Free ladder logic course (with a simulator you can use in your browser)

Ten steps to learn ladder logic

This is the order that works. The module numbers are from the Siemens TIA Portal course, where modules 8 to 21 teach it. Other brands cover the same ideas.

• Contacts and coils (module 8): the two symbols everything is built from.
• AND, OR and NOT in ladder form (module 9): series and parallel contacts.
• Seal-in (latching) circuits (module 10): a rung that holds itself on.
• Interlocks (module 10): stopping two outputs being on together.
• Timers, TON, TOF and TP (module 12): delays and pulses.
• Counters, CTU and CTD (module 13): counting parts and cycles.
• Set, reset and one-shots (module 14): acting on a change, not a level.
• Comparators (module 15): deciding on values.
• Analog scaling (module 18): turning a raw input into engineering units.
• HMI tags (module 21): a screen that reads and writes the same tags.

Practise in your browser

Write the exercises below in the EDWartens Lab PLC simulator. You can force inputs, go online and watch timers and counters run. It has a vendor-neutral editor and nothing to install, so the brand software stays in the course. There is also an IEC 61131-3 simulator for structured text and function blocks.

Eight exercises with solutions

Try each one before reading the solution. These are teaching examples. Never test logic on a live machine without isolation and a risk assessment.

• 1. Start-stop. Task: a START button runs a motor and a STOP button stops it. Solution: START (normally open) in series with STOP (normally closed) and the motor coil. The motor runs only while START is held.
• 2. Seal-in. Task: make the motor keep running after START is released. Solution: add a normally open contact of the motor coil in parallel with START. Once the coil is on, that contact holds the rung true, and STOP breaks it. Wire the physical STOP as normally closed so a broken wire stops the motor.
• 3. Traffic light. Task: red, red and amber, green, amber, repeating. Solution: one timer per step, each started when the previous step finishes, with the last one resetting the first. Each lamp is on during its own step. This is a small state machine.
• 4. Tank level. Task: switch a fill pump on at low level and off at high level. Solution: the low-level contact sets a latch that runs the pump and the high-level contact breaks it. Between the two levels the pump keeps its last state, which stops it chattering.
• 5. Bottle counter. Task: count bottles and pulse a diverter every 12. Solution: a counter (CTU) counts rising edges from the sensor, using a one-shot so one bottle is one count. When the count reaches the preset, the done bit pulses the diverter and a reset clears the count.
• 6. Two-hand control. Task: a press cycle that starts only if two buttons are pressed together. Solution: put both normally open buttons in series with the cycle output, and drop the output as soon as either is released. Real two-hand controls must use safety-rated devices, not standard PLC inputs. This exercise teaches the logic only.
• 7. Motor star-delta. Task: start a motor in star, then switch to delta after a few seconds. Solution: the main and star contactors start together, and a timer runs. When it ends, the star contactor drops and the delta contactor pulls in. Interlock star and delta so both can never be on together, and leave a short gap between them.
• 8. Alarm latch. Task: a high-temperature alarm stays on until acknowledged. Solution: the alarm condition sets a latch and a lamp. The acknowledge button resets the latch only if the condition has cleared, otherwise the alarm stays on.

Where to go next

Follow the PLC foundations path for one PLC, its networks, the HMI and the panel. Then try the PLC Programmer job simulation (days 1 to 5 are free, and it is practice, not employment). All free PLC courses are in one list.