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Learn PLCs and industrial automation
Free, plain-English guides to PLCs, ladder logic, wiring and the sensors and devices you find on every machine. Read how each one works, then practise it in the free online PLC simulator.
PLC basics
How a PLC works, how it is addressed, wired and programmed.
What is a PLC?A beginner guide to the PLC (programmable logic controller): what it is, its parts, the scan cycle, inputs and outputs, and why factories use PLCs…PLC addressing explainedHow PLC input and output addresses work in Siemens (I0.0 / Q0.0), Mitsubishi (X0 / Y0 octal), Allen-Bradley (I:0/0, tags) and TOYOPUC (X000 hex),…PLC wiring basicsHow to wire a PLC: 24 V DC power supply, input COM, sinking and sourcing inputs, relay and transistor outputs, fuses, flyback diodes and wire…Ladder logic basicsLearn ladder logic step by step: rails and rungs, normally open and closed contacts, coils, AND/OR logic, the start/stop seal-in circuit, set/reset…PLC timers and countersHow PLC timers (TON on-delay, TOF off-delay, TP pulse) and counters (CTU, CTD) work, with timing diagrams, presets, done bits and practical…Every way to wire a sensor to a PLCAll the ways to connect industrial sensors: PNP to sinking inputs, NPN to sourcing inputs, NPN on a PNP input with an interface relay, 2-wire…
Industrial sensors
How each type of sensor works, where it is used and how to wire it to a PLC.
PNP vs NPN sensorsThe difference between PNP (sourcing) and NPN (sinking) sensors, how to wire each to a PLC input, which input COM connection to use, and how to tell…Inductive proximity sensorHow an inductive proximity sensor detects metal, sensing distance and correction factors for steel, stainless, aluminium and copper, flush vs…Capacitive proximity sensorHow a capacitive proximity sensor detects plastic, wood, glass, powder and liquids, level sensing through a tank wall, sensitivity adjustment, and…Photoelectric sensors (photo-eyes)How photoelectric sensors work: through-beam, retro-reflective, polarized and diffuse (background suppression) types, light-on vs dark-on, range,…Limit switchWhat a limit switch is, actuator types (roller lever, plunger, rod), NO and NC contacts, positive opening for safety, and how to wire a limit switch…Reed switch and cylinder sensorsHow reed switches and magnetic cylinder sensors detect a pneumatic cylinder piston position, 2-wire vs 3-wire types, mounting in the T-slot, and…Ultrasonic sensorHow an industrial ultrasonic sensor measures distance with sound, blind zone and beam angle, switching vs analog 4-20 mA / 0-10 V outputs, and tank…Rotary encoderHow rotary encoders measure position and speed: incremental A/B quadrature and Z index, PPR and resolution, absolute single-turn and multi-turn…Pressure switches and pressure sensorsHow a pressure switch works, setpoint and differential (hysteresis), mechanical vs electronic switches, 4-20 mA pressure transmitters, and typical…Level sensors for tanksPoint and continuous level sensing: float switches, capacitive, conductive probes, vibrating fork, ultrasonic, radar and hydrostatic pressure, and…Push buttons and selector switchesHow industrial push buttons, illuminated buttons, selector switches and HAND-OFF-AUTO switches work, NO vs NC contact blocks, button colours, and how…Vision and colour sensorsHow colour sensors and vision sensors (smart cameras) inspect parts: teaching, triggering, OK/NG outputs, lighting, and how to connect them to a…RFID readers and tagsHow industrial RFID works: passive tags, LF / HF / UHF frequencies, read range, reading and writing data, and how an RFID reader sends tag data to a…Wireless sensorsHow wireless industrial sensors and receivers work, battery and energy-harvesting sensors, link monitoring and safe states, range, and typical uses…Temperature sensors and thermostatsHow thermostats, thermocouples (type K, J) and Pt100 RTDs measure temperature, how to connect them to a PLC, and on/off vs PID temperature…
Field devices
Contactors, overloads, safety circuits and the other parts in a control panel.
