Flashing light circuit with timers in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple ON-OFF timer logic

Given the following PLC timer logic snippet, what will be the output state of the light after 5 seconds if the input is ON continuously?

Timer TON with preset 3 seconds, input IN is ON from start, output Q controls the light.

PLC Programming

Timer1(IN:=TRUE, PT:=T#3S);
Light := Timer1.Q;

A Light is OFF

B Light remains undefined

C Light toggles every second

D Light is ON

💡 Hint

Think about how TON timer output Q behaves after preset time when input is ON.

The TON timer output Q becomes TRUE after the preset time (3 seconds) if input IN remains TRUE. Since input is ON continuously, after 5 seconds Q is TRUE, so Light is ON.

🧠 Conceptual

intermediate

Understanding timer types for flashing light

Which timer type is best suited to create a flashing light that turns ON and OFF repeatedly at fixed intervals in a PLC?

A TON (On-delay timer)

B TP (Pulse timer)

C TOF (Off-delay timer)

D TONR (Retentive on-delay timer)

💡 Hint

Consider which timer automatically resets and produces a pulse output.

The TP (Pulse timer) produces a fixed pulse duration output each time it is triggered, making it ideal for flashing lights that need to turn ON and OFF repeatedly.

🔧 Debug

advanced

Identify the error in flashing light timer logic

This PLC code is intended to flash a light ON for 2 seconds and OFF for 2 seconds repeatedly. What is the main error?

Code:
TimerOn(IN:=Start, PT:=T#2S);
TimerOff(IN:=TimerOn.Q, PT:=T#2S);
Light := TimerOn.Q AND NOT TimerOff.Q;

A Light logic causes it to never turn ON

B TimerOff input should be connected to Start, not TimerOn.Q

C Start input is never reset, so TimerOn never restarts

D Timers TON and TOF cannot be used together

💡 Hint

Check how the Start input affects the timers and if it resets the cycle.

Since Start input remains ON, TimerOn runs once and stays ON, TimerOff input depends on TimerOn.Q which stays ON, so TimerOff never resets. The cycle does not repeat, so light does not flash.

🚀 Application

advanced

Create a flashing light with adjustable ON and OFF times

You want to create a flashing light in a PLC where ON time and OFF time can be set independently using two timers. Which logic below correctly implements this behavior?

A Use two TON timers: TimerOn starts when TimerOff.Q is FALSE, TimerOff starts when TimerOn.Q is TRUE; Light := TimerOn.Q AND NOT TimerOff.Q

B Use one TP timer with preset ON time and a TOF timer with preset OFF time; Light := TP.Q AND NOT TOF.Q

C Use one TON timer with preset ON time and reset it manually; Light := TON.Q

D Use two TON timers: TimerOn starts when Light is OFF, TimerOff starts when Light is ON; Light := TimerOn.Q OR NOT TimerOff.Q

💡 Hint

Think about how to alternate timers to create ON and OFF periods.

Option A correctly uses two TON timers that trigger each other alternately to create independent ON and OFF times. Light is ON when TimerOn.Q is TRUE and TimerOff.Q is FALSE, producing the flashing effect.

📝 Syntax

expert

Identify the syntax error in PLC timer code

Which option contains a syntax error in this PLC timer code snippet intended to flash a light every 1 second?

Code snippet:
Timer1: TON(IN:=Light, PT:=T#1S);
Light := NOT Timer1.Q;

A Timer1 TON(IN:=Light, PT:=T#1S); Light := NOT Timer1.Q;

B Timer1 := TON(IN:=Light, PT:=T#1S); Light := NOT Timer1.Q;

C Timer1: TON(IN:=Light, PT:=T#1S); Light := NOT Timer1.Q;

D Timer1 TON(IN=Light, PT=T#1S); Light := NOT Timer1.Q;

💡 Hint

Check the syntax for timer declaration and assignment in PLC structured text.

Option A is missing the assignment operator ':=' or declaration syntax, causing a syntax error. Correct syntax requires either declaration with ':' or assignment with ':='.

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