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Industrial Automation Lab Experiments

Develop / test ladder program for traffic light control system


Develop/ test ladder program for traffic light control system.

Apparatus Required:

PLC software, NO and NC switches.


  1. Load the RS Logix software to the PC.
  2. Open the RS Logix software.
  3. Switch On the PLC trainer.
  4. Open the New project and draw the ladder logic program.
  5. Compile the program.
  6. Run the Program.
  7. Give force and observe the output.

Concept structure: Traffic Light Control

Traffic light is an arrangement of three colour lights. It is used for controlling the traffic at road junctions by providing a particular sequence of these lights. Different colour lights used are Red, Green and Amber (or Yellow). The red light indicates ‘STOP’, green light indicates ‘GO’ and amber light indicates ‘WAIT’ for red or green. Timers are devices that count increments of time. Traffic lights are one example where timers are used. In this example timers are used to control the length of time between signal changes. The lights in a traffic signal go through a sequence: Red,green,amber, Red,green,amber etc. When an ON/OFF switch is turned on and a START button is momentarily pressed, the red signal (R) lights for 30 seconds, then turns off and the green (G) lights for 25 seconds, then turns off and the Amber (A) turns on for 5 seconds and then turns off and back to the red. On the opposite street, the red light (R’) is on during the time the light on the other street is either green or amber. Transition from red to green to amber is accomplished by cascading timer circuits. The sequence then repeats itself.


Ladder diagram:



Red light will be ON for
Green light will be ON for
Amber light will be ON for


  1. number of timers are used in the ladder diagram of this experiment.


  1. When is the enable bit of timer instruction true?
  2. When does the done bit of timer change state?
  3. When is the output of a programmed timer energized?
  4. How RES instruction in relation with retentive PLC timer works?
  5. How accumulated count of programmed timer energized?