Magnetic contactor and overload relayHow a magnetic contactor (magnetic switch) lets a 24 V PLC control 230 V / 415 V motors, coil terminals A1/A2, main and auxiliary contacts, thermal…Emergency stop circuitsHow an emergency stop circuit works: NC positive-opening E-stop buttons, safety relays, dual-channel wiring, stop categories 0 and 1, manual reset,…Safety light curtainHow a safety light curtain protects operators: emitter and receiver, finger and hand resolution, OSSD outputs, muting and blanking, and the safety…Types of electric motorsThe motors used in automation: 3-phase induction motors, single-phase motors, DC motors, brushless DC, servo motors, stepper motors and gear motors,…Star-delta starterHow a star-delta (wye-delta) starter reduces motor starting current: star and delta connections, the three contactors KM1, KM2, KM3, the changeover…Guard door interlocks and two-hand controlHow guard door interlock switches (tongue, hinge, magnetic coded, with guard locking) and two-hand control stations protect operators, and how they…What is a VFD (variable frequency drive)?How a variable frequency drive (inverter) controls the speed of an AC motor: rectifier, DC bus and IGBT inverter, PWM, V/f and vector control, ramps,…Controlling a VFD from a PLCThe three ways a PLC controls a variable frequency drive: digital run / direction / preset-speed signals, an analog 0-10 V or 4-20 mA speed…How servo systems workWhat makes a servo system precise: the servo motor, its encoder and the servo amplifier, the three control loops (current, velocity, position),…Controlling a servo from a PLCHow a PLC commands a servo axis: pulse and direction outputs, EtherCAT / PROFINET / CC-Link motion networks, PLCopen motion function blocks…What is an HMI?How an HMI (human-machine interface) touch panel works with a PLC: tags, buttons, indicators, numeric entry, alarms, trends, recipes, user levels,…How to design a good HMI screenPractical HMI design rules from high-performance HMI (ISA-101) and alarm management (ISA-18.2): calm grey screens, colour only for abnormal states,…
PLC software
The programming software each PLC brand uses, and the languages they all share.
PLC programming software for every brandWhich software programs which PLC: Siemens TIA Portal, Mitsubishi GX Works3, Rockwell Studio 5000 and RSLogix 500, JTEKT TOYOPUC PCwin, Omron Sysmac…PLC programming languages (IEC 61131-3)The five PLC programming languages defined by IEC 61131-3: ladder diagram, function block diagram, structured text, sequential function chart and…Siemens TIA Portal (STEP 7)What TIA Portal is, STEP 7 Basic vs Professional, how a project is organised (devices, tags, OB, FB, FC, DB), the languages LAD, FBD, SCL and GRAPH,…Mitsubishi GX Works3 and GX Works2How Mitsubishi MELSEC PLCs are programmed: GX Works3 for iQ-R and iQ-F (FX5), GX Works2 and GX Developer for Q, L and FX3, devices X, Y, M, D, T, C,…Allen-Bradley software: Studio 5000, RSLogix 500 and CCWWhich Rockwell Automation software programs which Allen-Bradley PLC: Studio 5000 Logix Designer for ControlLogix and CompactLogix, RSLogix 500 for…TOYOPUC-PCwin (JTEKT)How JTEKT TOYOPUC PLCs are programmed with TOYOPUC-PCwin: hexadecimal X/Y addresses, M, K, L, V special relays and P edge relays, F-key ladder…Omron Sysmac Studio and CX-ProgrammerOmron PLC software explained: CX-Programmer (CX-One) for CP1, CJ2 and CS1 PLCs, Sysmac Studio for NJ/NX machine controllers with IEC variables,…CODESYS and Beckhoff TwinCAT 3What CODESYS is and which brands use it (WAGO, Schneider, ifm, Festo, Bosch Rexroth, Eaton), Beckhoff TwinCAT 3 in Visual Studio, soft PLCs on a PC,…
Learn by doing. Every topic here can be built and tested in the PLC Sim Lab Playground: Siemens, Mitsubishi, Allen-Bradley and TOYOPUC addressing, right in your browser. New to PLCs? The guided lessons and fault-finding challenges check your wiring and program automatically, and finishing them earns a free certificate.
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