Source: https://mokxi.com/teachers/plans/relays-and-motor-control Updated: 2026-11-12 Lesson plan # Relays, contactors and motor control lesson plan 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. 4 chapters, 7 lessons, about 4 hours, no board. Every lesson runs in the browser, with nothing to install. Start a class from this course The unit ## Objectives and materials ### By the end, students can - What a coil costs, the diode it needs, and the seal-in that remembers START. - Interlocks that never let two contactors close, three-way switching and timers. - The same starter run one scan at a time, as a PLC runs it. - A start/stop station with an emergency stop and pilot lamps. ### Materials for the whole unit - Arduino Uno R3 - Capacitor x 2 - Character LCD with I2C backpack - DC motor - Diode - DPDT relay x 2 - Filament lamp x 2 - Full-size breadboard - Half breadboard - LED x 3 - Limit switch, lever microswitch - Multimeter - NE555 timer - Pushbutton x 2 - Relay module - Resistor x 9 - Slide switch x 4 - Traffic light module - Transistor (NPN or PNP) x 2 Every part is already on the simulator’s bench, so there is nothing to buy or hand out. The counts are the most any one lesson uses, if you build it on real breadboards too. Chapter 1. About 1 h ## Relays and latches What a coil costs, the diode it needs, and the seal-in that remembers START. Lesson | Time | What students do 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. | 30 min - Run it: the STOPPED lamp - Throw the switch - What the coil costs - Drive the base hard - The diode that saves the transistor - A 5 V relay on a 3.3 V board 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. | 30 min - Run it: nothing moves - Press START and let go - Trace the seal-in - Press STOP - Trip the overload - Why STOP is normally closed ### Materials - DC motor - Diode - DPDT relay - Filament lamp x 2 - Full-size breadboard - Limit switch, lever microswitch - Pushbutton - Resistor - Slide switch - Transistor (NPN or PNP) ### Check for understanding Each student finishes "Latching start/stop circuits" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go. Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Why STOP is normally closed". Chapter 2. About 1 h 35 min ## Control circuits Interlocks that never let two contactors close, three-way switching and timers. Lesson | Time | What students do 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. | 35 min - Run it and turn on AUTO - The lamp on the utility - Cut the utility - Break before make - On the generator - Break the code on purpose - Why it matters outside the house 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. | 30 min - Run it: the stair light - Flip the bottom switch - And the top one - Find the live traveler - A third switch: the four-way - Before you touch a real switch 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. | 30 min - Run it: the lamp waits - Press the button - Watch the capacitor - Wait it out - Make it longer - On-delay and off-delay ### Materials - Arduino Uno R3 - Capacitor x 2 - DPDT relay x 2 - Filament lamp - Full-size breadboard - Half breadboard - LED x 3 - Multimeter - NE555 timer - Pushbutton - Relay module - Resistor x 9 - Slide switch x 4 - Transistor (NPN or PNP) x 2 ### Check for understanding Each student finishes "Timers: an off-delay on a 555" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go. Then ask each one to explain, in a sentence or two, the last thing it asked of them: "On-delay and off-delay". Chapter 3. About 35 min ## Ladder logic The same starter run one scan at a time, as a PLC runs it. Lesson | Time | What students do 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. | 35 min - Run it and read the ladder - Press START - The seal-in rung - One scan at a time - A fault that stays - Reset, but no restart - The E-stop ### Materials - Arduino Uno R3 - Character LCD with I2C backpack - DC motor - Half breadboard - Limit switch, lever microswitch - Pushbutton x 2 - Relay module - Slide switch x 2 - Traffic light module ### Check for understanding Each student finishes "Ladder-logic thinking" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go. Then ask each one to explain, in a sentence or two, the last thing it asked of them: "The E-stop". Chapter 4. About 40 min ## Boss project A start/stop station with an emergency stop and pilot lamps. Lesson | Time | What students do 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. | 40 min - Wire the E-stop into the feed - START it - The pilot lamps - Hit the E-stop - Release it: no restart - START again - Trip the overload - Lockout before you reach in ### Materials - DC motor - DPDT relay - Filament lamp x 2 - Full-size breadboard - Limit switch, lever microswitch - Pushbutton - Slide switch x 2 ### Check for understanding Each student finishes "Boss: a start/stop station with an emergency stop" on their own, without opening the hints. The simulator checks 8 of its 8 steps as they go. Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Lockout before you reach in". Your class ## Teach this unit as a class Start a class from this course and its 7 lessons are assigned in one go, in order. Students join with a six-character code or printed login cards, and each one’s work comes back to you. The class tools are free for 30 days for one class of up to 20 students, with no card. The lessons themselves are free either way. Start a class from this course See the demo class More units ## Other lesson plans Electronics basics 20 lessons, about 10 h. Embedded on the Arduino Uno 16 lessons, about 12 h. Embedded on the ESP32-C3 13 lessons, about 11 h. Embedded on the Raspberry Pi Pico 7 lessons, about 6 h. Build your own fight stick 5 lessons, about 4 h. Embedded on the STM32F411 6 lessons, about 6 h. C++ on a board 13 lessons, about 10 h. Digital logic and memory 4 lessons, about 4 h. Interactive devices on the Uno 9 lessons, about 8 h. From gates to a computer 23 lessons, about 16 h. Build a claw machine 8 lessons, about 6 h. 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