PLC based 4 Way Traffic Light Control System

We most often come across four way traffic jam in our city. This PLC ladder logic gives the solution to control city traffic  using programmable logic control.

4 Way Traffic Light Control

PLC based 4 Way Traffic Light Control System

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 using timer done bit at the end to run the program continuously.
  4. Program done in AB RSLogix 500 Software.

List of Inputs and Outputs

4O:0/0East GreenOutput
5O:0/1North RedOutput
6O:0/2West RedOutput
7O:0/3South YellowOutput
8O:0/4East YellowOutput
9O:0/5North YellowOutput
10O:0/6North GreenOutput
11O:0/7East RedOutput
12O:0/8West YellowOutput
13O:0/9West GreenOutput
14O:0/10South YellowOutput
15O:0/11South GreenOutput

Sequence of Operation

Below tabular column gives the Steps or sequence of outputs to turn ON the traffic system lamps (RED, GREEN, YELLOW)

PLC Traffic Light System

PLC Ladder Logic

4 Way Traffic Control
traffic light ladder diagram
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Logic Description


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


Starting the timer to turn on output East Green ,North-South-West are in red. Parallel circuits are added to turn ON/OFF the output in up next sequence.

RUNG 0002

Turning on North Yellow and East Yellow. Parallel circuits are added to turn ON/OFF the output in up next sequence.

Rung 0003

Turn on East Red and North Green.

Rung 0004-0005-0006-0007

Same procedures followed to turn on further outputs.( Refer Above Tabular column for sequence of operation)

RUNG 0008

Timer (T4:7) done bit is used to restart the cycle from beginning

Program runs continuously until STOP PB is pressed

Time delay of 10s has given using timers.

Note: We can reduce the number of timers using comparator block


The above explained 4 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 Interlocking function in AB PLC.

Author : Hema Sundaresan

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