The term ' Motion Detectors ' can be used to refer to any type of detection system used to detect motion; movement of any object or movement of human beings. However, it is mainly used to detect movement of human beings or, in other words, the presence of a body in a certain area. 
Figure 1: A representational image of an electronic device, motion detector
A motion detector is an electronic device that detects physical movement in a certain designated area/location and transforms the movement into an electrical signal.
Motion detection apps do this:
• Generate some stimulus and feel its reflection.
• Feel some signals generated by an object.
All motion sensors indicate the same thing; that some condition has changed. All sensors have some “normal” state. Some sensors only report when the “normal” state is disturbed, others also report when the condition returns to “normal”.
Motion sensors are commonly used in security systems as triggers for automatic lights or triggers for remote alarms and similar applications. Motion sensors work based on a wide variety of principles and are used in a wide variety of applications. Typical use might be on the exterior doors or windows of a building to monitor the area around the building. When they detect movement, they generate an electrical signal from which some actions are carried out. Some operate in the same way as an army radar scanner, while others work based on vibration, infrared radiation and even sound. All of these different types of motion sensors have different strengths and weaknesses, which are important to consider when making the decision to choose a particle motion detection sensor.
TYPES OF MOTION SENSING
Motion sensors are employed to detect different types of human movement. Some are intended for local event detection, others for area detection
• Local Sensing
Local detection involves detecting a movement at designated locations. Some of the motion sensors commonly used for this purpose are
o Visible/infrared light beam (LED/Laser)
o Contact switch
o Piezoelectric sensors
o Piezoresistive sensors, etc.
• Area Sensing
Area detection entails detecting a movement in a specific 'field of view (FOV)'. Motion sensors commonly used for such application are
o Active/passive infrared motion detector
o Ultrasonic motion detection sensor
o Step detection sensor
o Microwave Doppler Sensor
the video
Local motion detection
LOCAL MOTION SENSOR
• Motion detectors based on visible/infrared light beam (LED/Laser)
This type of motion sensor employs the interrupt beam principle; in a light source transmits a beam of light towards a distant receiver creating an “electronic fence”. Once a beam is broken/interrupted due to some opaque object, the output of the detector changes and the associated electronic circuit takes appropriate actions. The light source used can be a visible light source or an infrared light source. 
Fig. 2: A diagram demonstrating the principle of infrared light beam-based motion detectors
• Contact switch based motion detectors
These switches can be placed under the carpet and can be the simplest type. In the normal state, the switch is open, but when a person steps over the mat, the switches are closed. The status of the switch assists in detecting movement.
• Motion detectors based on piezoelectric/piezoresistive sensors
In place of contact switches, piezoelectric/piezoresistive sensors can be used. Piezoelectric sensors use piezoelectric effect. They generate electrical output on application of pressure. Piezoresistive sensors change their resistance with the application of pressure. The change in resistance is measured to calculate the applied pressure, i.e. the movement over the sensor.
AR and PIR motion sensors
MOTION DETECTION IN AN AREA
1. Active/Passive Infrared Motion Detectors
Infrared is the portion of the electromagnetic spectrum that lies between microwaves and visible light. Infrared has wavelengths longer than visible light but shorter than microwaves. Humans, at normal body temperature, radiate more strongly in the infrared, at a wavelength of approximately 10 µm. To detect this signal, a transducer is required that converts the infrared signal into an electrical output.
Active infrared (IR) motion detectors operate in conjunction with a radiation source. They are able to sense interruptions in the radiation they receive, usually caused by an intruder's body heat. To detect an intruder, therefore, the intruder must pass through the field generated by the radiation source. However, they are not very common.
Whether it is to detect “someone moving in the front yard” or “someone moving through a door”, Passive Infrared (PIR) sensor is usually the sensor of choice. There is a simple electronic device that is sensitive to “heat”, or rather infrared light emitted by warm or hot objects (like humans). Passive infrared (PIR) motion detectors also operate on the same general principle as active infrared motion detectors, but do not require a radiation source. Instead, they are able to detect changes in the ambient infrared radiation of the surrounding area. PIR detectors are the most widely used type of motion sensor.
Passive infrared (PIR) sensors react to infrared thermal energy emitted by people. PIR sensors are passive devices because they only detect radiation; they don't issue it. They are designed to be maximally sensitive to objects that emit thermal energy at a wavelength of about 10 microns (the peak wavelength of thermal energy emitted by humans). PIR sensors are strictly line-of-sight devices. They cannot “see” around corners and a person will not be detected if there is an obstruction, such as a divider, between the person and the detector.
The “logic” of the PIR Sensor is that it must detect 'significant changes' in the normal level of heat within the 'field' of your vision. The circuits that control it must be able to determine what is “normal” and then close a switch when the normal field changes, such as when a human walks in front of it. Must also be able to “tolerate” slow changes within the field and remember this as the new “normal”. Gradual changes, such as sunlight throughout the day, should not cause a false alarm.
