Pedestrian crossing: a chain of on-delay timers

A push button, a three-colour car signal and a walk signal: the classic exercise for learning how a chain of on-delay timers turns one button press into a timed sequence. Fully solved, in four PLC families.

▶ Open the finished solution in the Playground Loads the panel and the ladder program from the video. Press RUN PLC and try it.

Other PLC families: Mitsubishi Allen-Bradley TOYOPUC

The task

A road has a pedestrian crossing with a push button. The car signal is a three-colour tower light and the walk signal has two lamps, WAIT (red) and WALK (green). Normally the cars have green and the walk signal shows WAIT. When a pedestrian presses the button:

1. the cars keep green for 2 seconds, so a car that is already close can pass;
2. the cars get amber for 3 seconds;
3. the cars get red, and the walk signal shows WALK for 6 seconds;
4. WAIT comes back, and after 2 seconds of all red (so the last pedestrian can clear the road) the cars get green again.

A press while the sequence is running must not restart or lengthen it. This is one of the most common PLC exercises, because it teaches the single most useful pattern in sequential control: a chain of on-delay timers.

Inputs and outputs

Here with Siemens-style addresses; the other three PLC families are in the solution below.

Thinking it through

The button is only pressed for a moment, but the sequence takes 13 seconds. So the first job is to remember the request: the button sets a memory bit, and the bit stays on until the sequence is finished. A set / reset pair is clearer here than a seal-in, because the bit is switched on and off in two different places.

Next, the time steps. Instead of one timer with many comparisons, use four on-delay timers in a chain: the request bit starts the first timer, the done bit of each timer starts the next one. Each timer is one step of the sequence, so the times can be read and changed directly: 2 s, 3 s, 6 s and 2 s. When the last timer is done, it resets the request bit. That switches off the first timer, which switches off the second, and so on: the whole chain falls back to zero by itself, ready for the next press.

The lights then need no memory of their own. Each one is just a combination of timer bits:

Writing the outputs this way has a safety benefit: cars green and WALK can never be on together, because one needs the first timer off and the other needs the second timer on, and the second timer cannot run before the first is done.

The solution in four PLC families

The logic is identical; only the addresses, the instruction names and the way the timer preset is written change. Siemens counts presets in milliseconds, the FX-style Mitsubishi and TOYOPUC timers count in 0.1 s steps (K20 = 2 s), and the MicroLogix-style timers here use a 0.01 s time base. On Allen-Bradley the contact for a finished timer is its /DN (done) bit.

Siemens S7-1200 style

Mitsubishi MELSEC FX style

Allen-Bradley MicroLogix style

JTEKT TOYOPUC

Test it

1. Press RUN PLC. CARS shows green, WAIT is lit.
2. Press PUSH. Nothing changes for 2 s, then amber for 3 s.
3. Cars red and WALK green for 6 s. Open the monitor (Tag monitor) and watch the four timers count one after another.
4. WAIT comes back, 2 s later the cars are green again. Press PUSH during the sequence: nothing changes.

Common mistakes

• Using the button directly as the timer condition. The timer resets as soon as the button is released. Store the request in a bit first.
• Resetting the request too early, for example when WALK ends. The last timer then never finishes and the cars stay red. Reset only from the last timer of the chain.
• Giving each light its own set / reset. It works, but it is easy to leave a light on in a corner case. Deriving the lights from timer bits keeps them consistent automatically.
• Forgetting the all-red clearance. A real crossing gives slow pedestrians time to finish before cars move.

Variations to try

• Flash the WALK lamp during its last 2 seconds (an extra timer and a clock bit).
• Add a minimum green time for the cars, so pedestrians cannot stop the traffic every few seconds.
• Use the tower light's buzzer as an audible signal for blind pedestrians while WALK is on.

Try it yourself

Build it from scratch in the free PLC Sim Lab Playground, or open the finished solution and change it:

▶ Open the finished solution in the Playground Loads the panel and the ladder program from the video. Press RUN PLC and try it.

Other PLC families: Mitsubishi Allen-Bradley TOYOPUC

More: all PLC exercises · free PLC course with certificate · PLC timers and counters · ladder logic basics