Motion Detectors explained

High power lights that illuminate all night tend to consume a lot of power which results in large electricity bills. This is particularly true with outdoor flood lights that are bright and produce a lot of lumens. Also most solar lights aren’t able to work all night if they need to provide continuous illumination. To solve this problem a device called a motion sensor or motion detector is used together with a lighting fixture.

What is motion detector and motion detection?

A motion detector or motion sensor is a device that is used for detecting a movement in a certain range in front or around the detector. Usually it is used to detect motion of humans and is used for both safety and security purposes.

Motion detection is such a process, where a special sensor is used for detecting changes of different objects (mostly humans) in the sensors detection range that can either be an outdoor or an indoor area. The most basic motion detectors are able to detect movement of objects in an area from 2 – 5 meters (6.5 to 16 feet) up to 20 meters (70 feet) and more.

There are different types of motion detectors from infrared, optical to radio wave and others, and each of these types are used for specific occasions. For example, infrared sensors monitor changes in infrared wave or heat in the observable area and turns on a light, alarm or a security camera when changes in the infrared spectrum are detected. Detectors that use radio frequency detection sends out microwave signals to monitor a room and when an object walks in detection area the sensor detects changes in microwave field and signals the system that there is a moving object in the room. Video cameras can also be used for motion detection by algorithmically comparing images, when someone walks into the room the picture and pixels changes and special algorithm can detect these changes.

Where motion sensors are used?

Motion sensors are used together with some kind of other devices, such as security cameras, alarms or lightings. One of the most common usages of a motion sensor is together with an alarm system. These alarm systems are installed in both individual properties as well as commercial buildings to detect motion of unwanted trespassers. When a burglar walks into the detection range of the sensor it automatically starts the alarm. Motion detectors are also used together with security cameras, when motion is detected the security camera is turned on to catch the person walking by the observed location.

More advanced motion sensors are used in many new technologies, such as gaming consoles, smartphones, robotics and others.

automatic doors with motion sensor

Automatic doors with a motion sensor


Besides previously mentioned applications, motion detectors are often used together with lights and lighting systems both for security and safety purposes. When a person walks by an area which is monitored by the motion sensor, the light is automatically turned on at the moment, when a motion is detected. Motion activated lights are often used together with security cameras and alarms in security systems.

The most popular type of motion lights is motion activated flood light. These lights are used for both safety and security purposes. Motion activated outdoor flood lights are used for illuminating an outdoor area at night, starting from front door illumination to large parking lots. From safety aspect, these lights help to illuminate a dark area at night, so that a person can safely walk to desired place, and at the same time motion detector helps to save energy by turning the light on only, when someone walks by and turning it off after the person has safely walked to desired place. At the same time these lights can be used for security purposes to uncover a burglar from the dark and scare him away from your property.

Occupancy sensors are used in indoor places to detect if there is a movement in the room and turn light on or off accordingly. These sensors help to save both individuals and companies significant amount of money on electric bills each year.

Types of motion detectors

Motion detectors can be categorized in three types – active, passive and combined sensors. Each of these sensors respond to different actions and each are used in different situations.

Active sensors

Active sensors use technique that sends signals or waves across the room or other place, where the sensor is installed and monitors the changes in wave response time. When an object moves in the detection area, micro or other types of transmitted waves collide with the object in the room and responds back to the detector much quicker. Motion detector is able to understand these changes and send an electrical signal to the alarm system, light or other type of device that is connected to the motion detector. Active sensors are used to detect any kind of motion in detection range, which means that wind moved tree branches could be regarded as movement and detected by the sensor, which then could trigger the outdoor lighting. Active sensors that use microwaves for movement detection are mostly used in such applications as automatic doors on shopping malls and similar applications, but can also be found in house alarm systems and are used for indoor lighting systems. Active motion sensors are not the best solution for outdoor lighting as movement of random objects such as wind-blown things, smaller animals and even larger insects can be detected by the active sensor and lightning will be triggered.

