What are the different types of outputs for sensors?

All sensors have one or more (types of) outputs. An output generally sends the currently known measured value of the sensor. What that signal looks like depends on the output type. What is the difference between all these outputs and when is which output type used? This article explains the most common forms.

Sensors can have different types of outputs. The possible outputs of a sensor determine in which it can be used. Conversely, this also applies: the type of application determines which sensor you need.

In this article we discuss the different possible outputs. how these work and for which use cases these can be applied.

Digital vs. Analog

First, we make a distinction between two types of outputs: an analog and a digital output. A sensor with a digital output signals a logical value. In other words: Yes or No, 0 or 1, True or False, Valid or Invalid . A digital output is very well-suited to indicate the presence of an object (at a certain distance) or detecting whether a set limit value has been reached. Does the sensor "see" the object or not? Is the value reached or not? During a detection or non-detection the logical value changes from a 0 to a 1, or vice versa! Examples of digital (switching) outputs are PNP/NPN, relay, solid state relay and PushPull.

A sensor with an analog output is capable of giving a signal that is continuously partallel to the measured value. An analog signal is a signal that can register values without intervals. Think of a constantly fluctuating temperature in an outdoor location, such as the conveyor belts in the production of steel beams: the analog output changes parallel en mostly linear with the change of the measurement of the sensor. Another example is the change of a distance from 0 to 1.000 cm or a temperature drops from 200°C to 20°C. Examples of analog outputs are 0-10 Vdc, 4-20 mA, 0-5 Vdc or 0-20 mA.

Types of digital outputs: PNP or NPN

Sensors with a PNP or NPN switching contact use a transistor output. The type of transistor output determines whether the sensor switches PNP or NPN. Sensors with a PNP or NPN switching output feature at least three wires; A " + " (Pin 1 / brown wire), a " – " (Pin 3 / blue wire) and a switching wire (Pin 4 / black wire).

PNP switching output

NPN switching output

PushPull switching output

A PushPull output means that the switching component of a sensor consists of two transistors. This is a type of output in which it is possible to alternately switch PNP as well as NPN. The circuit is designed in such a way that any voltage between a certain limit will make the sensor switch NPN, while a lower voltage provides a PNP output. Sensors with a PushPull output are versatile to use in applications that require a PNP and NPN output. The advantage is also that there is no need for developing the same sensor but with an NPN or PNP output.

Solid State Relay (SSR) output

A solid state relay (SSR) is also known as an optocoupler relay or semiconductor relay. It is a type of relay without a mechanical switch, contrary to a more conventional relay. Conventional mechanical relays have the advantage of being able to switch higher power rates, but because of moving parts are susceptible to wear and tear. A solid state relay switches by use of a light-sensitive diode and is, because of this, free from wear and tear. In addition it is also capable of higher switching frequencies.

Normally Open, Normally Closed or Antivalent output

All sensors with a digital switching contact, whether it is a PNP/NPN, solid state or PushPull contact have the ability to switch Normally Open (NO) or Normally Closed (NC). In some cases the sensors only switch NO or NC. If a sensor is capable of switching NO as well as NC it is called antivalent.