Case Study - PLC Bridge Lighting Control System

Project Overview

This project implements a discrete, multi-state lighting controller using IEC 61131-3 Ladder Diagram logic. The system exposes three operational states — OFF, DIMMED, and FULL — driven by two momentary pushbutton inputs and enforced by interlocked rung logic. The exercise focuses on building deterministic state selection between mutually exclusive outputs, a foundational pattern in industrial mode-selection and lighting control routines.

Problem Statement

In industrial and operational environments, conflicting output commands can produce unsafe field conditions, contactor chatter, or damaged loads. A lighting controller offering multiple intensity levels must guarantee that no two driver outputs are energized simultaneously, regardless of operator input timing or button bouncing. The control strategy therefore requires a hardwired-style interlock implemented in software, ensuring deterministic selection between modes on every PLC scan cycle.

Solution

Implemented Control Logic:

PB_DIM → Normally Open (NO) momentary input requesting dimmed mode
PB_FULL → Normally Open (NO) momentary input requesting full mode
LIGHT_DIM → Discrete output coil driving the dimmed lighting circuit
LIGHT_FULL → Discrete output coil driving the full-intensity lighting circuit
Mutual Exclusion Interlock → Cross-wired NC feedback contacts preventing simultaneous energization of both outputs

Mutual exclusion was implemented by wiring a Normally Closed feedback contact of each output in series with the opposite rung's energizing path. When LIGHT_FULL is energized, its NC contact in the LIGHT_DIM rung is held open, blocking that rung from going true; the same relationship exists in the reverse direction. This software interlock mirrors the classical hardwired contactor-interlock pattern used in reversing starters and multi-mode drives, and ensures that any contention between the two pushbuttons resolves to a single active output on each scan cycle rather than driving both loads at once.

Project Screenshots

PLC Project Screenshot 1 — Variable declarations and I/O class assignment in the OpenPLC project tree

PLC Project Screenshot 2 — Two-rung Ladder Diagram showing cross-wired NC interlocks between LIGHT_DIM and LIGHT_FULL

PLC Project Screenshot 3 — Live debugger session validating mutual exclusion under sequential and simultaneous input conditions

Variable Declarations

VAR_INPUT
    PB_DIM  : BOOL := FALSE;
    PB_FULL : BOOL := FALSE;
END_VAR

VAR_OUTPUT
    LIGHT_DIM  : BOOL := FALSE;
    LIGHT_FULL : BOOL := FALSE;
END_VAR

Testing & Verification

Functional verification was carried out using the OpenPLC runtime simulator and integrated debugger. Input variables were toggled and force-set during live execution to drive the system through each defined state and confirm that the interlock held under both sequential and simultaneous input conditions.

Asserting PB_DIM energizes LIGHT_DIM while leaving LIGHT_FULL de-energized
Asserting PB_FULL energizes LIGHT_FULL and inhibits LIGHT_DIM via the cross-wired interlock
Simultaneous assertion of both inputs resolves to a single active output, never both — verifying the mutual exclusion holds at the rung level
Releasing both inputs returns the system to the OFF state with both output coils de-energized

Challenges & Engineering Decisions

The principal challenge was reasoning through the mutual exclusion topology — specifically, deciding to use each output's own NC feedback contact as the inhibit signal on the opposing rung, rather than relying purely on input-side logic. Input-only interlocks would have left a narrow scan-cycle window where both outputs could briefly energize before the logic resolved; routing the interlock through the output feedback contacts ensures the inhibit is always in lockstep with the actual coil state.

This project deepened my understanding of state-based control and showed how compact Ladder Diagram structures can implement safety- and contention-critical behavior that would otherwise require either hardwired relay interlocks or more complex state-machine code in a text-based language.

Key Learning Outcomes

Mutual exclusion and interlock design in Ladder Diagram
Normally Open vs Normally Closed contact selection and its physical wiring analogue
Coordinated control of multiple discrete outputs from a shared input set
Introductory state-machine thinking applied within a cyclic scan execution model
PLC runtime simulation, variable forcing, and edge-case verification
Industrial mode-selection patterns transferable to lighting, HVAC, and motor-direction control

Tools & Environment

OpenPLC Editor — IEC 61131-3 compliant programming environment
Autonomy Edge — runtime/deployment platform
Ladder Diagram (LD) — selected programming language for this routine
OpenPLC Runtime Simulator and integrated debugger for live variable forcing

Case Study: PLC Bridge Lighting Control System