Active microwave motion sensor operation

Active motion sensor operation. Source –

Passive sensors

Passive sensors or PIR sensors (passive infrared sensor) in opposite to active sensors does not send out anything, but sits in the corner of a wall or other place and waits for the infrared light to reach the sensor. Passive sensors scan the room or area it is installed for infrared heat that is radiated from living beings (actually heat is radiated from any object with temperature above absolute zero, most from humans). When a human walks into the detection area of a passive sensor, it detects heat emitted from that human and triggers alarm or turns on the light. These sensors would not be effective if they could get activated by a small animal or insect that moves in the detection range, however most passive sensors can be adjusted to pick up motion of object with certain level of emitted heat, for example adjusting the sensor to pick up movement only by humans.

Combined (Hybrid) sensors

Another motion sensor type that has become popular recently is combined or dual technology sensors that have a combination of both active and passive sensors. It activates light or alarm only in such occasion, when motion is detected by both active and passive sensor. Combined sensors are useful for alarm systems to reduce possibility of false alarm triggers. The most popular type of dual technology sensors combines a PIR and a microwave sensor. However this technology also has its downside. It cannot provide the same level of safety as separate PIR and microwave sensors, because alarm is triggered only when motion is detected by both sensors. So for example, if a burglar knows how to evade one of the sensors, his movement is going to be detected just by one sensor, no signal will be sent to the alarm system and it won’t be turned on.


Active, passive or combined sensors for outdoor and indoor lighting

When looking at motion activated flood lights you may have come across a fact that there are very few manufacturers that indicate what type of motion sensor is used for the light. However, this is because the largest part of lights (especially outdoor lights) has passive infrared motion sensors. And if you think more, it makes a lot of sense that passive motion sensors that use heat emitted from a human are a lot better suited for lighting than active sensors that sends and receives signals and detects changes in wave lengths. Active motion sensors can be found in some indoor lights and they are not the best solution for outdoor lighting as movement of random objects such as wind-blown things, smaller animals and even larger insects can be detected by an active sensor. The light will get triggered much more times without a purpose, which will result in larger electricity consumption, will degrade light bulbs faster and may disturb you and your neighbors at night.

If you see that motion activated light is commercialized as „PIR sensor light” or „Light with a PIR motion sensor” you can be sure that this type of light uses a passive motion sensor.



What a PIR detector is?

A PIR is a passive infrared motion sensor. It detects infrared energy, which is in a form of a temperature that is radiated from body of humans, animals and other objects. A PIR sensor can detect motion in certain distance and angle, so PIR sensors have different working areas.

PIR sensors are simple electronic devices and their only job is to detect changes of infrared radiation in form of a heat from detection area. Infrared light moves electrons in the sensors, which are captured and amplified in an electrical signal. This signal then is used to alert the alarm system or trigger a motion light to start illuminating. PIR sensor structure is pretty simple and consumes little energy and most basic sensors cost just few bucks, so they are widely used in security systems for homes and businesses as well as indoor and outdoor lights and lighting systems, and also are used in robotics and other industries.

A PIR motion detector

A PIR motion detector

PIR sensors can have different adjustable settings. For example, some motion detectors can be set to respond to energy in varying temperatures. For example a PIR sensor can be set to respond only to 10-12 μm (micrometer or micron) radiation, which is the average from a human body. This way the sensor will not detect motion of smaller animals like dogs and cats. Some sensors are so sensitive that they can pick up a heat emitted from birds and larger bugs and the light is turned on after motion detection.

In what application PIR detectors are used?

PIR sensors same as active sensors are mostly used in security alarm systems to detect thieves and trespassers. There are even some special pet immune motion sensors that are used in indoor alarm systems and can be set to ignore animals at specific weight. PIR sensors are often used together with active microwave sensors in security systems, to prevent false alarms triggered by one or other sensor.

Besides alarm systems, PIR sensors are often used in motion activated lights. This includes both indoor and outdoor lights. PIR detectors are very popular combination with outdoor flood lights, as they tend to consume lot of energy because of their brightness, and a motion sensor helps to save energy and money by turning the light on only when a motion is detected in front of the light. PIR sensors are also used in solar lighting. Solar lights work on batteries that get charged in day by sunlight and power the light at night. Based on the size of the battery lights can provide continuous illumination from couple of minutes to few hours and larger lights can even work for all night. To prolong the illumination period solar lights are often equipped with a motion sensor. With help of a motion sensor lights that have batteries with capacity of less than an hour or even less than a half an hour can provide illumination for all night if used in an area, where the light needs to be turned on just few times at night, for example, when used as a front door light.

