Flashing light circuit with timers in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a flashing light circuit using timers in PLC programming?

To create a light that turns on and off repeatedly at set intervals, often used for signaling or alerts.

beginner

Which type of timer is commonly used to create a flashing light effect in PLCs?

An ON-delay timer (TON) or an OFF-delay timer (TOF) can be used, but often an ON-delay timer is used to control the ON period, combined with logic to toggle the light.

intermediate

In a flashing light circuit, how do you make the light toggle ON and OFF continuously?

By using a timer to count a set time, then using a bit to toggle the light state each time the timer finishes, creating a repeating ON/OFF cycle.

advanced

What is the role of a 'retentive' timer in a flashing light circuit?

A retentive timer keeps its accumulated time even if the input turns off, useful for pausing and resuming the flashing cycle without resetting the timer.

intermediate

Why is it important to use a memory bit or flag in a flashing light circuit with timers?

Because the timer alone only measures time; a memory bit stores the current ON/OFF state of the light to toggle it correctly each cycle.

Which timer type is typically used to start counting when an input turns ON in a flashing light circuit?

A OFF-delay timer (TOF)

B Retentive timer (RTO)

C ON-delay timer (TON)

D Pulse timer (TP)

An ON-delay timer (TON) starts counting when the input turns ON, which is useful for timing the ON period of a flashing light.

What does a flashing light circuit typically use to remember if the light is currently ON or OFF?

A A memory bit or flag

B A counter

C A timer preset value

D An analog input

A memory bit or flag stores the current state (ON or OFF) so the circuit knows how to toggle the light each cycle.

How can you make a light flash faster in a timer-based flashing light circuit?

A Add more memory bits

B Increase the timer preset value

C Use a retentive timer

D Decrease the timer preset value

Decreasing the timer preset value shortens the ON and OFF intervals, making the light flash faster.

What happens if you use a non-retentive timer and the input turns OFF before the timer finishes?

A The timer keeps counting

B The timer resets to zero

C The timer pauses

D The timer doubles its count

A non-retentive timer resets its accumulated time to zero if the input turns OFF before completion.

In a flashing light circuit, what is the simplest way to toggle the light state after a timer finishes?

A Invert the memory bit controlling the light

B Use a counter to count flashes

C Reset the timer preset

D Use an analog input

Inverting the memory bit flips the light state from ON to OFF or vice versa, creating the flashing effect.

Explain how a flashing light circuit works using timers in a PLC. Include the role of timers and memory bits.

Think about how the timer and a bit work together to turn the light on and off repeatedly.

Describe the difference between a retentive and a non-retentive timer in the context of a flashing light circuit.

Consider what happens when the timer input turns off before the timer finishes.

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