Basic Knowledge of Infrared Pyroelectric Sensors and Common Issues in Their Use

　　A pyroelectric infrared sensor is a type of sensor that detects infrared radiation emitted by humans or animals and converts it into an electrical signal. As early as 1938, some researchers proposed using the pyroelectric effect to detect infrared radiation; however, this idea did not receive much attention at the time. It wasn't until the 1960s, with the rapid advancement of laser and infrared technologies, that research on the pyroelectric effect and the application of pyroelectric crystals were once again spurred forward. Today, pyroelectric crystals are widely used in infrared spectrometers, infrared remote sensing systems, and thermal radiation detectors, making them an ideal detector for infrared lasers. Their applications are now expanding rapidly into various automated control devices. Beyond their well-known uses in automatic stairwell switches and burglar alarms, these sensors hold great promise for even more diverse fields. For example, air conditioners and water dispensers that automatically shut down when no one is present in the room; televisions equipped with mechanisms that can detect when no one is watching or when viewers have fallen asleep and then automatically turn off; applications in monitor screens or automatic doorbells; and integration with cameras or digital photo cameras to automatically record the movements of animals or people—just to name a few. Based on your own creative ideas, you can combine these sensors with other circuits to develop even more advanced and innovative products or automated control systems.

　　Basic Knowledge of Pyroelectric Sensors

　　The pyroelectric effect is similar to the piezoelectric effect; it refers to the phenomenon in which a crystal surface becomes charged due to changes in temperature. A pyroelectric sensor is a temperature-sensitive sensor. It consists of ceramic oxide or piezoelectric crystal elements, with electrodes fabricated on both surfaces of the element. When the temperature within the sensor's detection range changes by ΔT, the pyroelectric effect induces a charge ΔQ on the two electrodes, resulting in a weak voltage ΔV between them. Because the sensor has a relatively high output impedance, a field-effect transistor is used inside the sensor to perform impedance transformation. The charge ΔQ generated by the pyroelectric effect will be neutralized and disappear as it combines with ions in the air; thus, when the ambient temperature remains stable and ΔT equals zero, the sensor produces no output. However, when a human body enters the detection zone, a temperature difference ΔT arises between the body’s temperature and the ambient temperature, triggering an output signal ΔT. If the human body remains stationary after entering the detection zone, the temperature no longer changes, and the sensor ceases to produce any output. Therefore, this type of sensor is designed to detect the movement of humans or animals. Experimental evidence shows that without an optical lens (also known as a Fresnel lens), the sensor’s detection range is less than 2 meters; but when an optical lens is added, the detection range can exceed 7 meters.

　　The following points should be noted during use:

　　The DC operating voltage must meet our specified requirements. Both excessively high and excessively low voltages can adversely affect the module’s performance. Moreover, the power supply must feature excellent voltage regulation and filtering. For instance, computer USB power supplies, mobile phone charger power supplies, and even older 9V stacked batteries cannot meet the module’s operational requirements. We recommend that customers use a transformer-based power supply, which should be regulated by a three-terminal voltage regulator chip and then filtered through capacitors of 220 μF and 0.1 μF before being fed to the module.

　　During debugging, keep your body as far away from the sensing area as possible. Even if your body isn't directly in front of the module, the module can still detect it if you're too close, resulting in continuous output. Additionally, during debugging, avoid touching the circuitry—doing so can also interfere with the module's operation. A more scientific approach is to connect an LED or a multimeter to the output terminal, cover the module with newspaper, and then leave the room. After waiting for about 2 minutes, check whether the module continues to produce output.

　　When the module is not connected to any load, it functions normally. However, once a load is connected, its operation becomes erratic. One possible reason is that the power supply has a very limited capacity, while the load consumes relatively high power. The voltage fluctuations caused by the load’s operation can trigger false activations in the module. Another reason is that when the load is powered on and starts operating, it generates interference. For example, inductive loads such as relays or electromagnets produce back electromotive forces; additionally, electromagnetic radiation emitted during the operation of the 315M transmitting board can also affect the module. The following solutions are recommended: A. Add an inductor filter to the power supply section. B. Use separate voltages for the load and the module—for instance, operate the load at 24V while running the module at 12V, with a three-terminal voltage regulator providing isolation between the two. C. Use a power supply with a larger capacity. Fourth, the human-sensing module can only function indoors, and its operating environment should be kept free from direct exposure to sunlight or strong artificial lighting. If the environment experiences significant radio-frequency interference, shielding measures should be implemented. In cases where there is strong airflow disturbance, close doors and windows or block air convection. The sensing area should ideally avoid being directly facing heating appliances or objects, as well as items or clothing that are easily moved by wind.

　　Fifth, the human-body sensing module is recommended to be installed in a sealed box; otherwise, it may continuously output a signal.

　　Sixth, if the detection angle of the human-body sensing module needs to be less than 90 degrees, you can achieve this by covering the lens with opaque tape or by trimming and reducing the size of the lens.

　　Seventh, the human-body sensing module employs a dual-element probe. The direction of movement of the human body’s hands, feet, and head is closely related to the sensitivity of the sensor, and the characteristics of the infrared module mean that the sensing distance cannot be controlled.

　　Eighth, the probe (PIR) in the module can be soldered onto the opposite side of the circuit board. Alternatively, the probe can be extended using a two-core shielded cable; it’s best if the cable length does not exceed 20 centimeters.