PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they are commonly found in appliances and gadgets used in homes or businesses such as security systems, automatic doors, intelligent lighting, garden lights, guest reception monitors, etc.
The HT7M21xx PIR motion sensor series come fully integrated with optical lenses, a passive infrared (PIR) sensor and DSP algorithms. These sensors include a wide range of features such as low power consumption, an I2C communication interface and DSP algorithms which improve the reliability of the PIR detector. The dimension is only 12.9mm(Width) x 12.8mm(Length) which is smaller than most other PIR sensors in market. The height differs by modules ranging from 13.35~14.96 mm.