A quick rise in temperature in the room can cause the PIR to go off while wind can move an object and trigger the microwave sensor. There is technology that incorporates both PIR and Microwave sensors to have less false alarms. Dual Tech Motion Sensors: How Dual Tech/Hybrid Sensors Work Its ability to penetrate material “see through walls” can cause frequent false alarms. Microwave sensors can cover a larger area than PIRs, but are more expensive and can be vulnerable to electrical interference. Figure 3 shows how the Microwave sensors work.įigure 3: Frequency shift on an object causes the signal to be amplified, detecting motion. If it does detect a frequency shift, the motion detector is activated. It sends out high radio frequency and measures the reflection off an object by sensing for a frequency shift. View Microwave Motion Sensors: How Microwave Sensors WorkĪs the name implies, Microwave sensors uses continuous waves of microwave radiation to detect motion, similar to how a radar speed gun works. (Source: Adafruit/Lady Ada) See related product Figure 2 shows the inside of the Fresnel lens, where you can see the “sections” on it.įigure 2: Inside a Fresnel lens. The optic physics behind the Fresnel lens is a whole other interesting area to look into if you have time, but in this case, it gives the PIR sensor a much larger detection area making it more efficient. The white plastic that you see on the PIR is called a Fresnel lens. Figure 1 from Adafruit shows a great example of how it works.įigure 1: PIR sensing heat source movement and outputting pulses. When the sensor senses a differential change between the two slots, this causes a pulse, which is what it detects as “movement”. The PIR has two slots made of a special material that is sensitive to infrared. PIRs have a pyroelectric sensor that detects levels of infrared radiation – everything emits some low-level radiation, but a human body emits a good amount of heat. The way it senses movement is by sensing the change in temperature between the background and a warm body. They are small, low power, easy to use, and inexpensive. The PIR is a motion sensor that you might have seen when you enter a restroom or an office space, usually having a white cover. Passive Infrared Sensors: What is a PIR Sensor? There are three types of motion sensors that are used frequently: Passive Infrared (PIR), Microwave, and Dual Tech/Hybrid. It can be an energy saver by shutting off lights in a building when it senses no motion anymore, which is used regularly in office buildings or restrooms. They can be used for burglary alarms or security cameras, activating these devices when it senses motion in the vicinity. Motion sensors are popular when it comes to security and energy efficiency.
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