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.
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.
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 diodeA 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 |
|
| Latching start/stop circuitsThe 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 |
|
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".
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 closeA 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 |
|
| Three-way switchingOne 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 |
|
| Timers: an off-delay on a 555A 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 |
|
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".
Ladder logic
The same starter run one scan at a time, as a PLC runs it.
| Lesson | Time | What students do |
|---|---|---|
| Ladder-logic thinkingThe 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 |
|
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".
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 stopThe 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 |
|
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".
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.