How to Wire a Safety Light Curtain? (NPN/PNP Wiring Diagrams + Common Issues)

How to Wire a Safety Light Curtain? (NPN/PNP Wiring Diagrams + Common Issues)

Summary

Incorrect wiring of a safety light curtain can not only cause it to malfunction but also create serious safety risks. This article, combined with detailed wiring diagrams, explains the wiring methods for the emitter and receiver, the differences between NPN and PNP output logic, normally open and normally closed circuits, common wiring issues, and practical application cases — helping you gain a comprehensive understanding of safety light curtain wiring.

How to Wire a Safety Light Curtain? (NPN/PNP Wiring Diagrams + Common Issues)

Structure and Function of Safety Light Curtains

A safety light curtain consists of a transmitter and a receiver. The transmitter emits infrared beams, while the receiver detects them to create an invisible safety barrier. When any object interrupts the beam, the light curtain sends a stop signal to prevent potential hazards. It is commonly used in applications such as presses, bending machines, automated lines, and robotic work cells.

NPN and PNP Output Logic in Safety Light Curtains

The light curtain OSSD output is available in two configurations — NPN and PNP. Choosing the correct type is critical for compatibility with PLCs or safety relays.
Your safety light curtain output must match the input type of the downstream control device. If you’re uncertain, it’s generally safer to choose PNP output, as most modern safety PLCs and relays support PNP (sourcing) inputs.
Output Type
State
Output Voltage
PLC Signal
NPN NO
Not triggered
Floating ≈ 24V
HIGH
Triggered
0V
LOW
NPN NC
Not triggered
0V
LOW
Triggered
Floating ≈ 24V
HIGH
PNP NO
Not triggered
0V
LOW
Triggered
+24V
HIGH
PNP NC
Not triggered
+24V
HIGH
Triggered
0V
LOW

Difference Between Normally Open (NO) and Normally Closed (NC) Outputs

The safety light curtain output signal can be either normally open (NO) or normally closed (NC):

▪️NO (Normally Open): No output signal in normal conditions; signal activates when the beam is blocked.
▪️NC (Normally Closed): Signal present in normal conditions; signal disappears when the beam is interrupted.

In industrial safety systems, NC outputs are often preferred because they can still indicate a “fault” (danger) condition in the event of wire disconnection or device failure, providing higher safety reliability.

Safety Light Curtain Wiring Diagram Explanation

In a safety curtain sensor connection, the transmitter requires only a power supply, while the receiver needs both power and output signal wiring. The following examples illustrate typical light curtain wiring diagrams.

Example 1: Emitter and Receiver with 7-pin Connectors

Emitter (7-pin)
1. Brown — +24 V DC
2. Blue — 0 V
3. Black — CP
4. White — NC
5. Green — NC
6. Red — CE
7. Yellow — GND

QCE safety light curtain NPN output wiring diagram
Receiver (7-pin)
1. Brown — +24 V DC
2. Blue — 0 V
3. Black — CP
4. White — OSSD1
5. Green — OSSD2
6. Red — CE
7. Yellow — GND
QCE safety light curtain PNP output wiring diagram
safety light curtain NPN normally closed wiring diagram
NPN normally closed (NC)
safety light curtain PNP normally closed wiring diagram
PNP normally closed (NC)
safety light curtain NPN normally open wiring diagram
NPN normally open (NO)
safety light curtain PNP normally open wiring diagram
PNP normally open (NO)

Example 2: Emitter with 3-pin, Receiver with 5-pin

Emitter (3-pin)
1. Brown — +24 V DC

2. Blue — 0 V

3. Yellow — GND
KT safety light curtain NPN output wiring diagram
Receiver (5-pin)
1. Brown — +24 V DC
2. Blue — 0 V
3. White — OSSD1
4. Black — OSSD2
5. Yellow — GND
KT safety light curtain NPN output wiring diagram
safety light curtain NPN normally closed wiring diagram
NPN normally closed (NC)
safety light curtain PNP normally closed wiring diagram
PNP normally closed (NC)
safety light curtain NPN normally open wiring diagram
NPN normally open (NO)
safety light curtain PNP normally open wiring diagram
PNP normally open (NO)
💡 Tip: Always refer to the manufacturer’s wiring diagram or user manual, as pin assignments may vary among different brands.

Safety Light Curtain Wiring Precautions

1. Use only DC 24V power — never connect AC power.
2. Ensure correct polarity to avoid circuit damage.
3. Choose either NPN or PNP, never both.
4. Keep signal cables away from high-voltage lines; use shielded cables if necessary.
5. Ensure reliable grounding to enhance anti-interference performance.
6. Confirm PLC input type (sinking or sourcing) before wiring.
7. After wiring, perform a practical blocking test to verify that the logic and outputs match the expected behavior.

Safety Light Curtain Wiring FAQ

Q1: Why doesn’t the safety light curtain power on?
Possible causes include insufficient supply voltage, reversed polarity, or loose connections.

Q2: Why is the output signal always 0V?
Check whether the correct NPN/PNP type is selected and verify compatibility with the PLC input.

Q3: Can NPN and PNP be used simultaneously?
No. Only one output type should be used; connecting both may damage the PLC input circuit.

Q4: Why is the output logic opposite of what I expected?
The safety curtain may use default NC logic. Refer to the product manual to adjust configuration if needed.

Safety Light Curtain Application Examples

Safety Light Curtain - Press Machine Guarding
Press Machine Guarding
Automatically stops the machine when an operator’s hand enters the hazardous zone.
Safety Light Curtain for Bending Machine Protection
Bending Machine Protection
Installed near the lower die to prevent accidental contact.
Safety Light Curtain for Automated Production Line
Automated Production Line
Protects entry points of packaging or inspection lines to prevent unauthorized access.
Safety Light Curtain for Robotic Work Cell
Robotic Work Cell
Installed around robot areas to replace traditional fences and enable flexible human-robot collaboration.

Related Safety Devices

QCE42-30-1230 2BB|Safety Light Curtain|DADISICK
Beam spacing: 30mm Number of optical axes: 42 Protection height: 1230mm Safety light curtain outputs (OSSD)2 PNP
QSA series|Multi-sided access protection Light Curtain|DADISICK
Beam spacing: 80mm Number of optical axes: 10 Protection height: 720mm Safety Curtain outputs (OSSD):2 PNP
Safety Relay LS-A Series
Economical safety relay, dual-channel safety monitoring circuit design, suitable for high-demand fields such as mechanical protection, automated production lines and robot systems.
Safety Relay Ter-A Series
Multifunctional safety relay, providing automatic/manual reset configuration and multifunctional configuration DIP switch, used for industrial field monitoring of various signals with high safety requirements.
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