Source: https://mokxi.com/learn/path/plc-programming Updated: 2026-11-11 All paths # PLCs and ladder logic How factory machines think: the scan cycle, contacts and coils, seal-in circuits, timers, counters and sequencers, ending on a crossing controller you design rung by rung. Arduino Uno R3 Free 8 lessons 7 h 5 chapters 3,129 XP Electrical ยท Advanced +250 XP 0 of 8 done Start the course free Read, solve, write: the scan Lesson plan for teachers What you will build ## 3 projects, built step by step 8 lessons in 5 chapters, about 7 hours on a simulated Arduino Uno R3. Free, in your browser, every step checked by the simulator. Chapter 1 project ### Contacts and coils The three instructions most ladder is made of: the normally open contact, the normally closed contact and the coil, plus set and reset coils and internal bits. Open it in the simulator Chapter 2 project ### The same rung in relays The three-wire motor control circuit with no board at all: a relay as the contactor, its own contact as the seal-in, and two pilot lamps you wire to it. Open it in the simulator Chapter 3 project ### Counters A packaging line that counts items into a box: a count-up counter, a preset you can change, a done bit that stops the feed, and an interlock that ignores strays. Open it in the simulator The map ## Chapter by chapter Each lesson opens when the one before it is done, and each chapter ends in a project. Chapter 1 ## The scan Read, solve, write: the loop every PLC runs, and the instructions on its rungs. Open 288 XP ### Read, solve, write: the scan A motor starter run the way a PLC runs it: every scan copies the inputs into a table, solves the rungs from that table, then writes every output at once. 0 of 6 steps Start What is in it Project Locked 576 XP ### Contacts and coils The three instructions most ladder is made of: the normally open contact, the normally closed contact and the coil, plus set and reset coils and internal bits. 0 of 6 steps Chapter 2 ## Latches and stops The seal-in rung, fail-safe stops and latched faults, in a program and in relays. Locked 265 XP ### Seal-in, stops and faults The rung every electrician knows: START latches the motor through its own contact, a fail-safe STOP and an E-stop break it, and a latched fault waits for a deliberate reset. 0 of 6 steps Project Locked 530 XP ### The same rung in relays The three-wire motor control circuit with no board at all: a relay as the contactor, its own contact as the seal-in, and two pilot lamps you wire to it. 0 of 6 steps Chapter 3 ## Timers and counters On-delay timers on a crosswalk and a counter that fills boxes. Locked 265 XP ### Timers On-delay timers on a crosswalk: a green that times out on its own, a fixed yellow, and a button that cuts the green short by loading a shorter preset. 0 of 6 steps Project Locked 576 XP ### Counters A packaging line that counts items into a box: a count-up counter, a preset you can change, a done bit that stops the feed, and an interlock that ignores strays. 0 of 6 steps Chapter 4 ## Sequencers A car wash that runs a table of timed steps, with an E-stop and a reset. Locked 326 XP ### Sequencers A drive-through car wash run as a step sequencer: a permissive to start, five timed stages from a table, an E-stop that drops everything, and a deliberate reset. 0 of 7 steps Chapter 5 ## Boss project A signal-controlled crossing with every idea in the course. Locked 303 XP ### Boss: a signal-controlled crossing A pelican crossing that uses everything: a latched request, minimum and maximum green timers, a fixed sequence, a flasher, and one lamp wired with no code at all. 0 of 7 steps Syllabus ## Every lesson in the course All 8 lessons, in order, each with what it covers. Any of them opens in the editor right away. Chapter 1 ### The scan - Read, solve, write: the scan A motor starter run the way a PLC runs it: every scan copies the inputs into a table, solves the rungs from that table, then writes every output at once. - Contacts and coils The three instructions most ladder is made of: the normally open contact, the normally closed contact and the coil, plus set and reset coils and internal bits. Chapter 2 ### Latches and stops - Seal-in, stops and faults The rung every electrician knows: START latches the motor through its own contact, a fail-safe STOP and an E-stop break it, and a latched fault waits for a deliberate reset. - The same rung in relays The three-wire motor control circuit with no board at all: a relay as the contactor, its own contact as the seal-in, and two pilot lamps you wire to it. Chapter 3 ### Timers and counters - Timers On-delay timers on a crosswalk: a green that times out on its own, a fixed yellow, and a button that cuts the green short by loading a shorter preset. - Counters A packaging line that counts items into a box: a count-up counter, a preset you can change, a done bit that stops the feed, and an interlock that ignores strays. Chapter 4 ### Sequencers - Sequencers A drive-through car wash run as a step sequencer: a permissive to start, five timed stages from a table, an E-stop that drops everything, and a deliberate reset. Chapter 5 ### Boss project - Boss: a signal-controlled crossing A pelican crossing that uses everything: a latched request, minimum and maximum green timers, a fixed sequence, a flasher, and one lamp wired with no code at all. Keep going ## The next course on your track PLCs and ladder logic is part of the robotics track, a ladder of courses from zero to expert. Next course ### Build a desk buddy robot Animated eyes on a round display that blink, feel, react to touch, tilt and light, ending with a robot that misses you. Open the next course Track ### Robotics track Motors, servos, sensors and PID control loops, then a rover, a claw machine and a balance bot, every lesson checked on a simulated board. See every course on the track Kits ## Build it on your bench too Every lesson runs in the simulator with nothing to buy. When you want the real thing, these kits have the parts. Have a kit already? ### Find your kit Pick the starter kit you own and run every lesson in its manual with the same parts on the same pins. See every kit Questions ## About PLCs and ladder logic Is PLCs and ladder logic free? Yes. All 8 lessons of PLCs and ladder logic are free and run in your browser, with nothing to install and no parts to buy before you start. Do I need a real Arduino Uno R3? No. Every lesson runs on a simulated Arduino Uno R3 with its parts on the canvas, and the simulator checks each step as you go. When you want to build it for real, the same wiring and code work on a real board. How long does PLCs and ladder logic take? About 7 hours: 8 lessons in 5 chapters, each one a few short steps. Your progress is kept, so you can stop after any lesson and pick up where you left off. What will I build? Each chapter ends in a project that puts it together: Contacts and coils, The same rung in relays and Counters. Where do I start? With "Read, solve, write: the scan", and each lesson builds on the one before it. New to circuits? Electronics basics is the course that starts from zero.