Motion detectors - PIR & HF explained
The most important technologies of motion sensors explained.

Do you want the light to automatically turn on when somebody approaches the front door or should the light turn on, when somebody is in the staircase? If that’s something you’ve been thinking about you need to install a motion sensor. By that time you’ll come across “PIR” or “HF” sensor technology.


A motion sensor’s job is to detect movement in its monitored area with a sensor. So such a device is literally predestined to turn on light. There are many types of motion sensors: a motion sensor can actively work with electromagnetic waves (e.g. HF or doppler radar), with ultrasonic (ultrasonic motion sensor) or passive with the infrared radiation of the environment (PIR, IR). There are also combinations of these. Many also feature a proximity switch and or even a built-in twilight switch. This turns the light on in response to the ambient light. After a specific timeframe (mostly adjustable) the light switches off again.

Differences between PIR and HF sensor technology

A motion sensor with PIR technology and a passive infrared sensor detects the invisible heat radiation of moving bodies. One of the advantages of RF against PIR technology is the precise detection at high temperatures and the detection through permeable materials. HF sensors work the best when people move head-on to the device. PIR detectors on the other hand function best when people move across the detector.

PIR sensor

The PIR Sensor (English: passive infrared) reacts to changing thermal radiation detected in the infrared range. People (and animals) send out body warmth in this waveband. This radiation is detected by the sensor. The detection angle of the PIR sensor is usually 120°. On a change in the sunlight of the PIR sensor does not react, only to objects that move past him. The sensor itself emits no energy, as is the case with active infrared sensors. Therefore, it’s also referred to as passive infrared sensor.


PIR-Sensor Bewegungsmelder

Under a dome of converging lenses, there is the sensor surface. The milky plastic dome is transparent to infrared light. Inside the sensor is a grid which serves to interrupt the permanent radiation, so that the sensor can distinguish whether light is required which means the light stays on until the object gets out of the detection angle or no heat signal gets returned.

Usually these motion sensors have built-in twilight switches, so that the light only comes on at twilight / darkness. Mostly 2 parameters can be set: 1. dury cicle: this means how long the light should stay on and 2. the ambient brightness: this means you can set at which darkness threshold the detector act.

High frequency sensor (HF)

The HF method proceeds in accordance with the principle of radar measurement. The HF sensor transmits electromagnetic waves with a frequency of 5.8 GHz. When the signal hits a stationary body – e.g. a piece of furniture – it is reflected at the same frequency. Although if a man crosses the detection area, it increases or decreases the frequency of the reflected signal. This change translates as a movement and the sensor switches the light on.

High frequency motion sensor

This means heat plays no role with this kind of sensor. Rays are emitted, reflected and report any changes in the distance. The detection can be horizontal, vertical, and also by thin layer made of glass. The fact that the sensitivity is very high the RF sensor is more suitable for indoor use.

These days there are also manufactures that use software-filtering to exclude bushes, trees, cats and such which makes the RF sensor smart and therefore also suitable for outdoor use.


The power consumption of a motion sensor

Each motion sensor has a different power consumption. The electrician or light planner needs the technical data of the respective device in order to determine the exact consumption. For the switching process (e.g. the command “light on” and the monitoring of the cover radius) only minimal energy is required. The required power consumption is in the lowest area. With a conventional energy meter the power consumption of a motion detector can not be measured.

For example, a common Steinel sensor has a stand-by consumption of approximately 0.8W. With this electricity consumption one would have annual electricity costs of about 2.- Euro depending on the provider. However it always depends on the individual device and of course, also on how often the motion detector is active. In the case of variants with a twilight sensor one can clearly delineate the action period. The individual who no longer wishes any activity could additionally add a time switch to the circuit which ensures no consumption in a certain time window, e.g. 1-5 o’clock in the morning. High-quality motion sensors for outdoor areas often have filters to exclude movements of bushes and animals or have the function to adjust the sensitivity on the device itself.


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