The modern world is filled with gadgets that activate in response to human motion. Automatic doors in elevators and shopping malls, burglar alarms in homes and shops, automatic lighting systems, and electronic amenities in washrooms are just a few examples of devices that switch between active and passive states based on human presence. Smart, right?
Now, what if we told you that behind this intelligent motion response is a tiny component measuring less than 2 cm? This small electronic device, known as a Pyroelectric or Passive Infrared sensor (PIR), is the focus of this article (Figure 1).
Every object with a temperature above absolute zero emits thermal energy through infrared radiation. Humans, for instance, radiate at wavelengths of nine to ten micrometers throughout the day. PIR sensors are specifically tuned to detect this IR wavelength, responding only when a human enters their range. The term “pyroelectricity” refers to generating electricity from heat, which, in this case, results in a small electric signal. Since PIR sensors do not generate infrared radiation themselves, they are classified as passive.
But how does a PIR sensor selectively detect human-emitted infrared radiation? What is its effective range? What internal components enable it to function? This Insight explores these questions and more, featuring the Panasonic 10m sensor — one of the smallest commercially available PIR sensors to date.
Figure 1. A Panasonic 10-mm IR sensor.
The Fresnel lens array
Figure 2. The sensor’s outer body.
Figure 2 shows a Panasonic 10m sensor, a PIR sensor unit enclosed in a translucent plastic chamber with a beehive-like cap. The design of the cap helps diffuse and direct infrared radiation toward the sensor. At the bottom of the chamber, an opening allows the sensor’s solder dip legs to protrude, enabling easy integration into circuits.
Figure 3. The beehive structure and curved segments are found on top of the sensor.
By closely examining the top region of the sensor, the beehive structure and curved segments become visible, as shown in Figure 3. These curved segments form a Fresnel lens array, which enhances the sensor’s detection zone by capturing more infrared radiation and focusing it onto a smaller point. This design improves detection stability and increases the maximum detection distance.
The Fresnel lens is crafted to be translucent (Figure 4), allowing it to capture only infrared radiation while filtering out unwanted visible light. The number of Fresnel lenses in the array can vary, and this particular sensor contains 20 lenses.
The inside plastic moldings
Figure 4. The internal design off the PIR sensor.
Figure 5. The overall shape and structure of the PIR sensor.
The Fresnel array-loaded cap is securely positioned over the base region of the plastic molding. However, no latch mechanism is holding it in place, making it an interesting challenge to remove the Fresnel lens array from the plastic assembly.
Beneath the Fresnel lens array, the PIR sensor is firmly embedded within the plastic moldings (Figure 5). Its placement is crucial, as it must receive the maximum amount of infrared radiation focused by the lens array. The sensor, positioned at the center of the moldings, is strategically placed where the converged infrared radiation is most concentrated.
The IR filter and TO5 metal can
Figure 6. The infrared filter of the sensor.
At the top of the sensor is the infrared filter, which appears as a square-shaped glass (Figure 6). This filter selectively allows only the desired wavelength to pass through, ensuring that the sensor responds accurately. Since this sensor is designed to detect human presence, it is tuned to a wavelength range of 8 to 14 micrometers — the range in which the human body emits infrared radiation (Figure 7).
Figure 7. The design specifications of the sensor.
The body of the sensor is a TO5 metal can structure, an industry-standard packaging format commonly used for small electronic modules such as transistors and sensors. The TO5 metal casing shields the internal circuitry from external factors like vibrations and electrical noise, ensuring stable and reliable operation.
The sensing element and the chip
Figure 8. The PIR sensor’s PCB.
By uncapping the sensor, you’ll find a small PCB (Figure 8) that houses the sensing module, amplifier, and comparator circuit. Figure 9 shows the top part of the PCB, where the sensing element is positioned. The sensing elements are typically made from ferroelectric ceramic (which contains lead) or lithium tantalite (a lead-free alternative).
Figure 9. The PCB’s amplifier and comparator.
Multiple sensing elements are used to enhance signal receptivity. This sensor is a quad type featuring four sensing elements arranged in an array. After passing through the infrared filter, the IR rays strike the sensing elements, generating an electrical charge. The magnitude of the charge is directly proportional to the amount of infrared radiation received by the element.
Once the charge is generated, it is transmitted to the amplifier circuitry before being processed by the comparator. Unlike earlier designs, the amplifier and comparator are embedded directly within the PCB, where these components are part of the external circuitry. Integrating them into the sensor module makes the circuit more compact and improves accuracy and precision, as the pre-defined settings are optimized for the IR radiation spectrum.
Comparators are specifically used in sensor modules that provide a digital output, as seen in this design.
The FET and the leads
Figure 10. The base plate and its components.
Figure 11: The processor unit and the embedded resistors.
The base plate and the remaining structure of the sensor house a Field Effect Transistor (FET). Although the charge produced by the sensing elements is amplified, it still generates voltages in the range of just 1mV. FETs are well-suited for operating at such low voltages and efficiently transmit the signal to the processor unit to which the sensor is connected.
Figure 10 provides a view of the base plate, while Figure 11 highlights the processor unit along with a few resistors mounted on it.
Figure 12. The multi-functional connecting leads of the PIR sensor.
At the bottom of the sensor are the connecting leads, which serve multiple functions. They not only allow the sensor to be securely soldered onto a circuit board but also handle the small power requirements needed for operation and transmit the output signal to the processing unit.
Figure 12 provides a detailed view of the lead placement, arranged similarly to a Junction Field Effect Transistor (JFET). The specific function of each lead is labeled in the image.
Filed Under: Insight
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