PIR detectors detect sudden, slight changes in temperature within the detection area; thus, when an intruder crosses or enters any zone, the resulting change in infrared energy is detected for alarm reporting. The best coverage is obtained if the mount is selected so that the likely direction of intruder movement is through the pattern.
PIR sensors employ a pyroelectric transducer to detect infrared radiation. The device converts IR energy into a voltage signal. A pyroelectric sensor is made of ceramic material that generates a surface charge when exposed to infrared radiation. As the amount of radiation changes, the charge also changes. The output voltage is a function of the amount of infrared (IR) radiation detected at the input. Because the sensor is sensitive to a wide range of radiation, a filter window is generally employed to limit incoming radiation and optimize human detection. Furthermore, to prevent the detector from being sensitive to vibrations, radio interference, sunlight and other background signals, two sensing elements are used in differential mode, causing any signal common to both elements to be cancelled.
A multifaceted Fresnel lens surrounds the transducer, compressing the light passing through it and focusing thermal energy on the detector. The Fresnel lens provides a cone of vision. This cone of vision has “dead” zones (areas where the detector will not see movement), but these are dispersed throughout the field of view. Thus, the lens views the area with a multitude of narrow, discrete beams or cones. However, these dead zones do not create detection problems. It would be very difficult for an intruder to move within the detector's field of view and not come into contact with active regions.
Fig. 3: A diagram showing motion detection in active infrared motion detectors
These lenses can be sensitive in the horizontal or vertical direction; most of them are horizontally sensitive. This implies that the lens will be more sensitive to the movement of a body horizontally across the field of view. Most PIRs are less likely to detect a body that is moving TOWARDS the sensor, therefore PIR sensors must be positioned so that the most likely movement in their field of view is horizontal to the “normal” position of the sensor unit. sensor. This must be taken into consideration when installing these sensors.
Fig. 4: A diagram illustrating the precise positioning of the PIR
Furthermore, care must be taken that PIR sensors are not pointed at surfaces that become “hot” periodically.
Ultrasonic motion detection sensors
2. Ultrasonic Motion Detection Sensors
These sensors use two different principles:
a) Flight time
This type of motion detector uses an electrostatic transducer that acts as a speaker and microphone. The transducer transmits a burst of ultrasonic pulses. The ultrasonic pulses bounce off an object and return to the sensor. The sensor measures the time between the rising edge of the shot and the rising edge of the echo. It uses this time and the speed of sound to calculate the distance to the object.
b) Doppler-based
In its simplest form, the Doppler theory states that as sound, light or even radio waves are reflected back by a moving object, the frequency of these waves will be different from the frequency when the object is stationary. As the object moves away, the frequency will decrease and if the object moves towards you, the frequency will increase. This change in frequency is due to the waves being stretched or compressed as they leave the moving object. Compressing the waves closer will have the effect of increasing the perceived frequency, while stretching the waves will make the frequency appear lower.
The basic sensor operating principle of these sensors is shown in fig. 
Figure 5: A figure showing the basic sensor operating principle of Doppler-based sensors
The ultrasound transmitter emits ultrasound waves continuously into the ambient space of the sensor. These waves are reflected from various objects and reach the ultrasound receiver. There is a constant interference figure if there are no moving objects in the positioning.
Ultrasonic motion detectors operate by tracking changes in sound frequency. Ultrasonic sensors activate a quartz crystal that emits ultrasonic waves throughout the space. This signal creates a standing wave pattern in the room or area in which it is installed. 
Figure 6: A representational image of ultrasonic motion detectors
The unit then detects the frequency of the reflected waves. The standing wave is disturbed by motion, so if there is motion, the frequency of the reflected waves will change slightly (Doppler effect). Any moving object changes the level and phase of the reflected signal, which changes the level of the summed received signal. Some sensors perform amplitude analysis of the reflected signal, while others perform frequency analysis of the reflected signal. The spectrum of the reflected signal emulates a Doppler effect.
Ultrasonic sensors operate at frequencies above human sensitivity (20 kHz). Typical operating frequencies are 25, 30 and 40 kHz. Ultrasonic sound waves cover the entire area continuously – there are no blind spots or gaps in the coverage pattern. For this reason, ultrasonic sensors are a little more sensitive to movement. For example, hand movement can be detected at a distance of about 25 feet, arm and body trunk detected up to 30 feet, and full body movement can be detected at over 40 feet, although these ranges differ significantly. for different products.
The frequency components of the moving object's velocity vector have a component in the direction of propagation of ultrasonic radiation. As ultrasound waves are reflected from windows, walls, furniture, etc., the sensor can detect movements of objects in any direction. To implement this principle, the sensor must perform Doppler Effect frequency shift selection and processing to detect moving objects.
