solution

  1. They are so many ways to write a program for traffic light control ex: sequencer output method but in this normal input, outputs and timers are used.
  2. Timers are used to give time delay for output to turn ON and OFF.
  3. Reset coil is used at the end to run the program continuously.
  4. Comparator blocks are used to reduce the number of timers used.
  5. Program done in AB RSLogix 500 Software.

List of Inputs and Outputs for Traffic Control System

simple traffic light plc design

PLC Program for 3-way Traffic control System

Traffic Light Control using PLC Ladder Logic
plc ladder diagram for traffic light control using timers
plc based traffic control system project

Below tabular column gives the Steps or sequence of outputs to turn ON.

traffic light plc programming

PLC Logic Description for 3-way Traffic Control System

RUNG000 :

This is a Latching rung to operate the system through Master Start and Stop PB.

RUNG001 and RUNG0002 :

Starting the timer to turn ON first output West Green so east and west should be in red.

Comparators in Parallel rung are used to turn OFF East red after 15 sec. Timer T4:2 timing bit in parallel contact used to turn ON East red again in 5th  and 6Th  Step. (Refer Above Tabular column for clarification)

RUNG 0003 :

Turning ON North Red up to 3rd step using T4:0 and T4:1’s timer timing bit and comparator blocks.

Rung 0004 :

Turn ON East yellow for 5 sec using comparator blocks. (Step 2nd)

Rung 0005-0006-0007-0008-0009-0010 :

The same procedures followed to turn ON further outputs. ( Refer Tabular column for a sequence of operation)

RUNG 0011 :

Reset coil is turned ON using T4:2 ‘s done bit to restart the cycle from beginning

The program runs continuously until STOP PB is pressed


The above-explained 3 ways traffic light control using PLC is for example only. It may vary from real-time. We can use this example program to understand the working of timers and comparator block function in AB PLC.

Author: Hema Deepan

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