Why Normally open (NO) contact in PLC Programming? - Purpose & Use Cases

The Big Idea

What if your machines could start exactly when needed, without you lifting a finger?

The Scenario

Imagine you have a factory machine that needs to start only when a button is pressed. Without automation, someone must watch and press the button every time, which is tiring and easy to miss.

The Problem

Manually watching and pressing the button is slow and can cause mistakes. If the operator is distracted or tired, the machine might not start on time, causing delays or errors in production.

The Solution

The Normally Open (NO) contact in PLC programming acts like a smart switch that closes the circuit only when the button is pressed. This lets the machine start automatically and reliably without constant human attention.

Before vs After

✗ Before

if button_pressed == True:
    start_machine()

✓ After

NO_contact = button_input
if NO_contact:
    start_machine()

What It Enables

It enables machines to respond instantly and safely to user actions without manual monitoring.

Real Life Example

In an assembly line, pressing a start button closes the NO contact, triggering the conveyor belt to move automatically, improving speed and safety.

Key Takeaways

Manual control is slow and error-prone.

Normally Open contact automates response to inputs.

This improves reliability and efficiency in machines.

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