Measurements based on the Doppler effect
Detection of motion is possible by using one of the fundamental phenomena present in almost all known types of waves. The Doppler effect consists in the change of the wavelength (and thus its frequency) reflected from an object that is in motion with respect to the source of the wave. In other words - the wave sent by a transmitter reaches an obstacle whose surface becomes, in a way, the source of a new wave. If the transmitter and the obstacle are stationary relative to each other, the reflected wave will have the same frequency (and length) as the source wave. The movement of the obstacle in relation to the transmitter (in the direction of propagation) causes decrease or increase of the frequency of the reflected wave - the direction of change is determined by the direction of movement (to/from the transmitter), while its intensity is determined by the speed of movement. What is important, this phenomenon takes place both for electromagnetic waves (radio, light and others) and for acoustic waves (including sound and ultrasound). In order to detect motion it is therefore sufficient to compare in some way the frequency of the wave received by the receiver (located next to the transmitter) with the frequency of the original wave sent by the device. This is how microwave distance sensors, operating in the 10 GHz band, have become increasingly popular in recent years. What is important, they detect both living beings (humans, animals) and inanimate matter (vehicles, moving parts of machines, etc.).
Passive PIR motion detectors
A completely different principle of operation is used for motion detectors based on the detection of infrared radiation emitted by all bodies with a non-zero temperature (zero Kelvin, or so-called absolute zero). PIR sensors consist of a semiconductor structure (containing a simple, several-element array of photodetectors sensitive to infrared radiation in the range of single micrometers), a special plastic lens (usually milk-coloured) and an electronic circuit, responsible for amplifying and processing weak signals from the sensor. If in the field of view (wide thanks to honeycomb lens) moves the object with a temperature different from the background (usually higher), some of the detectors will register a signal with a different amplitude than the rest - so to detect movement it is sufficient to use a system that compares signals from individual detectors (in the simplest case the classical comparator works here).
Gravity sensors in Botland offer
In our offer we have motion and distance sensors, based on electromagnetic waves, ultrasonic and infrared. DFRobot Gravity Microwave sensor v1.0 is perfect for detecting the movement of objects and people (or animals) in a way completely independent of their temperature. The sensor operates with a carrier wave at a frequency lying in the unlicensed band of 10.525 GHz and consumes an average supply current of 2 mA. The width of the observation field is 36 degrees vertically and 72 degrees horizontally. PIR sensors are also noteworthy - the company offers two versions: standard - with PCB size of 38 x 36 mm and miniature - with size of 30 x 22 mm. For applications requiring accurate distance measurement it is worth to use URM09 ultrasonic sensor, offering a wide range from 2 cm to 500 cm and allowing simultaneous temperature measurement from -10 °C to 70 °C.