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Sensor KnowHow

for a safe start and to clarify uncertainties

On this page you will find the basics about the sensors. This information will help you to make the right decisions. The information is kept simple and is clarified with drawings.
Of course, our technicians are always available for further questions or explanations via LiveCall, chat, email or on the phone.

The difference between PNP and NPN

The designation PNP and NPN at the sensors has nothing to do with the function (e.g. normally closed or normally open) of the sensor! It is about the wiring of the output transistor. A PNP sensor can have the function NO (normally open) or NC (normally closed).
PNP Ausgang
NPN Ausgang
PNP switching output
NPN switching output
The load at the switching output is connected to GND as reference point. When a signal change occurs due to the attenuation of the sensor, the supply voltage (U+) is "switched through". The current can flow from U+ through the transistor and through the load to GND.

The PNP switching output is preferred in Europe.
The load at the switching output is connected against U+ as reference point. When a signal change occurs due to the attenuation of the sensor, the ground (GND) is "switched on". The current can flow from U+ through the load via the transistor to GND.

The NPN switching output is preferably used in Asia and North America.

Normally open, normally closed and changeover contacts
The switching function of a sensor is defined by the terms normally open (NO), normally closed (NC) and changeover contact (NO/NC).

Ausgang Schliesser

Ausgang Öffner
Ausgang Wechsler

Normally open (NO)

Normally closed (NC)
Changeover (NO/NC)
A normally open contact establishes the electrical connection when the sensor is damped. In the idle state, the connection is open and no current flows.

NO = normally open

A normally closed contact disconnects an electrical connection when the sensor is damped and is closed in the idle state.

NC = normally closed
With a changeover contact, the connection is switched to another contact when the sensor is damped. When the damping is removed, the connection switches back to the normally closed contact.
Sensor functions
Not all sensors are the same. Each application requires the right type of sensor. The different designs offer different advantages and disadvantages. Here we explain the function of the individual sensor types and their advantages.
Triangulation, background suppressor
Diffuse reflecting objects are detected within the set switching range. Backgrounds or objects outside the switching range are electronically blanked.
Funktion Hintergrundausblender
Advantages of background suppressors

  • Detects diffuse surfaces within the sensing range
  • Minimal influence due to color differences
  • Objects outside setting range are not detected

Triangulation, foreground suppressor
The sensor is set to a reference surface. The switching window lies closely on the reference surface. The object blocks this optical axis, the sensor switches.
Funktion Vordergrundausblender
Advantages of Foreground Suppressors
  • Defined reference distance
  • Surface condition has no influence
  • Object non-uniformity has no influence
  • No close range
  • Detects also reflective surfaces

Convergent sensor
Convergent sensors detect objects only in a certain detection range (area).
Konvergenttaster Funktion
Advantages of Convergent sensor

  • Short distance sensor
  • Defined detection range
  • Surface condition has low influence
  • Mostly very small designs

Light barriers
Light barriers always consist of a transmitter and a receiver and can reliably bridge large distances. The well-focused light beam can penetrate dust and fog. The light beam must be clearly interrupted to obtain a reliable switching situation.
Funktion Lichtschranke
Advantages of Light barriers
  • Long distances
  • Pollution penetration
  • No close range

Retro-reflective sensors
Retro-reflective sensors are aligned and adjusted to a defined reflector. Such sensors are also well suited to detect transparent objects.
Funktion Reflexlichtschranke
Advantages of Retro-reflective sensors
  • Only one-sided wiring
  • Detects transparent objects
  • Defined switching zone
  • No close range

he optical diffuse sensor evaluates the surface reflection from the object. Depending on the reflectance, the switching distance is larger or smaller. Usually these sensors work with infrared light, laser or with visible red light.
Diffustaster Funktion
Advantages of Diffuse sensors
  • Detects most surfaces
  • Transmitter/receiver in one device
  • No close range

Coaxial sensor

Diffuse sensors, background suppressors or retro-reflective sensors can be influenced by other nearby design elements. This is mainly due to 2 reasons:

  • Due to the large aperture angle of the transmitter, light is reflected by other constructive parts. The receiver recognizes the light signal and switches!

  • Transmitter and receiver elements are placed constructively one above the other in the sensor. This has the consequence that always the transmitter or the receiver is in the shade.

In comparison, the coaxial LASER sensor has the transmitter and the receiver on the same axis. This means that the bundled and reflected light can reach the receiver on the same axis.

Optischer Koaxialtaster Prinzip
Advantages of Coaxial sensor
  • Transmitter and receiver on one axis
  • Can detect through holes and gaps
  • Very small close-range
  • Small light spot



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