Different Types of Motion Detectors

There are five basic types of motion detectors, and the active infrared (IR) and passive infrared (PIR) detectors are the most common types used. Other versions or types are the continuous wave (CW) radar, ultrasonic motion and vibration motion detectors. The main difference between active and passive types of motion detectors is that active detectors send out signals and search for motion, while passive detectors do not emit any signal. A passive detector reacts to a signal that is sent to the detector, such as noticeable body heat or sound waves. An active detector, however, sends out a pulse of ultrasonic sound waves and measures the reflection of the waves to detect motion.

Many motion detectors are used as switches on lights and alarm systems. By utilizing motion detectors, motion-sensing lights can be used to illuminate an area when a suspected intruder is present. While referred to as a motion light, it is, in reality, a light triggered by a motion detector. This type of system uses passive detector technology to identify motion. Used on lower-cost illumination and alarm switches, the passive system relies primarily on detected body heat to trigger a switch on the light or alarm, indicating someone or something has been detected in the vicinity of the sensor.


Higher-end motion detectors rely on active detection methods, such as ultrasonic sound-waves, to detect the presence of motion. This technology sends out sound waves and measures the time it takes for the waves to reach a known distance and return. When an intruder enters the midst of the sound waves, the time it takes the waves to return to the sender is shortened, so the sensor detects motion and trips a light or alarm switch, if so wired and programmed. The continuous-wave, radar motion detectors work on this same principal, however, this system uses radar waves in place of the ultrasonic sound waves.

Much like a police radar gun uses radar waves to detect the rate of time it takes a radar wave to reach a target and return to the gun, the presence of motion is also detected. The radar gun measures the changing time it is taking the waves to be sent out and returned and then calculates the rate of speed by the changing times. The motion detector needs only to sense the change in time for a single radar wave to return to detect motion. Vibration sensing motion detectors commonly use a network of sensors placed throughout an area to measure any vibration on or in the ground as detected motion.

Ultrasonic Motion Detector

An ultrasonic motion detector is a device that can detect movement of people or objects within a limited area. It senses motion by analyzing sound waves in its environment. Some just listen for sounds, while others send out ultrasonic signals and analyze how they are reflected back. This type of motion sensor is often used in a home security system to sound an alarm if anything moves. It can also be used to turn on lights or other devices when someone enters a room.

Active motion sensors generate sound waves in the ultrasonic frequency range, typically around 30 to 50 kilohertz (kHz). These frequencies are generally inaudible to humans and most animals and do not pass through most objects. The ultrasonic motion detector emits cone-shaped sound waves using a quartz transducer and listens to the echoes. When no objects are moving in the area, the pattern of sound and the time it takes to bounce back remain the same. If something moves, the detector senses that the level or phase of the returning sound waves has shifted slightly.

An active ultrasonic motion detector should try to eliminate as much noise or false data as possible. To do so, it often analyzes the Doppler Effect frequency variations in echoes, as well as amplitude changes. This type of detector is fairly complex and often includes a programmable microcontroller. In contrast, a passive motion sensor listens for suspect sounds like metal scraping or breaking glass. Unlike an active sensor, it does not generate any audio waves on its own.

Background noise and motion can often unintentionally trigger an ultrasonic motion detector. Birds, insects and wind gusts make it difficult to use one as an outdoor motion sensor. A low-cost active detector, such as a typical car security sensor, may only analyze variations in echo amplitude. Ignoring frequency variations can result in false alarms from this type of detector. Ringing telephones and other loud noises sometimes inadvertently trigger passive motion sensors.

Furniture, boxes and other large items can "hide" other objects in the area from an active ultrasonic motion detector. They can also absorb and distort sounds that are critical to a passive sensor. Breezes from air conditioners or open windows are often sensed as movement, too. Some motors and fans generate sounds in the ultrasonic range, which may also confuse motion sensors.

Using an ultrasonic motion detector that includes several active and passive sensors can improve results. Some systems combine an ultrasonic sensor with a passive infrared motion detector. Analyzing the sound and heat data together often gives a more accurate picture than either type would on its own.

Microwave Motion Detector

A microwave motion detector is a device that uses Doppler radar to detect and measure the movement of an object. For most people, the two most familiar forms of this mechanism are used in a home security system and in traffic law enforcement. Another familiar application is the use of a microwave motion detector to operate lighting devices upon a person entering or leaving an area.

