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How Do Motion Sensors Work? Types & Applications

Many of the most popular new IoT applications in smart homes and smart industries incorporate motion sensors. From security systems to lighting and more, motion sensors are a simple and effective way to save energy while gathering useful data. The most frequently used motion sensors are Passive Infrared (PIR), Microwave, and Dual Tech/Hybrid. Here’s how they work.

Motion Sensors Explained

Motion sensors are popular when it comes to security and energy efficiency. They can be used for burglary alarms or security cameras, activating these devices when it senses motion in the vicinity. 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. There are three types of motion sensors that are used frequently: Passive Infrared (PIR), Microwave, and Dual Tech/Hybrid.

Passive Infrared Sensors: What is a PIR Sensor?

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. They are small, low power, easy to use, and inexpensive. The way it senses movement is by sensing the change in temperature between the background and a warm body.

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 PIR has two slots made of a special material that is sensitive to infrared. When the sensor senses a differential change between the two slots, this causes a pulse, which is what it detects as “movement”. Figure 1 from Adafruit shows a great example of how it works.

How PIR Sensors Work 

Figure 1: PIR sensing heat source movement and outputting pulses. (Source: Adafruit/Lady Ada)

The white plastic that you see on the PIR is called 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. Figure 2 shows the inside of the Fresnel lens, where you can see the “sections” on it.


Fresnel Lens 

Figure 2: Inside a Fresnel lens. (Source: Adafruit/Lady Ada)

As the name implies, Microwave sensors uses continuous waves of microwave radiation to detect motion, similar to how a radar speed gun works. It sends out high radio frequency and measures the reflection off an object by sensing for a frequency shift. If it does detect a frequency shift, the motion detector is activated. Figure 3 shows how the Microwave sensors work.

How Microwave Sensors Work 

Figure 3: Frequency shift on an object causes the signal to be amplified, detecting motion. (Source: MikroElektonika)

Microwave sensors can cover a larger area than PIRs, but are more expensive and can be vulnerable to electrical interference. Its ability to penetrate material “see through walls” can cause frequent false alarms.

Dual Tech Motion Sensors: How Dual Tech/Hybrid Sensors Work

There is technology that incorporates both PIR and Microwave sensors to have less false alarms. 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. With the Dual Tech/Hybrid, both sensors will need to sense changes which will trigger it on, causing false alarms to be extremely low.

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