Source: https://mokxi.com/learn/path/relays-and-motor-control Updated: 2026-11-11 All paths # Relays, contactors and motor control Intermediate, the electrician's course: relay coils and flyback diodes, latching start/stop circuits, interlocks, three-way switching, timers and ladder logic, all at low voltage, finished with a start/stop station and an emergency stop you wire yourself. Free 7 lessons 4 h 4 chapters 2,086 XP Electrical ยท Intermediate +150 XP 0 of 7 done Start the course free Relay coils and the flyback diode Lesson plan for teachers What you will build ## 2 projects, built step by step 7 lessons in 4 chapters, about 4 hours on a simulated breadboard. Free, in your browser, every step checked by the simulator. Chapter 1 project ### Latching start/stop circuits The three-wire motor control circuit every electrician knows: a momentary START, a seal-in contact that holds the contactor in, a normally closed STOP, and an overload contact that drops it out. Open it in the simulator Chapter 2 project ### Timers: an off-delay on a 555 A 555 one-shot holding a relay in for 1.1 R C: the stairwell timer in eight parts, how the capacitor sets the time, and the on-delay and off-delay timers a panel uses. 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 ## Relays and latches What a coil costs, the diode it needs, and the seal-in that remembers START. Open 218 XP ### Relay coils and the flyback diode A bare relay coil on a transistor: how much current a coil takes, how hard to drive the base, why a diode sits backwards across every coil, and why a 5 V relay will not pull in on 3.3 V. 0 of 6 steps Start What is in it Project Locked 436 XP ### Latching start/stop circuits The three-wire motor control circuit every electrician knows: a momentary START, a seal-in contact that holds the contactor in, a normally closed STOP, and an overload contact that drops it out. 0 of 6 steps Chapter 2 ## Control circuits Interlocks that never let two contactors close, three-way switching and timers. Locked 249 XP ### Interlocks: two contactors that never both close A generator transfer switch at low voltage: an Uno runs the sequence, and each contactor coil is fed through the other one's normally closed contact, so the wiring itself makes both-on impossible even when the code goes wrong. 0 of 7 steps Locked 200 XP ### Three-way switching One light, two switches: the stairwell circuit, its two travelers, why either switch always flips the light, and how a four-way switch adds a third spot. 0 of 6 steps Project Locked 436 XP ### Timers: an off-delay on a 555 A 555 one-shot holding a relay in for 1.1 R C: the stairwell timer in eight parts, how the capacitor sets the time, and the on-delay and off-delay timers a panel uses. 0 of 6 steps Chapter 3 ## Ladder logic The same starter run one scan at a time, as a PLC runs it. Locked 267 XP ### Ladder-logic thinking The motor starter again, run the way a PLC runs it: read the inputs, solve the rungs top to bottom, write the outputs. A seal-in rung, a latched fault, a reset that never restarts, and an E-stop, drawn live on the Serial Monitor. 0 of 7 steps Chapter 4 ## Boss project A start/stop station with an emergency stop and pilot lamps. Locked 280 XP ### Boss: a start/stop station with an emergency stop The hardwired starter, finished the way a panel builder would: you wire a latching emergency stop into the control feed, then prove START, the seal-in, the E-stop, no restart on release, the overload and the RUN and STOPPED pilot lamps. 0 of 8 steps Syllabus ## Every lesson in the course All 7 lessons, in order, each with what it covers. Any of them opens in the editor right away. Chapter 1 ### Relays and latches - Relay coils and the flyback diode A bare relay coil on a transistor: how much current a coil takes, how hard to drive the base, why a diode sits backwards across every coil, and why a 5 V relay will not pull in on 3.3 V. - Latching start/stop circuits The three-wire motor control circuit every electrician knows: a momentary START, a seal-in contact that holds the contactor in, a normally closed STOP, and an overload contact that drops it out. Chapter 2 ### Control circuits - Interlocks: two contactors that never both close A generator transfer switch at low voltage: an Uno runs the sequence, and each contactor coil is fed through the other one's normally closed contact, so the wiring itself makes both-on impossible even when the code goes wrong. - Three-way switching One light, two switches: the stairwell circuit, its two travelers, why either switch always flips the light, and how a four-way switch adds a third spot. - Timers: an off-delay on a 555 A 555 one-shot holding a relay in for 1.1 R C: the stairwell timer in eight parts, how the capacitor sets the time, and the on-delay and off-delay timers a panel uses. Chapter 3 ### Ladder logic - Ladder-logic thinking The motor starter again, run the way a PLC runs it: read the inputs, solve the rungs top to bottom, write the outputs. A seal-in rung, a latched fault, a reset that never restarts, and an E-stop, drawn live on the Serial Monitor. Chapter 4 ### Boss project - Boss: a start/stop station with an emergency stop The hardwired starter, finished the way a panel builder would: you wire a latching emergency stop into the control feed, then prove START, the seal-in, the E-stop, no restart on release, the overload and the RUN and STOPPED pilot lamps. Keep going ## The next course on your track Relays, contactors and motor control is part of the electrical track, a ladder of courses from zero to expert. Next course ### AC, signals and filters Advanced, after Transistors and op-amps for real: sine, peak and RMS, reading a scope, RC and RL filters, frequency response, RLC resonance and active filters. Open the next course Track ### Electrical track Current and voltage you can see, then transistors, op-amps, filters, power and PCBs. Every circuit runs in a real circuit simulator in your browser. 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 Relays, contactors and motor control Is Relays, contactors and motor control free? Yes. All 7 lessons of Relays, contactors and motor control are free and run in your browser, with nothing to install and no parts to buy before you start. Do I need any parts? No. Every circuit is built on a simulated breadboard, and the simulator checks each step as you go. When you want to build it for real, the same parts and wiring work on a real bench. How long does Relays, contactors and motor control take? About 4 hours: 7 lessons in 4 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: Latching start/stop circuits and Timers: an off-delay on a 555. Where do I start? With "Relay coils and the flyback diode", and each lesson builds on the one before it. New to circuits? Electronics basics is the course that starts from zero.