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What's PIR sensor


Passive InfraRed – that is all we need to know. A PIR sensor passively detects infrared

radiation in its surrounding area. Since this technology is also used in fire detectors, sensors

are also called pyrosensors. They are called passive because they only analyse signals which

are transmitted from the object itself. In contrast to radar or ultrasound sensors an active

transmitter is not needed. Infrared is the wavelength beyond visible red light. Here

wavelengths around a few μm are analysed – the specific heat radiation which is emitted by

e.g. the human body. It is obvious that a PIR motion detector only reacts to objects that are

distinctly warmer than the surroundings, for example humans, animals or warm vehicles. It is

important to know that the sensor does not sense absolute heat radiation but only temporal and

local changes in radiation. That is required to avoid steady and immobile heat radiators to set

off the switching operation.

The sensor principle is based on infrared radiation causing a charge transfer on a special

membrane which resembles a capacitor. In order to perceive only changes in infrared

radiation (i.e. movements of an object) the dynamic states of charge of at least two equal

sensor elements are analysed, while “common mode signals” are compensated. This

compensation takes place by either series or parallel connection of these single elements while

the pyroelectric sensor membranes are polarized opposing each other. The advantage of this

system lies in the oppression of surrounding temperature influences which change temperature of both sensitive elements by convection, thermal conduction and occurring heat

radiation. A specific amplifying circuit with additional elaborate electrical filters is attached

for analysing very weak signals. Additional optical filters insure that only radiation required

for detecting movements is let through. Interfering radiation sources are also shielded


Usually specific lens systems (multi or Fresnell lenses), which focus optical irradiations from

different directions to the sensor, are added to complete the movement detector. This widens

the sensor detection range (horizontal and vertical angle) immensely. Such a wide detection

range, however, is often not desired. So in order to use the system for experimental purposes

removable or rather exchangeable lens systems are preferable.