Ⅰ: Classification of sensors

（1）Passive infrared (PIR) sensor: detects changes in infrared radiation from the human body or animals, suitable for security and automatic lighting.

（2）Active infrared (AIR) sensor: emits infrared and receives reflected signals, used for distance measurement or object detection (e.g. automatic doors)

（3）Ultrasonic sensors: measure distance by emitting ultrasonic waves and calculating the return time, suitable for obstacle avoidance and liquid level detection.

（4）Microwave (radar) sensors: utilize the microwave Doppler effect to detect moving objects, with strong penetration, suitable for complex environments.

（5）Photoelectric Sensor: Utilizes light reflection or shading to detect objects, commonly used in industrial automation.

（6）Capacitive/resistive sensor: detects objects close to or touching, used for touch screen, water level detection, etc.

Ⅱ：Sensor selection for different scenarios

1. Smart Home Scene

(a) Automatic lighting: this is usually used in aisles, stairwells, entrance halls, etc. These scenes are more commonly used. We generally use these scenes PIR human infrared sensor + radar microwave sensor. For example, our company where the model: GS-PIR18

(b) Smart Air Conditioner: usually, smart air conditioner is controlled by sensing the temperature and humidity, so we need sensors to automatically adjust the temperature and humidity of the room. So in this case we usually adapt the temperature and humidity sensor.

(c) Security Alarm: Security is usually to prevent the intrusion of thieves and bad guys, so we need to reduce the interference such as pets. So we usually choose microwave double identification sensor.

(d)  Automatic curtains: Automatically switch on and off according to light and temperature, usually choose light sensor + temperature and humidity sensor.

2. Industrial Automation Scenarios

(a) Detection of objects: usually this requires accurate detection of object position, so we usually choose photoelectric/ultrasonic sensors.

(b) Liquid level monitoring: in this case we need to detect the non-contact measurement of liquid height, choose ultrasonic/capacitive sensors.

(c) Assembly line counting: this requires accurate detection of objects passing through at high speed, infrared sensor can meet this demand.

(d) Robotic arm obstacle avoidance: In this case, high precision 3D environment sensing is needed, so we usually choose LiDAR (Laser Radar).

3. Security Monitoring Scene

(a) Glass breakage alarm: the sound frequency of glass is different from the frequency of other sounds, so by detecting the broken sound of a specific frequency to play the role of alarm, so the sound sensor will be selected.

Ⅲ. Key factors for choosing a sensor

1. Detection range: PIR is available for small indoor areas, microwave/radar is recommended for large or complex environments.

2. Anti-interference ability: industrial environment preferred photoelectric or ultrasonic, to avoid false triggering.

3. Response speed: security systems require millisecond response, while temperature and humidity sensing can be slightly slower.

4. Power consumption: battery-powered devices (such as wireless sensors) need low-power sensors.

5. Cost: household PIR is cheap, LIDAR and other high-end sensors cost more.