Perhaps we never got into a huge mall with automatic door open as we approach the door? that is one of the application components of the motion sensor in the electronics. Components that can detect movement commonly referred to as PIR or Passive Infrared Sensors.
PIR sensors can detect small movements of this and also feel if there is any man who moved out of range sensor.
For those of you who do not know this motion sensor PIR, its small, low power consumption and not easily worn away with quite a hefty hargayang. Also called PIR is also called Pyroelectric IR or motion. Not to be confused with the light sensor that yesterday I had a chance to discuss also, both are often used in a series of sensors. This is a form of censorship of PEARS:
How does it work? PIR motion Sensor works by measuring infrared energy that is detected by sensors that exist therein. They do not emit infrared itself so it is referred to as passive. Whereas in ordinary IR sensor, usually needed an infrared beam/laser as an indicator of the existence of light detected. Think about it like this way of working motion sensors:
Infrared energy is produced by heat, humans and animals including can produce heat even though small, an average rate of 9-10 micrometer infrared energy.
Usually range motion sensor this can reach 5 meters. Quite effective as a motion sensor is not it? Also note how the image above, this sensor PIR can't detect how much the number of people who were in the area range of the sensors, can only feel a change in heat energy/infrared only.
How much is a motion sensor PIR? If you are interested in based on the info I get the price could range anywhere between 200 thousands. Quite expensive, but in the hands of people who are creative, can be useful tools/missing 
.