The Challenge: Advent of Code Meets Industrial Automation

What happens when you combine Advent of Code with industrial control systems? Ladder logic solutions running on actual PLCs.

This year, I solved Advent of Code 2025 puzzles using ladder logic on an Allen-Bradley Micro870 PLC, with a FactoryTalk Optix HMI for visualization.

Github Repo with Code: https://github.com/ladderlogix/Advent-of-Code-2025

Hardware Setup

Day 1: Secret Entrance

The Problem

You arrive at the secret entrance to the North Pole base, but the password has changed. A safe with a dial (numbers 0-99) holds the answer. Your puzzle input contains rotation instructions like L68 (left 68 clicks) or R48 (right 48 clicks).

The dial starts at 50 and wraps around (left from 0 goes to 99, right from 99 goes to 0).

Part 1: Count how many times the dial ends on position 0 after a rotation.

Part 2: Count how many times the dial passes through or lands on position 0 during ALL rotations (including mid-rotation).

Video Demonstrations

Day 1 Part 1

Day 1 Part 2

Part 1 Solution

The algorithm processes 4,146 rotation instructions, tracking position on a circular 0-99 dial.

Rung 0 - Reset

Rung 1 - Start Processing

Rung 2 - Check If Complete

Rung 3 - Get Direction From Array

Rung 4 - Get Distance From Array

Rung 5 - Add (Direction = Right)

Rung 6 - Subtract (Direction = Left)

Rung 7 - Modulo Wraparound

Rung 8 - Handle Negative Values

Rung 9 - Count Zeros

Rung 10 - Advance Step

Part 2 Solution

Part 2 requires counting every time the dial crosses position 0, not just when it lands there. This adds significant complexity with 6 different cases to handle.

Rung 11 - Zero Crossing Calculation

This massive branching rung handles all the edge cases for counting zero crossings:

Rung 12 - Accumulate Zero Crossings

Rung 13 - Advance Step

Stay tuned for more days of Advent of Code solved with ladder logic!