The microwave motion detector typically is a single unit device that emits a pulse of microwave energy and then detects its reflection. Calculating the time required for the reflection and the change in frequency, it determines the distance, direction, and speed of the object being observed. Direction and speed are computed based on the Doppler effect. Most experience the Doppler effect when hearing a train or other vehicle approaching them; the sound seems to move higher in pitch as the object approaches and lower as it moves away.

In security applications, the microwave motion detector is usually paired with an infrared motion detector. This is to reduce the possibility of false alarms. The microwave motion detector may detect movement of an inanimate object behind a wall or door, such as a windblown tree limb, for example. If the infrared sensor also detects body heat, the likelihood that the object being detected is an intruder is increased, and the security system can execute the required function.

Choose the Best Portable Motion Detector

A portable motion detector is a device that alerts users when there is movement and which can be moved easily from location to location. These devices may be wireless and powered by batteries or must be plugged into electrical sockets. Some models emit chiming noises or alarm noises, while others can shine lights and deliver audio announcements. To choose the best portable motion detector, it can be helpful to think about your needs, since they can help you to determine which features you might benefit from the most. It can also be helpful to consider factors such as quality and price.

If you are a business owner, then the best portable motion detector may be one that makes a chiming noise when an individual passes by. These can be placed near front entrances or in certain rooms, alerting clerks when a customer is present. Many motion detectors come with different noise options. For example, you might be able to set the detector to an alarm setting. This can be useful for scaring away intruders.

Homeowners may have different portable motion detector needs. A useful driveway motion detector might shine a light when there is movement. This function can deter potential intruders. It can also serve as security for when you come home at night. A motion detector can shine light when you arrive, allowing you to see where you are going as you approach your house.

Infrared Motion Detector

An infrared motion detector uses infrared sensing to detect motion in a given area. This type of detector is often used as part of a larger security system, typically including the detection as a way of triggering alarms that indicate a potential issue to security or emergency services. The use of infrared technology usually allows such a detector to only pick up on heat and therefore more easily ignore non-living motion. An infrared motion detector can also be set to only pick up on certain levels of heat, making it possible to ignore small animals.

Infrared light is a spectrum of non-visible — for humans at least — light that is emitted by objects when they produce heat. Different levels of heat production can create varying levels of infrared emission, and other types of technology often use infrared beams to transmit information wirelessly. Television remote controls, for example, often used infrared light to send a signal between the remote and the television receiver. Body heat, from a living creature, produces infrared light and an infrared motion detector is able to pick up on this release of energy to track motion.

Typical motion detectors often suffer a flaw of picking up on any kind of motion. This means that someone with a motion detector outside his or her house will have cars that drive by trigger the motion detector, as well as tree limbs swaying in the wind. An infrared motion detector, however, only tracks motion that also emits infrared energy, and therefore is able to ignore non-living and cool organic materials.

This allows a security system utilizing an infrared motion detector to only trigger and activate a light, alarm, or other security system if there is motion from a living creature. Someone living in the woods, for example, may want a motion detector to activate a bright light to keep wild animals, such as raccoons, away from a house. The use of an infrared motion detector in this type of system would allow the activation of the light to scare away animals but not have it come on whenever a strong wind blew tree limbs around.

An infrared motion detector can also often be designed to only activate when certain levels of heat energy are detected. Such a detector can be set to only activate when heat levels from a living creature over a certain weight limit are in motion. For an indoor motion detector, this can allow someone to set the infrared detector to only activate an alarm if a person passes through a room, but ignore smaller animals like a cat or dog.

Passive Infrared Motion Detector

A passive infrared motion detector is a device that detects moving objects by detecting the infrared light they emit. They take advantage of the fact that all objects emit infrared radiation, in amounts that vary according to their temperatures. Like all passive detectors, a passive infrared motion detector works solely by detecting emissions produced from other objects and does not use emissions of its own for this purpose, unlike active motion detectors based on microwaves or ultrasonic pulses. These detectors are frequently used in security systems to detect intruders.

The infrared region of the electromagnetic spectrum encompasses radiation with wavelengths between 0.7 and 300 micrometers, shorter than the wavelength of visible light but longer than microwaves. Infrared light with a relatively short wavelength close to that of visible light, called “near infrared,” cannot be detected by human senses. “Far infrared,” with longer wavelengths, cannot be seen by the human eye but is felt as heat.

