motion detector

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A motion detector (also: "BWM") is an electronic sensor that detects movements in its immediate vicinity and can therefore work as an electrical switch.

A motion detector can work actively with: electromagnetic waves (HF, microwaves or Doppler radar ), with ultrasound ( ultrasonic motion detectors ) or, like the PIR sensor , passively based on the infrared radiation of the moving person and the environment.

The pyroelectric sensor (PIR sensor, P yroelectric I nfra r ed sensor) is the most commonly used type of motion detector. It reacts to small changes in temperature, for example when a person walks past the sensor.

Motion detectors using microwaves react optimally when the distance to the sensor changes.

Ultrasonic sensors are used less often due to the relatively complex technology.

Motion detector (PIR sensor) with a cylindrical Fresnel lens


Depending on the application, different types of sensors or combinations of several sensors are used.

light switch

Circuit board of a PIR movement module for a lamp control

An infrared-based motion detector based on a PIR sensor for motion detection usually also has an additional built-in twilight switch, which ensures that the actual motion detector only switches on the lighting when it is dark. If a heat source moves in front of the detector, it switches the lighting on for an adjustable period of time and off again after the set lighting time has elapsed. Most motion detectors have two, some even three, adjusters ( potentiometers )

  1. for the switch-on duration: defines how long the consumer should remain switched on when no more movement is detected
  2. for the ambient brightness (light-dark boundary): defines the darkness threshold from which the detector should switch
  3. for the proximity sensitivity (detection area): defines the distance from the detected object from which the sensor should trigger.

With most motion detectors, a timer for the switch-on time begins when the last motion is detected. If the detector then detects a new movement, it resets the counter to zero . This ensures that the consumer (e.g. lighting) is not switched off if a person stays in the detection area for a long time without moving

There are two pole and three pole switch types.

With two- pole motion detectors, their operating current flows through the lamp. This means that you do not need your own neutral conductor, as you get this via the downstream incandescent lamp. However, this only works reliably as long as this light source is an incandescent lamp. Two-pole motion detectors are therefore only suitable for ohmic loads (incandescent lamps) and do not necessarily work with energy-saving lamps.

Connection example

Three- pole motion detectors have their own neutral conductor connection and thus receive the power for their operation via two separate connections and therefore independent of the light source.

Presence detector

A presence detector is a sensor that is intended to detect the presence of people during sedentary and other quiet activities. For this purpose, higher quality detectors are used that react to even the smallest movements. These can be both HF presence detectors, which constantly emit low-power high-frequency waves (see above), and PIR detectors. Like PIR detectors, HF detectors also have setting options for switching duration and detection range, depending on the intended use.

Just like PIR detectors, HF detectors usually have an additional built-in twilight switch that ensures that the lighting is only switched on when it is dark. Presence detectors also usually have a second switching channel that only reacts to movements, for example to control ventilation and heating independently of the brightness.

Presence detectors are ideally mounted on the ceiling in order to enlarge the detection area and ensure that people are in the more sensitive so-called inner detection area. In larger rooms, they can be networked in groups to prevent the lighting from being switched off locally if there are temporarily no people in a part of the room.

Power consumption

The effective power consumption is in commercially available motion detectors typically less than 1  W , but falls through the capacitive power supply a reactive power to dealing with inductive loads such as transformers or motors compensate leaves.

Intrusion alarm systems

The motion detectors used for intruder alarm systems differ from the light motion detectors. In most cases, several pulses from the PIR sensor, multi-segment sensors (double or quad PIR sensors) or combinations of different sensors are used to avoid false tripping.

If the intrusion alarm system is switched on (armed), a movement in the detection area of ​​the detector triggers an alarm. The use of light / motion detectors to trigger intrusion alarm systems does not make sense because there is no sabotage protection and many motion detectors would switch on the connected consumer after failure and recovery of the supply voltage and in this case trigger a false alarm .


