How motion detectors see the world

People often say: That detector sees me. But in reality a detector can’t see, there is no camera on it. Everything works on the basis of an image on which different colours can be seen, and on which the software has to react or not on the basis of these colour changes. We will use some pictures to clarify what such an image of a detector can look like, so that you have an image of what a detector really “sees”.

The operation of a motion detector visualized

AJAX has various motion detectors in its range for both indoor and outdoor installation. The indoor detectors actually always have one sensor that looks at a certain image, and the outdoor detector such as the MotionProtect Outdoor has two sensors that compare both images, and based on this whether or not an alarm is generated. By using multiple sensors you get a much more reliable detection of unwanted people, but this is mainly applied to better assess whether it is a person or an animal. After all, the detector does not have to respond to dogs and cats, large birds or moving trees. In order to counteract all this erroneous detection, two images are always looked at in the outdoor detectors. If both images detect a certain discolouration of the image, i.e. a difference in heat at a certain height within the spectrum, an action is taken. In spite of all the measures that manufacturers of detectors take, there is still a chance that a detector detects a movement of something that you don’t want to be informed about. The fact that the neighbor’s cat walks in your garden is not something you would want to call the police for.

Help, my detector responds to my dog, or the neighbor’s cat.

If the wrong detector is selected, or the detector is not mounted at the correct location or mounting height, a false message may be caused by a pet. Even if the detector is set too sensitively or the correct detection distance is not programmed, this can lead to undesirable consequences.

Why do motion detectors react to animals and how can we avoid that?

After the installation of the motion detector at home, you expect an immediate reaction to outsiders. If a thief steps over the threshold or breaks in through the window, an alarm is triggered. And you can be sure that any high-quality alarm device can handle this task. But responding to movement is only half the job: an important quality of a detector is its ability to determine exactly what is moving. It must quickly and accurately detect a person, while also ignoring the natural presence of pets, so you and the security company can be sure: if the security system triggered the alarm, the threat is real and you need to react immediately.

How motion detectors see the world

This story tells how infrared motion detectors see the world, what technologies they use to distinguish a sheepdog from a villain, and what people with a pet should pay attention to when installing an alarm system in their home.

How infrared motion detectors work

Motion detectors based on infrared sensors capture the infrared radiation or, in other words, the heat.

The closer to the white, the stronger the radiation and the warmer the object.

Sometimes they say, “The motion detector saw a man.” However, these devices don’t see objects, animals or people; in fact, they only see data.

A special curved Fresnel lens focuses the infrared radiation on the pyro sensor of the detector. The sensor signals are then sent to the microprocessor, which converts them into digital data. This is followed by threat analysis and assessment, which is exactly the main difference between smart detectors and their primitive counterparts. With knowledge of the intensity of the infrared radiation and the dynamics of the source movement, software algorithms determine the nature of the threat and the detector decides whether to sound the alarm or ignore it. It is precisely here that the different alarm system manufacturers distinguish themselves, because believe us, there is a lot of difference in detection quality. We have compared many different brands of security systems, and the AJAX system belongs to the absolute top in detection quality and reliability.

How pet immunity is implemented in Ajax motion detectors


When the motion detectors MotionProtect, MotionProtect Plus, CombiProtect and MotionCam are properly installed and configured, they ignore animals that are up to 50 cm tall and weigh up to 20 kg. The maximum weight is defined as an understandable landmark, but in reality the size of the heat spot in the infrared spectrum is important. Keep in mind that detectors do not see objects, but analyse the infrared radiation of these objects. For example, a dog and cat looks like this on the image:

Motion detectors are less sensitive near the ground. Large pets are believed to be found there.

Motion detectors use the SmartDetect software algorithm developed by Ajax Systems to assess the threat. When motion is detected, SmartDetect analyzes the signal waveform of the infrared sensor and compares it to values specific to human movement. Due to the fact that the heat point of an animal is small and the signals enter the lower zone of the Fresnel lens (which emits less radiation to the sensor), the detector does not give an alarm.

Ajax motion detectors also use the temperature compensation system. It increases the sensitivity of the PIR sensor when the ambient temperature is close to the human body temperature (36°C) and decreases the sensitivity when the temperature contrast between the body and the environment is high. Such a system helps the detectors to remain effective throughout the temperature range and protects them from false alarms at low temperatures.


MotionProtect Outdoor detectors do not respond to animals up to 80 cm in height. To effectively ignore all typical outdoor disturbances, these detectors are equipped with two infrared sensors and read out signals from two areas in space. They only give an alarm if both sensors detect movement.

Like Ajax indoor detectors, MotionProtect Outdoor uses the SmartDetect algorithm – the first software-based level of protection against false alarms. The algorithm analyses and compares the data of each sensor. As a rule, the detector is ready to decide whether or not to raise the alarm at this stage. But if the situation is ambiguous, the spectrum analysis will cover the frequency components of the signals from two sensors. This anti-false alarm protection system, which is unique to safety detectors, was named LISA.

Points to consider for motion detectors to ignore animals

1. Ensure that the motion detector is installed at the optimum height.

If motion detectors are installed indoors below the recommended height, the animals move into the hypersensitive area. If a shepherd dog is walking on its hind legs or even a small spaniel is frolicking in the chair, the detector may sound an alarm.

Outside, the site of the protected area must be taken into account. If a large dog enters the detection area of both MotionProtect Outdoor lenses, the pet immunity function cannot distinguish it and the detector will sound an alarm.

2. Set the detector to the correct sensitivity.

When protecting areas with animals, MotionProtect, MotionProtect Plus, and CombiProtect detectors are set to medium or low sensitivity, depending on the size of the animal. The sensitivity of MotionProtect outdoor detectors is determined empirically: start with medium sensitivity and decrease it if a false alarm occurs.

3. Make sure that the animal cannot approach the lens of the room detector.

The closer the moving object gets to the lens, the larger the thermal spot becomes. If a maneuverable dog jumps to the level of the detector near the lens, an alarm will sound. The same goes for a cat that jumped on the case to get close to the detector.

To reduce the chance of unnecessarily calling the police because you think there is a burglar, use the new MotionCam motion detectors with visual alarm verification. In other words: Use a motion detector with a built-in camera that can not only give you a burglary report within seconds, but also directly display one or more photos on your mobile phone of the cause of this detection. When motion is detected, they take a photo or a series of 2-5 photos. Both you and the possibly linked security company know exactly what causes the alarm: a playful pet or a robber breaking into your home. Watch the video below to see how the AJAX MotionCAM motion detector works for indoor installation.

If you are looking for a reliable alarm system, from which you can expect many beautiful products in the future, then choose an AJAX alarm system.

We will be happy to advise you which products are most suitable for your living situation, and then make you an offer without any obligation so that you know what security costs you. Remember that with the AJAX system you are not stuck with monthly costs or other contracts that do not help you. AJAX is reliable and transparent.

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