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 simulatorAll 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.
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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.
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 simulatorCounters
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 simulatorThe 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.
- Open288 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
- ProjectLocked576 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.
- Locked265 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
- ProjectLocked530 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.
- Locked265 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
- ProjectLocked576 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.
- Locked326 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.
- Locked303 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.
The scan
- Read, solve, write: the scanA 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 coilsThe 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.
Latches and stops
- Seal-in, stops and faultsThe 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 relaysThe 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.
Timers and counters
- TimersOn-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.
- CountersA 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.
Sequencers
- SequencersA 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.
Boss project
- Boss: a signal-controlled crossingA 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.
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 courseRobotics 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 trackKits
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.
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 kitQuestions
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.