An object's heat causes it to emit part of that energy in the form of electromagnetic radiation, with higher temperatures producing greater emissions. At high temperatures, some of these emissions are in the visible spectrum, as can be seen in the filament of an incandescent light bulb or the glow of metal being worked by a blacksmith. All objects, however, emit at least some thermal radiation at infrared wavelengths, and since no matter has a temperature of absolute zero, even extremely cold objects have infrared emissions.


When objects in the vicinity of a passive infrared motion detector move relative to the detector, the infrared radiation reaching the detector from the direction of the motion changes. If an intruder enters the detector's range, the change in emissions caused by the presence of his or her body will be noticed. Since the detector is triggered by changes in infrared light reaching it from a particular direction rather than its absolute amount, a moving, low-emissions object will set off the detector just as a moving, high-emissions object would. Thus, concealing the intruder's body heat will not conceal the intruder.

The sensor of a passive infrared motion detector is built from pyroelectric crystals, which are substances that produce a temporary electrical voltage when their temperatures change due to the subtle shifts the changes cause in their crystalline structures. Commonly used pyroelectric materials include cesium nitrate (CsNO3) and gallium nitride (GaN). Changes in incoming infrared light caused by moving objects changes the pyroelectric crystals' temperatures, producing electric signals that are received and interpreted by the sensor's electronics to trigger the alarm. These components are placed in a housing with a plastic window that keeps most visible light out but is transparent to infrared light. The sensor can also contain devices, such as parabolic mirrors or specialized lenses called Fresnel lenses that focus the infrared light on its way to the pyroelectric crystal.

Motion Detector Schematic

If you want to build your own home security system, you need to know how to read a motion detector schematic. This displays all of the required components for building the motion detector circuit and shows how those components are connected to one another and the power supply. Such schematics are read just like any other schematic, since they use the same symbols. If you haven't read a motion detector schematic before, you can do so by learning to identify the components by their symbols and how complex components, such as integrated circuits (ICs), are identified. You'll also need to learn to identify the power supply and be able to piece together schematics for areas where some of the components are connected to one another indirectly.

Almost every schematic uses the same symbols to identify components. You can learn to identify these by using a beginner's electronics book or an online chart. When looking at the symbols, you should notice that lines protrude from one or more ends of the symbol. These lines represent a component's leads or pins. Occasionally, there are small variations in the symbols used in a schematic. For example, a resistor can be represented by either a zigzag or rectangle.


It is common for a motion detector schematic to include complex components, such as ICs. ICs, which are sometimes referred to as chips, are typically represented by rectangles and triangles. If you see a triangle, it means the IC is a specific type called an operational amplifier (op-amp). Each IC is also identified with a model name that typically consists of numbers and letters. The model name allows you to look up the IC and purchase the correct component.

The sensors, such as passive infrared (PIR) sensors and cameras, in a motion detector schematic might also be identified by rectangles. In some cases, the designer of the schematic might use a special symbol that he or she has designed. Typically, the sensors in a schematic can be identified by their part numbers or model names. If a part number is given, look it up in the circuit's parts list.

Usually, three or more pins of an IC or sensor are connected to other components. In a motion detector schematic, these pins are typically identified by their numbers, and sometimes by their functions. If a schematic identifies pins by their functions, you'll need to locate the component's datasheet, which should be available from the component's manufacturer. Occasionally, the pin numbers are listed out of order in a schematic, so it is important to pay close attention to the locations of the numbers.

Like all schematics, a motion detector schematic uses solid lines to indicate connections between components. It is common for the lines in a schematic to overlap, especially when one shows the design of a complex circuit. For example, you might notice that a series of horizontal lines are overlapping a series of vertical lines. The intersections of these lines don't always represent connections. Typically, a circle is used to identify connections between overlapping lines.

For an electronic circuit to function, it must be connected to a power supply consisting of a ground, also called earth, and a positive voltage. The ground is typically represented by three horizontal lines and the positive voltage is usually represented by a plus sign. Schematics also indicate the type of power required, such as direct current (DC) or alternating current (AC), and the number of volts required for proper functionality. The power supply is often displayed at the top or on the left side of a schematic. Large motion detector schematics might be broken up into several chunks within a page or across several pages. You can determine how the pieces of the schematic come together by looking for symbols, numbers, or words on the lines representing the connections. For instance, the first part of a schematic might contain an IC with numbered pins and the second part might contain additional components with numbered lines going into those components. In this case, the numbers represent the IC's pins.