Why Flashing light circuit with timers in PLC Programming? - Purpose & Use Cases

The Big Idea

What if your warning light could flash perfectly on its own, without you lifting a finger?

The Scenario

Imagine you have to manually turn a warning light on and off every few seconds to alert workers in a factory. You stand there, pressing the switch on and off repeatedly, trying to keep the timing just right.

The Problem

This manual method is tiring and distracting. You might miss the timing, causing the light to stay on too long or too short. It wastes your time and can lead to safety risks if the light doesn't flash correctly.

The Solution

Using a flashing light circuit with timers automates this process. The timer controls the light to turn on and off at precise intervals without any human effort, ensuring consistent and reliable flashing.

Before vs After

✗ Before

Press switch ON
Wait 2 seconds
Press switch OFF
Wait 2 seconds
Repeat

✓ After

Timer ON for 2 seconds -> Light ON
Timer OFF for 2 seconds -> Light OFF
Loop automatically

What It Enables

This lets machines handle repetitive timing tasks perfectly, freeing people to focus on more important work.

Real Life Example

In a factory, a flashing warning light signals when a machine is running or needs attention, automatically controlled by timers to keep workers safe without manual effort.

Key Takeaways

Manual flashing is tiring and error-prone.

Timers automate on/off cycles precisely.

Automation improves safety and efficiency.

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