Normally open (NO) contact in PLC Programming - Mini Project: Build & Apply

Normally Open (NO) Contact Simulation in PLC Programming

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to control a motor using a Normally Open (NO) contact switch. The motor should turn on only when the NO contact is closed (activated).

🎯 Goal: Build a PLC program that simulates a Normally Open (NO) contact controlling a motor output. The program will read the input state of the NO contact and set the motor output accordingly.

📋 What You'll Learn

Create a variable NO_contact to represent the Normally Open contact input state.

Create a variable motor to represent the motor output state.

Use the NO_contact variable to control the motor output.

Print the state of the motor output as Motor is ON or Motor is OFF.

💡 Why This Matters

🌍 Real World

Normally Open contacts are common in industrial control systems to start machines only when a button or switch is pressed.

💼 Career

Understanding how to simulate and program NO contacts is essential for PLC programmers and automation engineers working with factory equipment.

1

Create the input variable for the Normally Open contact

Create a boolean variable called NO_contact and set it to False to represent the Normally Open contact being initially open.

PLC Programming

# Create a boolean variable NO_contact and set it to False
# Your code here

Hint

Think of NO_contact as a switch that starts off open, so its value is False.

2

Create the output variable for the motor

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

PLC Programming

NO_contact = False
# Create a boolean variable motor and set it to False
# Your code here

Hint

The motor starts off stopped, so motor should be False.

3

Control the motor output using the NO contact input

Set the motor variable to the value of NO_contact to simulate the Normally Open contact controlling the motor.

PLC Programming

NO_contact = False
motor = False
# Set motor to the value of NO_contact
# Your code here

Hint

The motor should be ON only when the NO contact is closed (True).

4

Print the motor output state

Write a print statement that displays Motor is ON if motor is True, otherwise print Motor is OFF.

PLC Programming

NO_contact = False
motor = False
motor = NO_contact
# Print 'Motor is ON' if motor is True, else print 'Motor is OFF'
# Your code here

Hint

Use a simple if-else expression inside the print statement to show the motor state.

Practice

1. What does a Normally Open (NO) contact do in a PLC circuit?

easy

A. It remains open until activated, then closes to allow current flow.

B. It remains closed until activated, then opens to stop current flow.

C. It always stays closed regardless of activation.

D. It always stays open regardless of activation.

Solution

1. Step 1: Understand NO contact default state

   A Normally Open contact is open by default, meaning no current flows through it initially.

2. Step 2: Behavior when activated

   When the NO contact is activated (e.g., button pressed), it closes, allowing current to flow.

3. Final Answer:

   It remains open until activated, then closes to allow current flow. -> Option A

4. Quick Check:

   NO contact = open by default, closes when activated [OK]

Hint: NO means open until pressed, then closes [OK]

Common Mistakes:

• Confusing NO with Normally Closed (NC) contact
• Thinking NO contact is closed by default
• Assuming NO contact always stays open

2. Which of the following is the correct syntax to represent a Normally Open (NO) contact in a PLC ladder logic program?

easy

A. --[/]-- symbol

B. |/| symbol

C. | | symbol

D. --[ ]-- symbol

Solution

1. Step 1: Identify NO contact symbol

   The Normally Open contact is represented by two parallel vertical lines: | |.

2. Step 2: Differentiate from NC contact

   The Normally Closed contact uses |/| or --[/]-- symbols, which are different.

3. Final Answer:

   | | symbol -> Option C

4. Quick Check:

   NO contact = | | [OK]

Hint: NO contact looks like two parallel lines | | [OK]

Common Mistakes:

• Using NC contact symbol for NO contact
• Confusing ladder logic symbols
• Mixing up contact and coil symbols

3. Given the following ladder logic snippet with a Normally Open (NO) contact named Start_Button controlling a coil Motor, what happens when Start_Button is pressed?

--| |--( )--
  Start_Button  Motor

medium

A. Motor coil de-energizes, turning the motor OFF.

B. Motor coil energizes, turning the motor ON.

C. Nothing happens; the motor state remains unchanged.

D. Motor coil toggles ON and OFF rapidly.

Solution

1. Step 1: Analyze NO contact behavior when pressed

   Pressing the Start_Button closes the NO contact, allowing current to flow.

2. Step 2: Effect on coil Motor

   Current energizes the Motor coil, turning the motor ON.

3. Final Answer:

   Motor coil energizes, turning the motor ON. -> Option B

4. Quick Check:

   NO contact pressed = coil energized [OK]

Hint: Pressing NO contact energizes coil [OK]

Common Mistakes:

• Assuming coil de-energizes when NO contact closes
• Thinking NO contact does not affect coil
• Confusing coil with contact behavior

4. In the ladder logic below, the Normally Open (NO) contact Sensor is supposed to start the Conveyor motor. However, the motor never starts even when the sensor is triggered. What is the likely error?

--|/|--( )--
 Sensor Conveyor

medium

A. The contact is Normally Closed (NC) instead of Normally Open (NO).

B. The coil Conveyor is wired incorrectly.

C. The sensor is not physically connected.

D. The power supply is off.

Solution

1. Step 1: Identify contact type in code

   The contact symbol |/| represents a Normally Closed (NC) contact, not Normally Open (NO).

2. Step 2: Effect of NC contact on motor start

   Since the sensor is triggered (activated), the NC contact opens, breaking the circuit and preventing the motor from starting.

3. Final Answer:

   The contact is Normally Closed (NC) instead of Normally Open (NO). -> Option A

4. Quick Check:

   Wrong contact type = motor won't start [OK]

Hint: Check contact symbol: |/| means NC, not NO [OK]

Common Mistakes:

• Confusing NC and NO contact symbols
• Assuming coil wiring is always the issue
• Ignoring sensor physical connection

5. You want to design a control circuit where a Normally Open (NO) start button must be pressed to start a machine. Which logic setup correctly uses the NO contact to achieve this?

hard

A. Use the NO contact in series with the motor coil; pressing the button breaks the circuit to start the machine.

B. Use the NO contact in parallel with the motor coil; pressing the button breaks the circuit to start the machine.

C. Use the NO contact in parallel with the motor coil; pressing the button energizes the coil to start the machine.

D. Use the NO contact in series with the motor coil; pressing the button energizes the coil to start the machine.

Solution

1. Step 1: Understand NO contact behavior in start circuit

   A Normally Open start button is open by default, meaning no current flows through it initially (machine OFF).

2. Step 2: Effect of pressing NO start button

   When the NO button is pressed, it closes, allowing current to flow and energizing the motor coil to start the machine.

3. Final Answer:

   Use the NO contact in series with the motor coil; pressing the button energizes the coil to start the machine. -> Option D

4. Quick Check:

   NO series press = coil energized to start [OK]

Hint: NO start button in series energizes coil when pressed [OK]

Common Mistakes:

• Placing NO contact in parallel for start function
• Assuming pressing NO button breaks circuit to start
• Confusing NO and NC contacts in control circuits