Traffic light sequence in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Traffic Light Sequence Timing Output

Given the following PLC ladder logic snippet controlling a traffic light sequence, what is the total time (in seconds) the green light stays on during one full cycle?

PLC Programming

(* Ladder logic timing for green light *)
Timer T1: preset = T#30s
Timer T2: preset = T#5s
(* Green light ON during T1 only *)

A 0

B 30

C 5

D 35

💡 Hint

Focus on the timer preset that controls the green light ON duration.

The green light is ON only during Timer T1 which is set to 30 seconds. Timer T2 is for a different light phase.

🧠 Conceptual

intermediate

Understanding Traffic Light State Transitions

In a PLC program controlling a traffic light, which of the following best describes the role of a state machine?

A It detects hardware faults in the PLC.

B It measures the speed of passing vehicles.

C It controls the brightness of the traffic lights.

D It manages the sequence of light changes by moving through defined states.

💡 Hint

Think about how the traffic lights change from green to yellow to red.

A state machine defines and controls the order of traffic light phases by transitioning through states.

🔧 Debug

advanced

Identify the Error in Traffic Light Sequence Code

This PLC structured text code snippet is intended to cycle traffic lights: Green -> Yellow -> Red -> Green. What error causes the red light to never turn ON? code: IF Green THEN Yellow := FALSE; Red := FALSE; TimerGreen(IN:=TRUE, PT:=T#30s); IF TimerGreen.Q THEN Green := FALSE; Yellow := TRUE; END_IF; ELSIF Yellow THEN TimerYellow(IN:=TRUE, PT:=T#5s); IF TimerYellow.Q THEN Yellow := FALSE; Green := TRUE; END_IF; END_IF;

A Red light is never set TRUE in any condition.

B TimerGreen is never started.

C Yellow light stays TRUE indefinitely.

D Green light never turns FALSE.

💡 Hint

Check where the red light variable is assigned TRUE.

The code never assigns TRUE to Red, so it never turns ON.

🚀 Application

advanced

Designing a Pedestrian Crossing Signal in PLC

You want to add a pedestrian crossing button to the traffic light PLC program. When pressed, it should extend the red light duration by 10 seconds. Which approach correctly implements this?

A Add a condition that adds 10 seconds to the red light timer preset when the button is pressed.

B Reset the green light timer to zero immediately when the button is pressed.

C Turn off the yellow light permanently when the button is pressed.

D Ignore the button press during the red light phase.

💡 Hint

Think about how to increase red light duration dynamically.

Extending the red light timer preset when the button is pressed delays the green light, allowing safe pedestrian crossing.

📝 Syntax

expert

Correct Syntax for Traffic Light Control in Structured Text

Which of the following PLC structured text snippets correctly sets the traffic light states so that only one light is ON at a time in the sequence Green -> Yellow -> Red?

A

SWITCH State
  CASE 1:
    Green := TRUE;
    Yellow := FALSE;
    Red := FALSE;
  CASE 2:
    Green := FALSE;
    Yellow := TRUE;
    Red := FALSE;
  CASE 3:
    Green := FALSE;
    Yellow := FALSE;
    Red := TRUE;
END_SWITCH

B

IF State = 1 THEN
  Green := TRUE;
  Yellow := FALSE;
  Red := FALSE;
ELSIF State = 2 THEN
  Green := FALSE;
  Yellow := TRUE;
  Red := FALSE;
ELSIF State = 3 THEN
  Green := FALSE;
  Yellow := FALSE;
  Red := TRUE;
END_IF

C

CASE State OF
  1: Green := TRUE; Yellow := FALSE; Red := FALSE;
  2: Green := FALSE; Yellow := TRUE; Red := FALSE;
  3: Green := FALSE; Yellow := FALSE; Red := TRUE;
END_CASE

D

IF State == 1 THEN
  Green := TRUE;
  Yellow := FALSE;
  Red := FALSE;
ELSE IF State == 2 THEN
  Green := FALSE;
  Yellow := TRUE;
  Red := FALSE;
ELSE IF State == 3 THEN
  Green := FALSE;
  Yellow := FALSE;
  Red := TRUE;
END_IF

💡 Hint

Check the correct syntax for CASE and assignment operators in structured text.

Option C uses correct CASE syntax and assignment operator ':='; others have syntax errors like wrong operators or keywords.

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