Why motion detectors react to animals and how to avoid it

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Why motion detectors react to animals and how to avoid it

After installing motion detectors at home, you expect an instant response to outsiders. If a thief steps over the threshold or breaks in through the window, an alarm will be triggered. And you can rest assured that any high-quality alarm device can handle this task. But responding to movement is only half the battle: an important quality of a detector is the ability to determine what exactly is moving. It must quickly and accurately detect a person, while also ignoring the natural interference and pets, so that you and the security company are sure: if the security system has raised the alarm, the threat is real and you need to respond immediately.

This article tells how infrared motion detectors see the world, what technologies they use to distinguish a sheepdog from a villain, and what animal owners should remember when installing an alarm system at home.

How infrared motion detectors work

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

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


Sometimes they say: “The motion detector saw a man.” However, these devices do not see objects, animals, or people; in fact, they only deal with data. 

A special curved Fresnel lens focuses infrared radiation onto the detector’s pyrosensor. Then the sensor signals are sent to the microprocessor, which converts them to digital data. This is followed by threat analysis and assessment, which is precisely the key difference between smart detectors and their primitive counterparts. Knowing the intensity of infrared radiation and the dynamics of the source movement, software algorithms determine the nature of the threat, and the detector decides whether to raise an alarm or ignore it.

How pet immunity is implemented in Ajax motion detectors


When properly installed and configured, indoor motion detectors MotionProtect, MotionProtect Plus, CombiProtect and MotionCam ignore animals that are up to 50 cm in height and weigh up to 20 kg. The maximum weight is defined as an understandable orienting point, but in reality, the size of the heat spot in the infrared spectrum is of importance. Keep in mind that detectors do not see objects but instead analyze the infrared radiation of these objects.

ajax pet immunity

Motion detectors are less sensitive near the ground. That’s where large pets are assumed to be found.

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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 with values ​​specific to human movements. Due to the fact that the heat spot of an animal is small and the signals enter the lower zone of the Fresnel lens (which transmits less radiation to the sensor), the detector does not raise an alarm.

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


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

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Like Ajax indoor detectors, MotionProtect Outdoor uses the SmartDetect algorithm — the first software level of protection against false alarms. The algorithm analyses and compares data from each sensor. As a rule, the detector is ready to make a decision whether to ignore or raise an alarm at this stage. But if the situation is ambiguous, the spectrum analysis will be involved for frequency components of the signals of two sensors. This anti-false alarm protection system unique to security detectors was named LISA.

See also: MotionProtect Outdoor — an outdoor motion detector that reacts only to real threats

Points to consider for motion detectors to ignore animals

1. Make sure that the motion detector is installed at the optimum height.

The MotionProtect, MotionProtect Plus, CombiProtect and MotionCam indoor detectors are installed at a height of 2.4 m. Curtain-type motion detectors such as MotionProtect Curtain when used in rooms with pets should be mounted upside down at a height above pet’s growth.


Guides: MotionProtect, MotionProtect Plus, CombiProtect, MotionCam, MotionProtect Curtain

MotionProtect Outdoor detectors are fixed at a height of 0.8–1.3 meters. The line of sight of the upper lens should be parallel to the ground plane. Even an imperceptible line of sight deviation at long distances will shift the field of view of the upper lens above a person or direct it to the ground. Therefore, it is important to conduct three detection zone tests: of the upper lens (correct mounting), the lower one (human detection range), and then of both lenses.

Guides: MotionProtect Outdoor

If indoor motion detectors are installed below the recommended height, animals move in the hypersensitive area. If a sheepdog walks on its hind legs or even a small spaniel frolics in the chair, the detector may raise an alarm.

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Outdoors, the terrain of the protected area must be taken into account. If a large dog enters the detection area of both MotionProtect Outdoor lenses, pet immunity function may not be able to handle it and the detector will raise an alarm.

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2. Set the detector to appropriate sensitivity.

When protecting rooms 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 lower it if there are false alarms.

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

The closer the moving object gets to the lens, the greater its thermal spot becomes. If an agile dog jumps to the detector level in close proximity to the lens, an alarm will be triggered. The same goes for a cat that jumped on the closet to get close to the detector.

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To eliminate the slightest chance of worries and vain trips of patrols due to triggering on animals, use the new MotionCam motion detectors with visual alarm verification. When motion is detected, they take a shot or a series of 2-5 shots. Both you and the security company know exactly what caused the alarm: a playful pet or a robber breaking into your house.

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