A pyroelectric infrared sensor, also known as a PIR sensor, is a semiconductor sensor used to detect temperature changes. PIR sensors are based on the eponymous pyroelectricity, a property of some piezoelectric semiconductor crystals. A change in temperature ΔT leads to a measurable change in the electrical voltage. PIR sensors, unlike other temperature sensors, do not react to a certain temperature level that is constant over time, but only to the change in temperature.
PIR sensors are used, among other things, in motion detectors to detect the heat radiation emitted, for example by living beings such as humans, at a distance of a few meters. This can be used to trigger various actions, such as activating lighting or triggering an alarm message. Further applications are NDIR-gas analysis, as a sensor in IR flame detectors and in low-temperature pyrometers.
A motion detector is an electronic sensor that detects movement in its immediate vicinity and can therefore function as an electrical switch. bA motion detector can work actively with: electromagnetic waves (RF, 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.
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The pyroelectric sensor (PIR) is the most commonly used type of motion detector. It reacts to small changes in temperature, such as 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 frequently due to the relatively complex technology.
Construction of a PIR Sensor
The pyroelectric material is a thin (< 40 μm) polarized crystal. In order to make use of the pyroelectric effect, electrodes must be applied on the opposite sides. When radiation hits the pyroelectric material, it is absorbed, the resulting temperature difference ΔT causes a change in the polarization of the crystal: the change in thermal radiation causes a change in the electric potential. With the help of the applied electrodes, the electrical signal can be measured via an amplifier with a high-impedance input – such as a charge amplifier. Commercially available PIR sensors for motion detectors consist of two adjacent sensor elements with dimensions ranging from 1 mm to 2 mm. The absorption coating used has the highest sensitivity in the mid-infrared at wavelengths in the range of 5 μm to 14 μm. The sensors are typically housed in a hermetically sealed transistor package, such as the TO-5 design, along with a preamplifier stage in the form of a junction field-effect transistor (JFET).
Above the active sensing element, a simple optics is placed to improve the detection of changes and the range. Typical is a spherical or cylindrically curved Fresnel lens made of plastic. On the one hand, the design of this lens can influence the spatial area of effect of the sensor and adapt it to the respective application, and on the other hand, the steps of the Fresnel lens in combination with the two sensor elements lead to a spatial fanning out of the sensitivity ranges, which improves the detection of spatial changes in heat sources due to steeper transitions. The Fresnel lens can be divided into individual sector elements and into several partial lenses for fanning out.
Due to the fanning out, when an object that is warm compared to the background passes through the sensor’s detection area, there is an amplified temperature change on the sensor, since the warm object is initially detected by only one sensor element through the lenses. With further movement, the temperature change detects the second sensing element, while at the same time the first sensing element loses the object in the sensing. The two sensor elements are internally connected in such a way that one sensor element generates a positive voltage pulse, the other sensor element a negative voltage pulse. As a result, the signal change during movements due to the formation of differences is roughly doubled due to the fanning out.
Electrically, PIR sensors in motion detectors have three connections, which, like a junction field-effect transistor, are referred to as a gate circuit, as a source, with the function of the output, as a drain, for the positive supply connection and gate as a ground connection.
In the case of PIR sensors in flame detectors, the flame also modulates the radiation flux with a typical flickering frequency between 1 Hz and 5 Hz, as is the case in motion detectors. In NDIR gas analysis, modulation takes place either electrically by switching the radiation source on and off or mechanically. The latter can be realized by means of a folding mirror, a folding panel or a chopper. Mechanical modulation is also used in pyrometry.
Since pyroelectric sensors are used to detect temperature changes, these sensors are virtually wavelength-independent and can detect radiation ranging from low ultraviolet radiation (wavelength around 100 nm) to the visible and infrared wavelength range to terahertz radiation with a wavelength of 1 mm. The prerequisite for this is that the pyroelectric crystal has a suitable absorption coating. If materials with wavelength-dependent absorption coefficient are chosen for the electrodes, a certain wavelength selectivity is achieved.
