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Digital input wiring (NPN vs PNP sensors) in PLC Programming - Practice Questions

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Challenge - 5 Problems
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Sensor Wiring Mastery
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🧠 Conceptual
intermediate
2:00remaining
Understanding NPN Sensor Wiring
Which statement correctly describes how an NPN sensor is wired to a PLC digital input?
AThe sensor switches the input line to ground when active, requiring the PLC input to be connected to a positive voltage through a pull-up resistor.
BThe sensor switches the input line to positive voltage when active, requiring the PLC input to be connected to ground through a pull-down resistor.
CThe sensor switches the input line to positive voltage when active, requiring the PLC input to be connected directly to positive voltage without any resistor.
DThe sensor switches the input line to ground when active, requiring the PLC input to be connected directly to ground without any resistor.
Attempts:
2 left
💡 Hint
Think about which side the sensor connects to when it activates and how the PLC input detects that.
🧠 Conceptual
intermediate
2:00remaining
Understanding PNP Sensor Wiring
Which statement correctly describes how a PNP sensor is wired to a PLC digital input?
AThe sensor switches the input line to positive voltage when active, requiring the PLC input to be connected to ground through a pull-down resistor.
BThe sensor switches the input line to ground when active, requiring the PLC input to be connected to positive voltage through a pull-up resistor.
CThe sensor switches the input line to positive voltage when active, requiring the PLC input to be connected directly to positive voltage without any resistor.
DThe sensor switches the input line to ground when active, requiring the PLC input to be connected directly to ground without any resistor.
Attempts:
2 left
💡 Hint
Consider which voltage level the sensor provides when active and how the PLC input detects it.
💻 Command Output
advanced
2:00remaining
Identify the Resulting Input State
Given a PLC input wired to an NPN sensor with a 24V supply and a pull-up resistor, what is the input voltage when the sensor is active?
PLC Programming
Supply voltage = 24V
Sensor active = input connected to ground
Pull-up resistor present

What is the voltage at the PLC input terminal?
AApproximately 24V (high state)
BApproximately 0V (low state)
CApproximately 12V (mid-level voltage)
DVoltage fluctuates rapidly
Attempts:
2 left
💡 Hint
When the sensor connects the input to ground, the voltage drops to what level?
💻 Command Output
advanced
2:00remaining
Identify the Resulting Input State for PNP Sensor
Given a PLC input wired to a PNP sensor with a 24V supply and a pull-down resistor, what is the input voltage when the sensor is inactive?
PLC Programming
Supply voltage = 24V
Sensor inactive = input not connected to positive voltage
Pull-down resistor present

What is the voltage at the PLC input terminal?
AApproximately 24V (high state)
BVoltage fluctuates rapidly
CApproximately 12V (mid-level voltage)
DApproximately 0V (low state)
Attempts:
2 left
💡 Hint
When the sensor is off and the pull-down resistor is connected, what voltage does the input line have?
🚀 Application
expert
3:00remaining
Troubleshooting Sensor Wiring
A technician wired a PNP sensor to a PLC digital input but the PLC never detects the sensor as active. The sensor is confirmed working and powered. Which wiring mistake is most likely causing this issue?
AThe sensor output is connected to the PLC input, but the PLC input is wired with a pull-down resistor as required.
BThe sensor output is connected to the PLC input, but the sensor power supply is reversed.
CThe sensor output is connected to the PLC input, but the PLC input is wired with a pull-up resistor instead of a pull-down resistor.
DThe sensor output is connected to the PLC input, but the sensor is wired as NPN instead of PNP.
Attempts:
2 left
💡 Hint
Think about what happens if the pull resistor polarity does not match the sensor type.

Practice

(1/5)
1. What is the main difference between NPN and PNP sensors in digital input wiring?
easy
A. NPN sensors pull the input to ground; PNP sensors supply positive voltage.
B. NPN sensors supply positive voltage; PNP sensors pull the input to ground.
C. Both NPN and PNP sensors supply positive voltage.
D. Both NPN and PNP sensors pull the input to ground.

Solution

  1. Step 1: Understand NPN sensor behavior

    NPN sensors connect the input to ground (0V) when active, sinking current.

  2. Step 2: Understand PNP sensor behavior

    PNP sensors connect the input to positive voltage (e.g., 24V) when active, sourcing current.

  3. Final Answer:

    NPN sensors pull the input to ground; PNP sensors supply positive voltage. -> Option A

  4. Quick Check:

    NPN sinks, PNP sources = A [OK]
Hint: Remember: NPN sinks (ground), PNP sources (positive) [OK]
Common Mistakes:
  • Confusing which sensor sinks or sources current
  • Assuming both sensors behave the same
  • Mixing up voltage levels for NPN and PNP
2. Which wiring connection is correct for a PNP sensor to a PLC digital input?
easy
A. Sensor output connected to PLC input; sensor negative to +24V; sensor positive to +24V.
B. Sensor output connected to 0V; sensor negative to PLC input; sensor positive to +24V.
C. Sensor output connected to PLC input; sensor negative to 0V; sensor positive to +24V.
D. Sensor output connected to PLC input; sensor negative to +24V; sensor positive to 0V.

