NOT logic with NC contact in PLC Programming - Mini Project: Build & Apply

NOT Logic with NC Contact

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to control a motor. The motor should run only when a safety switch is not pressed. The safety switch is a Normally Closed (NC) contact, which means it is closed (allowing current) when not pressed, and opens (breaks current) when pressed.Your task is to write a PLC ladder logic script that uses NOT logic with the NC contact to control the motor.

🎯 Goal: Build a PLC ladder logic script that uses the NOT logic with a Normally Closed (NC) contact named SafetySwitch_NC to control a motor coil named Motor. The motor should be energized only when the safety switch is not pressed.

📋 What You'll Learn

Create a variable named SafetySwitch_NC representing the NC contact input.

Create a variable named Motor representing the motor coil output.

Use NOT logic with the NC contact to control the motor.

Print the motor state as ON or OFF based on the logic.

💡 Why This Matters

🌍 Real World

In industrial automation, safety switches often use Normally Closed contacts to ensure machines stop immediately if a safety device is triggered. Understanding NOT logic with NC contacts helps program safe machine controls.

💼 Career

PLC programming skills with safety logic are essential for automation technicians and engineers to design and troubleshoot industrial control systems.

1

Create the NC contact input variable

Create a boolean variable called SafetySwitch_NC and set it to True to represent the Normally Closed contact being closed (not pressed).

PLC Programming

# Create the NC contact input variable
# Your code here

Hint

Remember, NC contact is True when not pressed.

2

Create the motor coil output variable

Create a boolean variable called Motor and set it to False initially to represent the motor being off.

PLC Programming

SafetySwitch_NC = True
# Create the motor coil output variable
# Your code here

Hint

The motor starts off as off, so set Motor to False.

3

Apply NOT logic with the NC contact to control the motor

Set the Motor variable to the NOT of SafetySwitch_NC to represent that the motor runs only when the safety switch is pressed (NC contact opens). Use Motor = not SafetySwitch_NC.

PLC Programming

SafetySwitch_NC = True
Motor = False
# Apply NOT logic to control the motor
# Your code here

Hint

Use the not operator to invert the NC contact state.

4

Print the motor state

Print Motor ON if Motor is True, otherwise print Motor OFF. Use print("Motor ON") or print("Motor OFF") accordingly.

PLC Programming

SafetySwitch_NC = True
Motor = False
Motor = not SafetySwitch_NC
# Print the motor state
# Your code here

Hint

Use an if statement to check Motor and print the correct message.

Practice

1. What does a Normally Closed (NC) contact represent in PLC logic?

easy

A. The circuit is ON when the switch is NOT pressed

B. The circuit is ON only when the switch is pressed

C. The circuit is always OFF

D. The circuit toggles ON and OFF automatically

Solution

1. Step 1: Understand NC contact behavior

   An NC contact means the circuit is closed (ON) when the switch is not pressed.

2. Step 2: Compare options with NC contact logic

   Only The circuit is ON when the switch is NOT pressed correctly states the circuit is ON when the switch is NOT pressed.

3. Final Answer:

   The circuit is ON when the switch is NOT pressed -> Option A

4. Quick Check:

   NC contact = ON when NOT pressed [OK]

Hint: NC means circuit closed when switch is NOT pressed [OK]

Common Mistakes:

• Confusing NC with Normally Open (NO) contact
• Thinking circuit is ON when switch is pressed
• Assuming NC contact toggles automatically

2. Which of the following is the correct PLC ladder logic symbol for a Normally Closed (NC) contact?

easy

A. --|&|--

B. --| |--

C. --|/|--

D. --|X|--

Solution

1. Step 1: Recall PLC contact symbols

   Normally Open (NO) contact is --| |--, Normally Closed (NC) contact is --|/|--.

2. Step 2: Match symbol to NC contact

   --|/|-- shows --|/|--, which is the correct NC contact symbol.

3. Final Answer:

   --|/|-- -> Option C

4. Quick Check:

   NC contact symbol = --|/|-- [OK]

Hint: NC contact symbol has a slash / inside brackets [OK]

Common Mistakes:

• Choosing NO contact symbol instead of NC
• Confusing symbols with AND/OR logic
• Using invalid or made-up symbols

3. Given this PLC ladder logic snippet with an NC contact labeled Stop controlling output Motor:

--|/|--(Stop)----( )--(Motor)

What happens to Motor when the Stop button is pressed?

medium

A. Motor turns OFF

B. Motor toggles ON and OFF rapidly

C. Motor turns ON

D. Motor state does not change

Solution

1. Step 1: Understand NC contact effect when pressed

   Pressing the NC contact opens the circuit, breaking the path.

2. Step 2: Determine output state

   Since the circuit is open, the Motor output coil is not energized and turns OFF.

3. Final Answer:

   Motor turns OFF -> Option A

4. Quick Check:

   Pressing NC contact = circuit open = output OFF [OK]

Hint: Pressing NC opens circuit, output turns OFF [OK]

Common Mistakes:

• Assuming pressing NC turns output ON
• Thinking output toggles automatically
• Ignoring NC contact behavior

4. A PLC program uses an NC contact named EmergencyStop to stop a conveyor motor. The motor never stops when the emergency button is pressed. What is the most likely error?

medium

A. The PLC is powered off

B. The NC contact is wired as Normally Open (NO)

C. The emergency button is not pressed long enough

D. The output coil is missing

Solution

1. Step 1: Analyze symptom

   Motor never stops when emergency button pressed means circuit does not open.

2. Step 2: Identify wiring error

   If NC contact is wired as NO, pressing button closes circuit instead of opening it, so motor stays ON.

3. Final Answer:

   The NC contact is wired as Normally Open (NO) -> Option B

4. Quick Check:

   Wrong contact type wiring = motor won't stop [OK]

Hint: Check contact type wiring if stop fails [OK]

Common Mistakes:

• Blaming output coil without checking input wiring
• Assuming button press duration causes issue
• Ignoring PLC power status

5. You want to design a safety stop circuit using an NC contact SafetySwitch and an output coil MachineRun. Which ladder logic rung correctly implements the NOT logic with the NC contact to stop the machine when the switch is pressed?

hard

A. --| |--(SafetySwitch)----( )--(MachineRun)

B. --| |--(MachineRun)----( )--(SafetySwitch)

C. --|/|--(MachineRun)----( )--(SafetySwitch)

D. --|/|--(SafetySwitch)----( )--(MachineRun)

Solution

1. Step 1: Identify correct NC contact usage

   Safety stop uses NC contact so circuit is closed (MachineRun ON) when switch NOT pressed.

2. Step 2: Match ladder logic rung

   --|/|--(SafetySwitch)----( )--(MachineRun) uses NC contact --|/|-- controlling MachineRun output correctly.

3. Final Answer:

   --|/|--(SafetySwitch)----( )--(MachineRun) -> Option D

4. Quick Check:

   NC contact before output coil = safety stop logic [OK]

Hint: Use NC contact before output coil for safety stop [OK]

Common Mistakes:

• Using NO contact instead of NC for safety stop
• Reversing contact and coil positions
• Confusing contact labels with output coil