Traffic light sequence in PLC Programming - Step-by-Step Execution

Concept Flow - Traffic light sequence

Start

↓

Red ON

↓

Wait Red Duration

↓

Red OFF, Green ON

↓

Wait Green Duration

↓

Green OFF, Yellow ON

↓

Wait Yellow Duration

↓

Yellow OFF

↓

Loop back to Red ON

The traffic light cycles through Red, Green, and Yellow states, each with a wait time, then repeats.

Execution Sample

PLC Programming

RED = True
WAIT(5)
RED = False
GREEN = True
WAIT(5)
GREEN = False
YELLOW = True
WAIT(2)
YELLOW = False

This code turns on Red light, waits 5 seconds, switches to Green, waits 5 seconds, then Yellow for 2 seconds.

Execution Table

Step	Red	Green	Yellow	Action	Wait Time (s)
1	ON	OFF	OFF	Turn Red ON	0
2	ON	OFF	OFF	Wait Red duration	5
3	OFF	ON	OFF	Turn Red OFF, Green ON	0
4	OFF	ON	OFF	Wait Green duration	5
5	OFF	OFF	ON	Turn Green OFF, Yellow ON	0
6	OFF	OFF	ON	Wait Yellow duration	2
7	OFF	OFF	OFF	Turn Yellow OFF	0
8	ON	OFF	OFF	Loop back to Red ON	0

💡 The sequence loops indefinitely, cycling through Red, Green, Yellow with specified wait times.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	After Step 7	After Step 8
RED	OFF	ON	OFF	OFF	OFF	ON
GREEN	OFF	OFF	ON	OFF	OFF	OFF
YELLOW	OFF	OFF	OFF	ON	OFF	OFF

Key Moments - 3 Insights

Why does the Red light turn OFF before the Green light turns ON?

Because the sequence must avoid having Red and Green ON simultaneously to prevent conflicting signals, as shown in steps 3 and 4 of the execution_table.

Why is there a wait time after turning each light ON?

The wait time simulates how long the light stays ON before changing, ensuring safe traffic flow, as seen in the 'Wait Red duration' and similar rows in the execution_table.

What happens after the Yellow light turns OFF?

The sequence loops back to turning the Red light ON again, restarting the cycle, as shown in step 8 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the Green light at Step 5?

A OFF

B Blinking

C ON

D Unknown

💡 Hint

Check the 'Green' column at Step 5 in the execution_table.

At which step does the Yellow light turn ON?

A Step 7

B Step 3

C Step 5

D Step 1

💡 Hint

Look for the step where 'Yellow' changes from OFF to ON in the execution_table.

If the wait time for Green is increased to 10 seconds, which step's wait time changes?

A Step 2

B Step 4

C Step 6

D Step 8

💡 Hint

Find the step labeled 'Wait Green duration' in the execution_table.

Concept Snapshot

Traffic light sequence cycles through Red, Green, Yellow lights.
Each light turns ON, stays ON for a set time, then turns OFF.
Sequence repeats indefinitely for continuous traffic control.
No two conflicting lights are ON simultaneously.
Wait times control how long each light stays ON.

Full Transcript

This visual execution trace shows how a traffic light sequence works in PLC programming. The sequence starts by turning the Red light ON and waiting 5 seconds. Then Red turns OFF and Green turns ON for 5 seconds. Next, Green turns OFF and Yellow turns ON for 2 seconds. After Yellow turns OFF, the cycle loops back to Red ON. Variables RED, GREEN, and YELLOW track which light is ON or OFF at each step. The wait times simulate how long each light stays ON to control traffic safely. The sequence repeats indefinitely, ensuring a continuous traffic light cycle.

Practice

1. What is the main purpose of a traffic light sequence in PLC programming?

easy

A. To control the order and timing of red, yellow, and green lights

B. To measure the speed of vehicles

C. To detect weather conditions

D. To count the number of cars passing

Solution

1. Step 1: Understand traffic light function

   Traffic lights manage traffic by showing red, yellow, and green lights in a set order.