Step-based detection
3. Motion detection based on step detection
People walking generate unique acoustic and Doppler signatures of footsteps that can be used for human detection and recognition to differentiate them from other moving objects. Acoustic footstep signatures have broadband frequency response from a few Hertz to ultrasonic frequencies and generate vibrations and sound through an interaction of the foot and supporting surface.
There are two characteristic frequency bands in the vibratory and sonic responses of footstep signatures. The first frequency band is generated by a force normal to the support surface and is concentrated in a low frequency range below 500 Hz. The second frequency band is generated by tangential force (friction) and located in a high frequency range, above 1 kHz up to ultrasonic frequencies. These frequencies can be measured and analyzed for human detection purposes.
Another method to detect human movement through footstep detection is to measure vibrations directly. Walking/running generates vibrations on the surface where there is movement. Accelerometers can be used to measure these accelerations. Analysis of these accelerations can be done to detect movement. Click to learn more about what an accelerometer is.
Microwave motion detectors
4. Microwave Motion Detectors
Microwave motion detectors use the philosophy of continuous wave (CW) radar for motion detection. Similar to Doppler-based ultrasonic motion detections, these detectors use a transmitter to emit frequencies, but in the microwave range. The signals reflect off objects in the surrounding area. The detector is then able to track disturbances at these frequencies.
Microwave energy is developed by microwave oscillators and is radiated by antenna systems directionally or uniformly in all directions based on specific application requirements. The radiated energy fills the surrounding area with microwave energy. Any movement in this area creates the Doppler Effect; the reflected waves will have a higher frequency if the object is approaching or lower if it is moving away. Detectors don't care about directions, frequency changes are what they care about.
These sensor systems can be monostatic, in which the emitter and receiver are contained in one unit, or bistatic, in which the emitter and receiver are housed in separate units. Each type has advantages and disadvantages for the user. Monostatic units have limited range coverage compared to bistatic sensor. Bistatic sensors are also more prone to false alarms.
Fig. 7: A figure representing the range coverage of the monostatic sensor and the bistatic sensor
Microwave beams can penetrate walls and therefore allow detection of movement behind walls. Because the sensors are extremely sensitive to movement, they are also subject to other false alarms. Objects blown by the wind can trigger alarms. Even fluorescent lighting, which emits detectable light particles, can trigger a false alarm. Although microwave beams can penetrate most types of surfaces, metal is not among them. Therefore, the sensor may detect motion where detection is not desirable and not detect motion where it is desirable.
These detectors are more sensitive than PIR detectors and generally cost more.
Microwave PIR Motion Detectors
5. Combined Microwave-PIR Motion Detectors
To reduce the likelihood of false alarms, PIRs are increasingly combined with a microwave-based Doppler detector to form a dual-technology motion detector. Microwave-based detectors are more sensitive to targets moving toward the detector and less sensitive to targets moving slowly across their field of view, while PIR detectors are sensitive only to targets moving across their field of view. vision. By combining the two systems, the detector can be very difficult to avoid and less prone to false trigger events. These two types of sensors complement each other, with one being more sensitive in the area where the other is less sensitive.
Various motion detection systems
VARIOUS MOTION DETECTION SYSTEMS
Various types of detection systems, namely infrared, microwave and ultrasound devices, are employed to detect movements of humans or objects; each has its pros and cons, with sensitivity and range being the important parameters.
Since microwaves can pass through dielectric materials, microwave sensors are effective in large apartments/buildings. But these sensors are relatively expensive and their radiation is unhealthy for living organisms.
Infrared sensors are characterized by high sensitivity, low cost and are widely used. However, these sensors can generate false alarm signals if heating systems are active or if the speed of temperature change exceeds some threshold level.
Ultrasonic motion detection sensors are characterized by low power consumption, higher cost (compared to PIRs) and high sensitivity and provide greater coverage than PIR detectors. However, increased sensitivity means that ultrasonic sensors are more susceptible to false triggers due to any movement in space.
Dealing with false alarms is a challenge for all these motion detectors. PIR detectors use two elements in differential mode to cancel out unwanted air movements, vibrations, etc. Additionally, filters are used to limit IR radiation. Ultrasonic and microwave detectors adjust receiver gain to decrease sensitivity to false echoes. To reduce sensitivity to Doppler generated due to moving parts of air conditioners, etc., filters are used to limit the frequency range for operations without false alarms. However, despite all these precautions, it is not possible to completely avoid false alarms. To improve performance without false alarms, dual technology detectors are the solution. Although unique detection systems can detect movements that do not exist, any movement does not go unnoticed by these detectors. Therefore, occasional false alarms are not a cause for very serious concern and are therefore used with their limitations.
TYPICAL APPLICATIONS
Motion detection is used in various applications viz.
• Automated lighting system
• Occupancy sensors
• Vehicle Alert System
• Smart spotlights
• Burglar alarm
And many others.