PLC Logic by Hand: Contacts & Coils, Seal-In Circuits, the Scan Cycle, Edge Detection, Timers, Counters, Step Sequences & Alarm Handling

What you'll build

What you will be able to do

Each lab ends with a concrete result that you build yourself and the checker confirms, cell by cell.

• Solve four rungs of a toy filling station for 12 recorded input states: normally open and normally closed contacts as 0/1 cells, series as AND, parallel as OR, one exclusive OR, and a count of how often each coil switches on.
• Run a toy conveyor motor through 12 scans of start and stop presses: first without a holding contact, then as a reset-dominant and a set-dominant seal-in, and count where the two versions disagree.
• Solve the same two rungs of a toy sorting conveyor in two orders, count the scans in which the lamp lags one scan behind the motor, and turn measured scan times into simplified worst-case response times.
• Catch the rising and falling edges of a held push button and a bottle sensor over 14 scans and see why counting edges instead of held scans counts every press and every bottle exactly once.
• Run three toy timers over 10 scans of a fill request with uneven scan times: an on-delay that resets when the request drops, an off-delay that keeps its output on after the request drops, and a retentive timer that keeps its time until a reset.
• Count bottles on a toy packing conveyor over 18 scans – up on the part sensor's rising edges, down on the reject sensor's, with a reset – split the count into boxes with MOD and total three shifts from a counter that wraps around.
• Step a fictional mixing tank through Idle, Fill, Mix and Drain over 10 scans: each scan looks up the transition condition of the active step, the outputs come from a step table and the step times are counted from the sequence table.
• Run a toy labelling machine through 10 scans: latch three alarms until they are acknowledged, pack them into an alarm word, find the first-out alarm and the activations, gate the start with a permissive and follow the machine state scan by scan.