Solution

  1. Step 1: Identify PNP sensor wiring

    PNP sensors source positive voltage to the PLC input; sensor positive terminal connects to +24V, negative to 0V.

  2. Step 2: Confirm correct PLC input connection

    Sensor output wire connects directly to the PLC digital input to supply voltage when active.

  3. Final Answer:

    Sensor output connected to PLC input; sensor negative to 0V; sensor positive to +24V. -> Option C

  4. Quick Check:

    PNP sources +24V to input = D [OK]
Hint: PNP sensor positive to +24V, output to input, negative to 0V [OK]
Common Mistakes:
  • Reversing sensor positive and negative connections
  • Connecting sensor output to 0V instead of PLC input
  • Mixing NPN wiring rules with PNP sensors
3. Given this PLC input wiring with an NPN sensor, what will the PLC input read when the sensor is active?
Sensor output connected to PLC input; sensor negative to 0V; sensor positive to +24V.
medium
A. PLC input reads HIGH (24V)
B. PLC input reads an error signal
C. PLC input reads fluctuating voltage
D. PLC input reads LOW (0V)

Solution

  1. Step 1: Understand NPN sensor active state

    When active, NPN sensor pulls output to 0V (ground), sinking current.

  2. Step 2: Determine PLC input voltage

    Since sensor output connects to PLC input and is pulled to 0V, PLC input reads LOW (0V).

  3. Final Answer:

    PLC input reads LOW (0V) -> Option D

  4. Quick Check:

    NPN active pulls input LOW = B [OK]
Hint: NPN active means input pulled to ground = LOW [OK]
Common Mistakes:
  • Assuming NPN sensor outputs positive voltage when active
  • Confusing sensor wiring polarity
  • Thinking input voltage fluctuates instead of stable LOW
4. A technician wired a PNP sensor but the PLC input never reads HIGH. Which is the most likely wiring mistake?
medium
A. Sensor output connected to 0V instead of PLC input.
B. Sensor positive connected to 0V instead of +24V.
C. Sensor negative connected to +24V instead of 0V.
D. Sensor output connected to PLC input correctly.

Solution

  1. Step 1: Review correct PNP wiring

    PNP sensor output must connect to PLC input to supply positive voltage when active.

  2. Step 2: Identify wiring mistake

    If sensor output is connected to 0V instead of PLC input, input never receives voltage, so PLC reads LOW always.

  3. Final Answer:

    Sensor output connected to 0V instead of PLC input. -> Option A

  4. Quick Check:

    Output must connect to input, not 0V = A [OK]
Hint: Output wire must go to PLC input, not ground [OK]
Common Mistakes:
  • Connecting sensor output to 0V instead of input
  • Swapping sensor positive and negative wires
  • Assuming PLC input reads HIGH without proper wiring
5. You have a PLC input configured for sourcing (PNP) sensors but only NPN sensors are available. How should you wire the NPN sensor to ensure the PLC reads the input correctly?
hard
A. Connect the NPN sensor output to 0V and sensor positive to PLC input.
B. Use an external pull-up resistor to +24V on the PLC input and connect the NPN sensor output to the input and sensor negative to 0V.
C. Connect the NPN sensor output directly to +24V and sensor negative to PLC input.
D. Wire the NPN sensor as a PNP sensor without any changes.

Solution

  1. Step 1: Understand PLC input expecting sourcing (PNP) sensor

    PLC input expects positive voltage from sensor output when active.

  2. Step 2: Adapt NPN sensor output for sourcing input

    NPN sensor sinks current (pulls to 0V), so add pull-up resistor to +24V on PLC input to provide voltage when sensor is inactive.

  3. Step 3: Wire NPN sensor output to PLC input and sensor negative to 0V

    This wiring allows PLC input to read HIGH when sensor is inactive (pull-up voltage) and LOW when sensor is active (pulled to ground).

  4. Final Answer:

    Use an external pull-up resistor to +24V on the PLC input and connect the NPN sensor output to the input and sensor negative to 0V. -> Option B

  5. Quick Check:

    Pull-up resistor adapts NPN sinking to sourcing input = C [OK]
Hint: Add pull-up resistor to adapt NPN sensor for PNP input [OK]
Common Mistakes:
  • Wiring NPN sensor directly without pull-up resistor
  • Connecting sensor output to +24V directly
  • Ignoring PLC input type requirements
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