⚙️ PLC Programming Exercise: Traffic Light Control System for a Pedestrian Crossing

1. Introduction

In this exercise, you’ll program a PLC to control a traffic light system with pedestrian crossing functionality. The system ensures safe pedestrian crossing by managing traffic light sequences using timers and input from a pedestrian push button.

Real-World Application:

• Used in urban traffic management for pedestrian safety at intersections.
• Common in industrial facilities where both foot and vehicular traffic intersect.

2. Learning Objectives

By completing this project, you will:

• Learn to design sequential control logic using ladder diagrams.
• Understand the use of timers (TON) for time-based transitions.
• Interface input devices (push buttons) and output devices (lights, buzzers) with the PLC.
• Implement interlock and safety mechanisms in logic design.

3. Tools and Components

Hardware:

• PLC (Siemens, Allen-Bradley, Delta, etc.)
• Traffic Lights (Red, Yellow, Green for vehicles)
• Pedestrian Lights (Walk, Don’t Walk indicators)
• Pedestrian Push Button
• Buzzer (for pedestrian crossing alert)
• Power supply (24V DC)

Software:

• PLC Programming Software (e.g., TIA Portal, RSLogix, WPLSoft)
• Optional: PLC Simulation Software (e.g., Factory I/O)

4. Key Concepts and Definitions

• Sequential Control: Process where operations occur in a defined sequence using timing and input events.
• TON (Timer ON Delay): Activates output after a preset delay once the input is true.
• Interlock: Logic that prevents unsafe states (e.g., both traffic and pedestrian green lights ON).
• Push Button Input: Trigger used to initiate the pedestrian crossing sequence.

5. Step-by-Step Programming Guide

Logic Flow Overview:

1. System powers on, vehicle green light is ON.
2. When a pedestrian presses the push button, the current vehicle light sequence completes.
3. Lights switch: vehicle red ON, pedestrian walk ON, and buzzer ON for a set duration.
4. After the crossing time ends, system resets to vehicle green and pedestrian don’t walk.

Wiring Assignments (Suggested PLC I/O)

• I0.0 – Pedestrian Push Button
• Q0.0 – Vehicle Green Light
• Q0.1 – Vehicle Yellow Light
• Q0.2 – Vehicle Red Light
• Q0.3 – Pedestrian Walk Light
• Q0.4 – Pedestrian Don’t Walk Light
• Q0.5 – Buzzer

Ladder Logic Breakdown

Initial State – Vehicle Green ON:

• At power-on, vehicle green light (Q0.0) is ON.
• Pedestrian don’t walk light (Q0.4) is ON.

Pedestrian Request:

• When push button (I0.0) is pressed, trigger a latching bit to start the crossing sequence.

Sequence Control with Timers:

1. Vehicle Green to Yellow:
   • Turn OFF green, turn ON yellow for 3 seconds using TON.
2. Vehicle Yellow to Red:
   • Turn OFF yellow, turn ON red for 5 seconds.
3. Pedestrian Walk & Buzzer ON:
   • After vehicle red, turn ON pedestrian walk (Q0.3) and buzzer (Q0.5) for 7 seconds.
4. Reset to Initial State:
   • Turn OFF pedestrian indicators and buzzer.
   • Turn ON vehicle green (Q0.0) and pedestrian don’t walk (Q0.4).

Safety Interlock:

• Ensure vehicle green and pedestrian walk are never ON simultaneously.

6. Testing and Debugging Tips

• Simulate each state transition step-by-step to validate correct timer durations.
• Verify that pedestrian requests during active crossing are ignored (use a latch reset).
• Ensure the buzzer activates only during pedestrian crossing.
• Use indicator lights on PLC software to monitor I/O status.

7. Project Extensions

• Add a countdown timer display for pedestrian crossing using an HMI.
• Introduce a night mode where the system switches to flashing yellow when traffic is low.
• Incorporate emergency vehicle override that switches all lights to red temporarily.
• Add sensor input to detect vehicle presence and adjust green light duration dynamically.

Let me know if you’d like example ladder diagrams, PLC code snippets for a specific brand, or a visual representation of the light sequence—happy to provide more details!