A traffic light is the classic first PLC sequencing project: three timed outputs that cycle Green → Yellow → Red forever. Below is the full ladder diagram, the I/O table and the timing chart — and because this is a live scenario, you can write the logic and watch the lights run in your browser without installing anything.
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The finale. Everything you've learned — outputs, NC contacts, AND/OR logic, seal-in, SET/RST, timers — comes together in a 4-way traffic light controller. NS and EW alternate phases. Each green is 10 s, yellow is 3 s, and a 1-second all-red clearance prevents conflicting greens. This is a real control problem — sequence logic, timing, safety. Get it green and you've graduated the PLC-101 curriculum.
POWER_ONMaster power enable
BOOL · %I0.0
NS_REDNorth-South red lamp
BOOL · %Q0.0
NS_YELNorth-South yellow lamp
BOOL · %Q0.1
NS_GRNNorth-South green lamp
BOOL · %Q0.2
EW_REDEast-West red lamp
BOOL · %Q0.3
EW_YELEast-West yellow lamp
BOOL · %Q0.4
EW_GRNEast-West green lamp
BOOL · %Q0.5
All test cases run automatically when you submit. Assertions are hidden until you pass.
NS green -> yellow -> red, then EW green -> yellow -> red
Safety check — conflicting greens would cause collision
A single-direction traffic light needs three digital outputs — Green, Yellow and Red lamps — and a Start input to enable the cycle. The PLC runs three on-delay (TON) timers in a chain so each light is on for a fixed time before handing over to the next.
A typical sequence is Green for 30 seconds, Yellow for 5 seconds, then Red for 35 seconds, after which the cycle repeats. Each timer's done bit both turns off its own light and starts the next light's timer.
The program is built from three timer rungs that chain together. The Green rung runs its TON while the Red timer is not done; when the Green timer finishes it enables the Yellow timer, and when Yellow finishes it enables the Red timer. When the Red timer finishes the chain resets and Green starts again.
This is the timer-based method — the most common and most readable way to program a traffic light in ladder logic. Each light is driven by the done (.DN) bit of the timer in front of it in the sequence.
The timing diagram makes the sequence obvious: exactly one lamp output is energised at any moment, and the transitions happen as each timer reaches its preset. Reading the chart top to bottom you can see Green hand over to Yellow, Yellow to Red, and Red back to Green.
When you build this in the simulator, the timing chart is what you are verifying — that the lights never overlap and that each stays on for the right duration.
A 4-way intersection adds a second direction (North/South and East/West) that must be interlocked so the two directions are never both green. The cleanest approach is a state machine: each phase (NS green, NS yellow, EW green, EW yellow) is one state, and a master timer steps through them in order.
The state table below shows the four phases and which lamps are energised in each. Because only one phase is active at a time, the cross-direction interlock is automatic.
The logic is identical in Siemens TIA Portal — only the addressing changes. Inputs are %I0.0 style and outputs %Q0.0, and the Siemens TON uses an IEC timer block (with a TIME preset like T#30s) rather than a tag-based timer with a PRE value. The Green/Yellow/Red chain works exactly the same way.
If you learn the sequence here in IEC or Allen-Bradley style, porting it to Siemens is just a matter of swapping the timer block and the I/O addresses.
If a light stays on and the sequence never advances, the timer for the next light is not being enabled — check that you are using the previous timer's done bit (.DN), not its timing bit (.TT), to advance the chain.
If a timer never counts, its rung is false: confirm the enabling contact (the Start input on the first rung, or the prior timer's done bit on later rungs) is actually true during the scan.
Traffic light control using a PLC is the textbook way to learn timed sequencing because the whole program is just a chain of on-delay timers feeding each other. The Start input enables the first timer; each timer's done bit turns off its own lamp and starts the next; and when the last timer finishes, the chain wraps around and the cycle repeats forever. There is no operator interaction once it starts — the timers carry the sequence on their own, which is exactly why a traffic light plc program is the classic first project for a new PLC programmer.
Scaling from one direction to a 4 way traffic light plc ladder logic just means treating each intersection phase (NS green, NS yellow, EW green, EW yellow) as one state stepped by a master timer, so the two directions are never green together and the cross-direction interlock is automatic. The whole program is runnable and auto-graded right here in your browser — and to keep a copy of this traffic light PLC ladder diagram (I/O table, single-direction rungs and the 4-way state table) beside you while you build, press Ctrl/Cmd+P on this page and choose Save as PDF.
Three on-delay timers (TON) and their done bits, plus output coils for the Green, Yellow and Red lamps. Each timer's done bit turns off its own light and enables the next timer in the chain.
Set each TON preset to how long that light should stay on — for example Green 30 s, Yellow 5 s, Red 35 s. In Allen-Bradley that is the PRE value; in Siemens it is the IEC timer's TIME preset such as T#30s.
Yes. This page is a live browser scenario — write the ladder logic, press Run, and watch the lamps cycle on a simulated intersection. No PLC, no install and no licence required.
A 4-way intersection is best built as a state machine: one state per phase (North/South green, North/South yellow, East/West green, East/West yellow) stepped by a master timer, so the two directions are never green at the same time.
Exactly the same logic with Siemens addressing: %I and %Q I/O and IEC TON timer blocks with TIME presets (e.g. T#30s). The Green → Yellow → Red timer chain is identical to the Allen-Bradley or IEC version.
A single-direction light needs one input (Start) and three outputs (Green, Yellow, Red lamps). A 4-way intersection doubles the outputs to six lamps across the two directions.
A single-direction traffic light PLC ladder diagram is three chained TON timer rungs: the Green rung runs a TON enabled while the Red timer is not done; the Green done bit enables the Yellow TON; the Yellow done bit enables the Red TON; and the Red done bit resets the chain so Green restarts. Each lamp output is driven by the done bit of the timer ahead of it. To keep a copy of this exact ladder diagram, press Ctrl/Cmd+P on this page and choose Save as PDF.
Traffic light control using a PLC is done with a chain of on-delay (TON) timers: a Start input enables the first timer, each timer's done bit turns off its lamp and starts the next, and the final timer's done bit wraps the chain back to the start so the Green → Yellow → Red cycle repeats indefinitely. Set each timer's preset to that lamp's duration. You can write and run exactly this program in the browser scenario on this page.
A 4 way traffic light PLC ladder logic is best written as a state machine with one state per phase — North/South green, North/South yellow, East/West green, East/West yellow — stepped in order by a master timer. Because only one phase is active at a time, the two directions are never green together and the cross-direction interlock is automatic. The live scenario on this page includes the 4-way state table so you can see which lamps are energised in each phase.
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