From what parts PIR sensor consists?

The basis of all PIR sensors is practically the same, only few parts such as lenses are different, which is why there are PIR sensors with wider and shorter detection ranges as well as different sensitivity settings.


The PIR motion detector consists of such parts:

  • PIR sensor – The PIR sensor is the part which is responsible for measuring the infrared light emitted from an object in its detection area. The sensor is protected from outer factors such as humidity by a metal casing. On top of the casing there is a small surface with infrared-transmitting material under which is the infrared sensor. The sensor (usually there are two sensors) generates electricity from the infrared radiation or heat.
  • Lens – The lens is what can be seen from the outside of the detector. Lens is much wider than the sensor and its job is to bend light and focus it on the infrared sensor. Lens is made from a plastic which allows light with certain wavelengths to reach the sensor under the plastic cover. The large lens is made from multiple smaller Fresnel lenses, which can capture more light on a smaller lens surface than regular lenses. Lens also protects the sensor from outer objects like dust getting to the sensor. To sum up, lens of the motion detector is responsible for couple of tasks:
    • Focus the light on the infrared sensor, so it can cover much larger area;
    • Filter the light to capture the right wavelengths, for example to detect movement only by humans and not by smaller animals;
    • Protect the sensor from outer impacts and objects.
  • Electric circuit – This consists of some resistors, capacitors, chips and adjusters. There can be couple of adjusters inside a motion detector light:
    • Sensitivity adjuster – regulates a detection range of the sensor;
    • Timer to set the time of how long the light needs to illuminate after motion has been detected by the sensor;
    • Timer to set the time of how long the light needs to be turned off after no further movement is detected.
PIR micro chip

PIR micro chip. Source –

How PIR detects motion?

Infrared wavelength is longer than a wavelength of a visible light, so infrared light cannot be seen by humans, but it can be captured by motion detectors. Not all materials that can transmit visible light like glass of windows are able to transmit infrared light. However materials such as silicon or different types of plastic are good for transmitting infrared waves and can also be used to filter the correct wavelengths of the infrared light.

A PIR sensor detects changes in amount of infrared radiation in its detection area. Infrared radiation basically is heat coming from objects such as humans and the amount of infrared radiation mostly depends on the temperature of the object.

PIR motion sensor operation

PIR motion sensor operation. Source –

For example, when a human walks into the sensors detection range, the temperature in that range changes from a temperature of room objects (if used indoors), which is lower, to a temperature of a human body, which is higher and when a human walks out of the detection range the temperature changes back to the temperature of room objects. A PIR sensor detects these changes in temperature and transfers them into electrical signals which are passed to the alarm system that gets activated or turn on a light that starts to illuminate.

Advantages of motion sensing lights over regular lights

Motion sensors are common options for flood lights and solar lights.  They offer numerous advantages for both high powered outdoor flood lights as well as solar lights, starting from energy savings to improving the lifetime of certain parts of lighting fixtures when compared to regular lights. Here are few advantages of motion activated lights over regular lights.

Motion activated flood light

Motion activated flood light

Lower energy consumption and lower electricity costs

If you have ever owned a regular flood light, you must know how much energy such a light can consume when illuminating nonstop for all night. The interesting part is that in most applications we actually don’t need the light to provide continuous illumination all night long, but we need the light to be turned on in a certain situation, mostly when someone is in the area, which light should provide illumination. In these situations motion activated sensors are very useful. Motion sensor will trigger the light only when someone is moving in its detection range and will turn the light off afterwards, this may be one or multiple times at night based on the area the motion light is installed. Obviously a light that illuminates for 8 full hours will consume a lot more electricity than the same light that will get triggered just few times at night and will illuminate for couple of hours or even less. Lower energy consumption results in lower electricity bills and based on the power and brightness of the fixture as well as the intensity of usage the savings on electricity may be colossal. And because most motion sensors cost just few bucks and motion activated lights cost practically the same as regular lights, solar motion light will pay itself off in the first days to month of usage.

