Flashing light circuit with timers in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the timer when the input is ON.

PLC Programming

IF input_signal = [1] THEN
    start_timer()
END_IF

Drag options to blanks, or click blank then click option'

AFALSE

B1

C0

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALSE instead of TRUE to start the timer.

Using numeric values like 0 or 1 instead of TRUE.

The timer starts when the input signal is TRUE (ON).

2fill in blank

medium

Complete the code to reset the timer when the reset button is pressed.

PLC Programming

IF reset_button = [1] THEN
    reset_timer()
END_IF

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ATRUE

B0

C1

DFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALSE instead of TRUE for reset condition.

Confusing numeric values with boolean.

The timer resets when the reset button is pressed (TRUE).

3fill in blank

hard

Fix the error in the timer comparison to check if the timer has finished.

PLC Programming

IF timer_elapsed_time [1] preset_time THEN
    light := TRUE
END_IF

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A==

B<

C>=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>=' causes light to turn on too early.

Using '==' may miss the exact match if time skips.

The light turns ON when the timer elapsed time is greater than or equal to the preset time.

4fill in blank

hard

Complete the code to toggle the light state after the timer finishes.

PLC Programming

IF timer_done = [1] THEN
    light := NOT light;
END_IF

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ATRUE

BFALSE

C;

D:=

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALSE for timer_done condition.

Missing semicolon causes syntax error.

The light toggles when the timer is done (TRUE). The semicolon ends the statement.

5fill in blank

hard

Complete the code to create a flashing light loop with timer reset and start.

PLC Programming

IF light = [1] THEN
    reset_timer();
    start_timer();
END_IF

Drag options to blanks, or click blank then click option'

ATRUE

B;

DFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if light is TRUE instead of FALSE.

Missing semicolons after commands.

Using newline instead of semicolon to end statements.

When light is FALSE (OFF), reset and start the timer. Each statement ends with a semicolon.

Practice

1. What is the main purpose of a timer in a flashing light circuit in PLC programming?

easy

A. To increase the brightness of the light

B. To measure the voltage of the light circuit

C. To create a delay that controls when the light turns on or off

D. To permanently keep the light on

Solution

1. Step 1: Understand timer function in PLC

   Timers in PLCs are used to create delays or measure time intervals before an action occurs.

2. Step 2: Apply timer role in flashing light circuit

   In a flashing light circuit, the timer delays turning the light on or off to create the flashing effect.

3. Final Answer:

   To create a delay that controls when the light turns on or off -> Option C

4. Quick Check:

   Timer delays control flashing = B [OK]

Hint: Timers delay actions to create flashing effects [OK]

Common Mistakes:

• Thinking timers increase brightness
• Confusing timers with voltage measurement
• Assuming timers keep lights permanently on

2. Which of the following is the correct syntax to start a TON timer named T1 with a preset of 5 seconds in a PLC program?

easy

A. TON T1 (IN=Start, PT=5s);

B. TON(T1, Start, 5);

C. TIMER TON T1 SET 5

D. TON T1(IN:=Start, PT:=T#5s);

Solution

1. Step 1: Recall standard TON timer syntax in PLC

   The TON timer uses named parameters with := and time format T# for preset time.

2. Step 2: Match syntax to options

   TON T1(IN:=Start, PT:=T#5s); uses correct parameter names and time format: TON T1(IN:=Start, PT:=T#5s); others are incorrect or incomplete.

3. Final Answer:

   TON T1(IN:=Start, PT:=T#5s); -> Option D

4. Quick Check:

   Correct TON syntax uses := and T# time format = D [OK]

Hint: Use := for parameters and T# for time in TON timers [OK]

Common Mistakes:

• Using parentheses without := for parameters
• Writing time as plain number without T#
• Incorrect keyword order or missing parameters

3. Given this PLC ladder logic snippet for a flashing light:

TON T1(IN:=Start, PT:=T#2s);
IF T1.Q THEN Light := NOT Light; T1(IN:=FALSE); END_IF;

What happens to the Light output after 2 seconds of Start being TRUE?

medium

A. Light turns ON and stays ON

B. Light toggles once then stops

C. Light remains OFF permanently

D. Light toggles its state every 2 seconds

Solution

1. Step 1: Analyze timer and toggle logic

   The timer T1 starts when Start is TRUE. After 2 seconds, T1.Q becomes TRUE, toggling Light once.

2. Step 2: Check timer reset behavior

   After toggling, T1 is set to FALSE (stopped), so no further toggling occurs.

3. Final Answer:

   Light toggles once then stops -> Option B

4. Quick Check:

   Timer reset stops repeated toggling = A [OK]

Hint: Timer reset stops repeated toggling; toggles only once [OK]

Common Mistakes:

• Assuming continuous toggling without timer reset
• Thinking light stays ON permanently
• Ignoring timer reset effect

4. Identify the error in this PLC code snippet for a flashing light:

TON T1(IN:=Start, PT:=T#3s);
IF T1.Q THEN Light := NOT Light; END_IF;
T1(IN:=Start);

medium

A. Timer input IN is assigned after checking T1.Q, causing logic error

B. Timer preset time PT is missing units

C. Light output is not toggled correctly

D. Timer T1 is not declared

Solution

1. Step 1: Check order of timer input assignment

   The timer input IN must be assigned before checking T1.Q to update timer status correctly.

2. Step 2: Identify impact of wrong order

   Assigning IN after checking T1.Q means T1.Q is stale, causing incorrect toggling behavior.

3. Final Answer:

   Timer input IN is assigned after checking T1.Q, causing logic error -> Option A

4. Quick Check:

   Assign timer input before reading output = A [OK]

Hint: Assign timer input before reading output to avoid stale data [OK]

Common Mistakes:

• Assigning timer input after reading output
• Forgetting time units in PT
• Misunderstanding toggle logic

5. How would you modify a PLC flashing light circuit using a TON timer to make the light flash continuously every 1 second?

hard

A. Use a TON timer with 1 second preset and toggle light when timer done, then reset timer input

B. Use a TON timer with 1 second preset and never reset timer input

C. Use a TON timer with 1 second preset and toggle light only once when Start is TRUE

D. Use a TON timer with 2 second preset and toggle light every 1 second

Solution

1. Step 1: Understand continuous flashing logic

   To flash continuously, the light must toggle every timer cycle and the timer must restart each cycle.

2. Step 2: Apply timer reset and toggle

   Toggle light when timer finishes (Q=TRUE), then reset timer input to restart timing for next cycle.

3. Final Answer:

   Use a TON timer with 1 second preset and toggle light when timer done, then reset timer input -> Option A

4. Quick Check:

   Toggle + reset timer input = continuous flashing [OK]

Hint: Toggle light and reset timer input each cycle for continuous flash [OK]

Common Mistakes:

• Not resetting timer input causing no repeat
• Using wrong preset time for desired flash rate
• Toggling light only once