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.
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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.
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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.
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.
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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.
- Open218 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.
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- ProjectLocked436 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.
- Locked249 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.
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- Locked200 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
- ProjectLocked436 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.
- Locked267 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.
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Chapter 4
Boss project
A start/stop station with an emergency stop and pilot lamps.
- Locked280 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.
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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.
Relays and latches
- 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.
- 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.
Control circuits
- 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.
- 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.
- 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.
Ladder logic
- 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.
Boss project
- 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.
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.
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 courseElectrical 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.
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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.
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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.