Minimum required connections
  • Sabotage contact (NC contact): opening of the housing is reported to the intrusion alarm center
  • Alarm contact (NC contact): Triggering of the infrared sensor is reported to the burglar alarm center
  • Supply voltage: mostly 12  V direct voltage via the control center with emergency power supply from a lead-acid battery , more rarely also 24 V
Additional connections
  • Walk test function: Input via which the burglar alarm center can switch on the LED to indicate motion detection in walk test mode
  • Alarm memory : If several motion detectors are switched in a zone, the motion detector that triggered the first is saved

Duo PIR detector

Some models have two separate PIR sensors. An alarm is only triggered if both react at the same time. This will reduce the risk of false alarms.

Dual detector

Some models combine a PIR sensor with a microwave sensor (radar motion detector) or an ultrasonic motion detector. These combinations are usually referred to as dual detectors .

Detector with anti-masking

For use in areas with high security requirements (according to EN50131-2-4, grade 3), the motion detectors are equipped with additional detection of a cover (e.g. by masking off, spraying with hair or color spray). The detection of the cover is reported to the alarm system through an additional output.

Detectors with animal immunity

Animal-resistant motion detectors are only available for burglar alarm systems. The triggering behavior of these motion detectors is more sluggish than normal motion detectors and the lower detection area is somewhat less sensitive. The installation guidelines for animal-resistant motion detectors must be followed precisely in order to avoid false alarms on the one hand and to ensure detection sensitivity on the other. At the moment [as of?] No animal-resistant motion detector has VdS approval.

Video surveillance

Outside, special motion detectors are used to switch on the recording in video surveillance systems. Motion detectors have clear advantages here compared to classic camera motion detection through pixel analysis. They also work in the dark and are not triggered by changing light conditions (e.g. vehicle headlights, fast-moving clouds). In this way, the existing recording capacity is used more effectively and a search for possible incidents is made easier.

Traffic lights

At traffic signals and on motorways PIR sensors detect the volume of traffic for control of the traffic signal, for triggering a traffic jam warning or issue a directive speed. These sensors react to the thermal radiation that is emitted through the body by the heat from the vehicles' engines. PIR sensors are an alternative to induction loops and, unlike those, do not require any intervention in the road surface - they are often attached to traffic light masts or bridges. Construction site traffic lights often also have PIR sensors, so if there is no traffic from one side, the oncoming traffic can get green for longer.


  • Adam Merschbacher: Security analysis for households. VdS Schadenverhütung Verlag, Cologne 2002, ISBN 3-936050-03-1
  • Adam Merschbacher: Security Analysis for Businesses. VdS Schadenverhütung Verlag, Cologne 2003, ISBN 3-936050-04-X
  • Hanno Schaumburg: sensors . Teubner, Stuttgart 1992, ISBN 3-519-06125-2 , ( materials and components of electrical engineering 3).
  • Sven Bär, Jürgen Kitz, Raymond Kleger, Christian Kluge: Energy efficiency through presence detectors and motion detectors Rommert Verlag, Gummersbach 2012, ISBN 978-3-941276-04-8

Web links

Commons : motion detectors  - collection of pictures, videos and audio files

Individual evidence

  1. Detectors with pyroelectric sensors were available to private users by the early 1980s at the latest. A "three-dimensional infrared passive motion detector" for use in connection with an alarm system cost 290 DM in 1983  (source: Conrad Electronic (publisher): Conrad Electronic catalog E 84. 1983, p. 393 (infrared room sensor PA-3010). ) , which corresponds to EUR 276 in today's purchasing power .
  2. Dual motion detector security wiki
  3. DIN EN 50131-2-4: 2008-10 Alarm systems - Intrusion and hold-up alarm systems - Part 2-4: Requirements for passive infrared dual alarms and microwave alarms; German version EN 50131-2-4: 2008
  4. How does the integrated motion detection work?