What you can learn here
What a PLC is
A PLC (programmable logic controller) is an industrial computer that reads sensors and push buttons, runs a program and drives contactors, valves and pilot lights. It replaced relay panels because it can be reprogrammed without rewiring. You’ll find it in packaging plants, mining, water pumping, lifts and almost any automatic machine.
Ladder diagram
Ladder logic (LD in the IEC 61131-3 standard) draws the program like a relay wiring diagram: two vertical rails and horizontal rungs with contacts and coils. That is why it is the first language taught in technical schools: an electrician reads it like a drawing. Contacts in series form an AND function and in parallel an OR.
The scan cycle
A PLC doesn’t react instantly: it reads every input, runs the whole program and then writes the outputs, over and over, within a few milliseconds. Understanding the scan explains why a very short pulse can go unnoticed, why rung order matters and what happens with a “double coil”.
Latching (seal-in)
A start button only gives a pulse; to keep the motor running, a contact of the output itself is placed in parallel with it. The start/stop circuit with a seal-in is the basis of almost every industrial control, and also of the interlock between two contactors that must never close at the same time, such as the up and down contactors of a door.
Timers and counters
TON delays switching on (staggered starts, traffic lights, star-delta) and TOF delays switching off (fans that keep cooling after the motor stops). The CTU and CTD counters count parts, cycles or batches, and comparators let you make decisions based on a timer or count value.
Safety: a hard-wired NC emergency stop
The emergency stop always uses a normally closed contact: if a wire breaks or a terminal comes loose, the machine stops instead of being left with no way to stop it. In addition, the mushroom button must cut the power through hard wiring, in series with the contactor coils or through a safety relay, without relying on the program alone: a standard PLC can fail or be left with an output stuck on. In the program you use an NO contact of that input (it is 1 when everything is fine) and, when the button is released, the machine must not restart by itself: Start has to be pressed again. IEC 60204-1 and ISO 13850 detail these requirements.
From the simulator to the real panel
The I0.0, Q0.0, M0.0, T0 and C0 addresses follow the style of many compact PLCs. Other brands use different names (for example %I0.0 or I:0/0), but the logic is the same. Before loading a program into a real machine, check the wiring diagram, the datasheets and the safety standards: this lab is educational.
Keep learning with the Circuit Lab, the MCU Lab for microcontrollers, the KUKA arm and its welding cell, the three-phase power calculator, the torque and power calculator or the Electrical and Mechatronics paths.