2. Step 2: Identify PLC role

   PLC programs control the timing and sequence of these lights to keep traffic safe and flowing.

3. Final Answer:

   To control the order and timing of red, yellow, and green lights -> Option A

4. Quick Check:

   Traffic light sequence = control lights order and timing [OK]

Hint: Traffic lights cycle colors in order to manage traffic flow [OK]

Common Mistakes:

• Confusing traffic light sequence with vehicle speed measurement
• Thinking traffic lights detect weather
• Assuming traffic lights count cars

2. Which of the following is the correct way to represent a state for the green light in a PLC program?

easy

A. state = GREEN

B. state = YELLOW

C. state = RED

D. state = OFF

Solution

1. Step 1: Identify the green light state

   The green light state should be named clearly as GREEN to represent that light.

2. Step 2: Match option to green state

   Only state = GREEN assigns the state variable to GREEN, which is correct for the green light.

3. Final Answer:

   state = GREEN -> Option A

4. Quick Check:

   Green light state = GREEN [OK]

Hint: Use clear names like GREEN for green light state [OK]

Common Mistakes:

• Using RED or YELLOW for green light state
• Using OFF which means no light
• Confusing state names

3. Given this PLC pseudocode snippet for a traffic light sequence:

timer = 0
state = 'RED'
if timer < 10:
  light = 'RED'
elif timer < 15:
  light = 'GREEN'
else:
  light = 'YELLOW'

What will be the value of light when timer is 12?

medium

A. 'YELLOW'

B. 'RED'

C. 'GREEN'

D. Error

Solution

1. Step 1: Check timer conditions for 12

   12 is not less than 10, so first if is false. 12 is less than 15, so second condition is true.

2. Step 2: Determine light value

   Since timer < 15 is true, light is set to 'GREEN'.

3. Final Answer:

   'GREEN' -> Option C

4. Quick Check:

   timer=12 <15 -> light='GREEN' [OK]

Hint: Check conditions in order, timer 12 fits second condition [OK]

Common Mistakes:

• Choosing 'RED' because timer is close to 10
• Choosing 'YELLOW' by ignoring conditions order
• Thinking code causes error

4. This PLC code snippet is intended to cycle traffic lights but does not work correctly:

state = 'RED'
if state == 'RED':
  state = 'GREEN'
elif state == 'GREEN':
  state = 'YELLOW'
elif state == 'YELLOW':
  state = 'RED'

What is the main problem?

medium

A. The state variable is misspelled

B. The state changes only once because conditions are checked once

C. The code uses wrong comparison operators

D. The code lacks a timer to delay state changes

Solution

1. Step 1: Analyze state change logic

   The code changes state immediately but only once; no loop or repeated check.

2. Step 2: Identify missing repeated execution

   Without looping or timer, state changes once and stops, so no cycling happens.

3. Final Answer:

   The state changes only once because conditions are checked once -> Option B

4. Quick Check:

   Single run code = no cycling [OK]

Hint: State must update repeatedly, not just once [OK]

Common Mistakes:

• Thinking variable name is wrong
• Believing comparison operators are incorrect
• Ignoring need for timing or looping

5. You want to create a PLC program that cycles traffic lights with these timings: red for 10 seconds, green for 15 seconds, yellow for 3 seconds. Which approach best implements this sequence?

hard

A. Use separate timers for each light but no state variable

B. Use a single timer that counts total time without resetting

C. Use random delays between light changes

D. Use a state machine with a timer that resets after each state duration

Solution

1. Step 1: Understand timing requirements

   Each light needs a specific duration before switching to the next.

2. Step 2: Choose best control method

   A state machine with a timer that resets after each state's time allows precise control and cycling.

3. Final Answer:

   Use a state machine with a timer that resets after each state duration -> Option D

4. Quick Check:

   State machine + timer reset = correct timing control [OK]

Hint: Reset timer each state for exact light durations [OK]

Common Mistakes:

• Using one timer without reset causes wrong timing
• Random delays break predictable sequence
• No state variable causes confusion in sequence