LN-21R-02 Magnetic Switch LN21R02 Brand New Sealed
UNIQUC LN-21R-02 | Cylinder Magnetic Reed Switch Sensor
2-Wire / 5-240V AC/DC / IP67
Position Detection on the Cylinder Body
The LN-21R-02 is a magnetic reed switch sensor designed to detect the position of a pneumatic cylinder piston by sensing the permanent magnet embedded in the piston. When the piston reaches the mounted sensor's position along the cylinder body, the magnet's field closes the reed switch contact and the sensor's LED illuminates to signal the piston is at that position.
The reed switch operates on the principle of a sealed glass envelope containing two ferromagnetic contacts. The piston magnet's field causes the contacts to attract and close when the piston is within range. When the piston moves away, the field weakens and the contacts spring back open. This mechanism requires no external power supply for the sensing element — the reed switch itself is a passive contact that simply opens and closes. The 2-wire wiring connects in series with the load circuit, keeping installation straightforward.
The 5-240V AC/DC operating range is the main practical advantage of the reed switch type over transistor-output (NPN/PNP) variants. A single LN-21R-02 works directly in 24V DC PLC input circuits, older 110V AC control circuits, or 240V AC systems without signal converters or interface relays. This range covers the control voltage standards found in a wide mix of industrial machine generations.
LN-21 Series — Type Variants
Within the LN-21 sensor family, the type suffix identifies the sensing technology and output type:
| Model |
Sensor Type |
Wiring |
| LN-21R |
Reed switch |
2-wire |
| LN-21N |
NPN transistor |
3-wire |
| LN-21P |
PNP transistor |
3-wire |
The Reed switch R-type operates across the full AC/DC voltage range. The NPN and PNP transistor types are limited to 5-30V DC but offer higher switching frequency and no contact wear. For most single-cycle end-of-stroke position detection applications, the reed switch type is adequate. For high-cycle applications or where switching frequency exceeds 200 Hz, the NPN or PNP transistor variants are preferred.
Key Specifications
| Parameter |
Value |
| Part Number |
LN-21R-02 |
| Sensor Type |
Reed switch (2-wire) |
| Operating Voltage |
5-240V AC/DC |
| Max. Switching Current |
100 mA |
| Voltage Drop |
3.5V max |
| Indicator |
LED (red) |
| Max. Frequency |
200 Hz |
| Protection |
IP67 |
| Temperature |
−10°C to +70°C |
| Cable |
Ø4.0mm, 2-core PVC |
FAQ
Q1: The PLC input shows the cylinder sensor always ON, even when the cylinder is fully retracted. What should be checked?
Confirm the sensor is mounted at the correct position along the cylinder body — if mounted too far from the piston's retracted position, the piston magnet may still be within the sensor's detection range at full retract. Also check that a strong external magnetic field from adjacent wiring or equipment is not holding the reed contacts closed. If both checks are clear, the reed contact has welded shut and the sensor must be replaced.
Q2: The 2-wire LN-21R-02 shows 3.5V voltage drop. Does this affect PLC input reading?
The 3.5V maximum drop applies when the reed contact is closed and current flows through the sensor. For a 24V DC PLC input, the output terminal delivers approximately 20-21V when active — well above the minimum ON-state voltage for any standard industrial PLC input card. Voltage drop is only relevant when the supply voltage is at the low end of the operating range (5V DC); at 24V DC it has no practical effect.
Q3: Can the LN-21R-02 be used on hydraulic cylinders as well as pneumatic cylinders?
Yes, provided the hydraulic cylinder's piston includes a permanent magnet. The LN-21R-02 detects the magnetic field from the piston magnet regardless of the fluid medium. Hydraulic cylinders designed for magnetic sensor use carry embedded ring magnets in the piston. Verify the cylinder's documentation confirms magnetic sensor compatibility before mounting.
Q4: What is the maximum switching frequency of the LN-21R-02 and when does it become a limitation?
The LN-21R-02 reed switch type has a maximum switching frequency of 200 Hz. At 200 Hz, each sensing event occupies 5 milliseconds — sufficient for the vast majority of pneumatic cylinder position detection where cycle times are in the range of 0.5-5 seconds. For very fast cylinders cycling more than 200 times per second, the NPN or PNP transistor-output LN-21N or LN-21P variants with 1000 Hz maximum frequency are appropriate.
Q5: The sensor's LED illuminates but the connected PLC input does not respond. What is the likely cause?
The LED illuminates from the sensor's internal current — its indication reflects the reed contact state. If the LED is on but the PLC input is off, check the circuit wiring polarity and confirm the minimum load current requirement is met. Reed switch sensors in 2-wire configuration require a minimum current flow through the load (the PLC input) to operate correctly. If the PLC input's impedance is too high (input current below the sensor's minimum switching current), the reed contact may close without sufficient current to register at the PLC input. Verify input current against the sensor's specification.
