Flashing light circuit with timers in PLC Programming

Introduction

A flashing light circuit with timers helps control lights to turn on and off repeatedly at set intervals. This is useful for signaling or alerting in machines or buildings.

To make a warning light flash on a machine to show it needs attention.

To create a blinking light at a pedestrian crossing for safety.

To signal an alarm by flashing a light in a factory.

To control decorative lights that turn on and off automatically.

To test timer functions in a PLC system.

Syntax

PLC Programming

TON TimerName(IN:=Condition, PT:=TimeValue, Q=>Output, ET=>ElapsedTime);

// TON is an ON-delay timer
// IN: input to start timer
// PT: preset time (how long to wait)
// Q: output becomes true after PT
// ET: elapsed time since timer started

Timers in PLCs often use standard function blocks like TON (Timer ON delay).

You set the input to start the timer and the preset time for delay.

Examples

Starts Timer1 when StartButton is pressed; output Timer1Done is true after 1 second.

PLC Programming

Timer1(IN:=StartButton, PT:=T#1s, Q=>Timer1Done);

Self-oscillating timer: pulses output true briefly (~PLC scan time) every 500ms.

PLC Programming

FlashTimer(IN:=NOT FlashTimer.Q, PT:=T#500ms, Q=>FlashTimer.Q);

Sample Program

This program uses a timer and reset pulse to toggle the light on and off every 500 milliseconds, creating a flashing effect.

PLC Programming

(* Flashing light using TON timer in Structured Text *)
VAR
  FlashTimer: TON;
  LightOutput: BOOL := FALSE;
  ResetPulse: BOOL := FALSE;
END_VAR

FlashTimer(IN := NOT ResetPulse, PT := T#500ms);

IF FlashTimer.Q THEN
  LightOutput := NOT LightOutput;
END_IF;

ResetPulse := FlashTimer.Q;

(* Output LightOutput controls the flashing light *)

Important Notes

Make sure the timer is reset properly to create a continuous flashing effect.

The preset time (PT) controls how fast the light flashes.

Use boolean variables to control the light output for easy connection to hardware.

Summary

A flashing light circuit uses timers to turn lights on and off repeatedly.

TON timers delay turning on outputs after an input signal.

Toggle the light output when the timer finishes to create flashing.

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