Solar lights

Other types of lights that benefit hugely from motion sensors are solar lights. You will see much more motion activated solar lights than ones without a motion sensor, and there is a good reason for that. Solar lights have built in batteries with varying capacities, solar lights with larger lead acid or lithium-ion batteries can provide continuous full night illumination, while medium and smaller light that have lower capacity Ni-MH or Ni-Cd batteries likely won’t be able to provide energy for longer than an hour or two. This is why most solar lights, especially solar flood lights, are equipped with motion sensors. Solar lights are mostly used in such areas that do not require a nonstop illumination, but needs to be illuminated few times at night, such as front door lights that need to provide illumination for few minutes so you can safely walk to the house at night. Also, when used for security purposes to scare unwanted trespassers off your property, whether it is a bugler or some kind of animal in your backyard. It would not make sense to use a regular flood light that would illuminate nonstop all night in these situations. These are just some basic example how solar lights benefit from a motion sensor. Same as with regular motion activated lights, the largest benefits from motion sensors for solar lights is energy conservation and ability to provide longer illumination times, while running on batteries.

Solar motion lighting

Solar motion lighting

Longer lifetime for parts and bulbs

Because motion activated lights do not require to illuminate for such long periods of time as regular lights, they not only save more energy but also help to extend life span of bulbs used in the lighting fixture. This can be attributed to practically any light bulb (except CFL as continuous turning on and off will decrease the lifespan of the bulb), even LED diodes that have much higher life span of 25000-50000 hours and more will benefit in long run.

More practical to use in certain applications

Motion activated lights are simply more practical to use in some applications than regular lights, and opposite around. Motion lights will benefit in such applications that require illumination only when a person walks by a certain area or stays in that area for fixed amount of time. Motion lights will also benefit areas, where it is inconvenient or impossible to install a light switch. Areas where motion lights proves to be useful are front door lights, staircase lights, garden and backyard lights, lights that provide illumination on sidewalks when walking by a personal house or a commercial property, lights on basements or other places with insufficient light in a house and other similar cases.


Blogger, editor, developer who loves green living. Interested in photovoltaics and solar lighting. Reviewing solar products since 2013.

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10 Responses

  1. Vishal Srivastava says:

    Hi, Does the PIR sensor light work during the daytime?

  2. Joel Snyder says:

    After several days of heavy rain, we noticed that our outdoor motion detector floodlight stays on all day. It will not switch between auto/test/manual when we switch the power off/on. I notice about a 1/4 inch of water on the inside of the lens. Could this be the problem? Will the water eventually dry up? Is there a way to speed up the process?

    • Arthur says:

      Water could be the problem yes. I would suggest you to drain the water inside of the lens and see if that helps. If not then you can also try cleaning the motion sensor of your floodlight or reset the lamp by following the instructions provided in the manual, if your light came with one. If that doesn’t help you might need to replace the sensor altogether.

  3. Mark says:

    We have a PIR on our electric gates and they open constantly even when the black bird wonders by! Is there a PIR that has the ability to ignore anything smaller than a human? or car or whatever

    • Arthur says:

      I would suggest you switch to a microwave sensor, ultrasonic sensor or a combined sensor (one that combines infrared with ultrasonic or microwave sensor) for your electric gate since these types of sensors only respond to larger objects without reacting to birds and other small creatures.

  4. Dan Mills says:

    Hi Arthur: We have an outdoor motion sensor that triggers an LED flood light. However the area that we want illuminated is also an area where our boiler discharges hot steam to the exterior of the house. So all night when the steam is discharged every hour the LED flood light turns on. Do know if there is a motion sensor that is immune to hot steam and can help me detect people but not steam? Thank you for your expertice!!

    • Arthur says:

      Since PIR motion sensors detect Infra Red patterns, there is no wonder that your lamp lights up every time the steam is discharged. The easiest and most permanent solution here would be to just move your motion sensor LED to a different location. Another way to solve this is to aim the sensor away from the place where the steam is discharged or place a bit of black tape on the side of the sensor where the heat is discharged so it doesn’t “see” the boiler steam. There also are motion sensors that are adjustable and will ignore the presence of a heat source up to a certain volume that you might want to look into. And finally, a hybrid motion sensor that will trigger the light only when both the PIR and Microwave sensors are tripped might provide a good alternative to your current PIR one, especially it is adjustable.

  5. Johraine L. Mesis says:

    Hi Arthur, we are grade 9 students that is conducting a research topic, specifically its experimental. So, is it possible to connect a PIR sensor to a water spray that will spray if detected a pest. If possible can you explain it to us on how to do it 🙂 thaank you.

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