Whether it is really a cat or a cat burglar trying to sneak into your house at night, a motion detector is a more prudent device to have around, rather than trying your luck with a baseball bat. The trick is in knowing what type of motion detector to use at what point, since there are so many varieties of them and that could be confusing. It helps to know how some of the more common types of motion detectors work.
Typically, there are two types of motion detectors – passive type and active type. The differentiation depends on whether a detector is injecting energy into the environment for detecting a change. Active types inject energy into their immediate environment, whereas, passive types do not. Both devices are simple electronic components.
Active type motion detectors can use light, microwaves or sound for detecting movement. The most common type of active motion detector is a beam of light crossing the door with a photo sensor on the other end. As soon as a person breaks the beam of light, the photo sensor detects the change for light reaching it and either rings a bell or flashes a light.
Many places have automatic door openers. These can detect when someone passes near and opens the door in response. A device above the door sends out bursts of microwave radio energy at periodic intervals. A sensor waits for detecting reflected energy. When a person moves into the range of the microwave energy bursts, the amount of reflected energy changes or the time taken for the reflections to arrive changes and the box triggers an arrangement that opens the door.
Pyroelectric sensors or Passive Infrared detectors can sense the heat given off by a human. To make the sensor sensitive to the temperature of a human body, the sensor must be capable of sensing skin temperatures of around 93°F or 37°C. Such sensors are typically sensitive to the infrared energy wavelengths of the range 8-12micrometers, since the human body radiates wavelengths between 9 and 10 micrometers.
To prevent the sensors triggering false alarms for example, a sidewalk cooling off at night, a pyroelectric type motion detector detects only rapid changes in its field of view. That makes these sensors insensitive to a person standing still. However, the amount of infrared energy changes rapidly when a person is moving or walking by, enabling the sensor to detect it easily.
Since infrared energy is a form of light, a plastic lens can very easily bend or focus it. That is how these sensors have a wide field of view. Most detectors have one or sometimes two sensors within them looking for changes in infrared energy. However, infrared sensors installed within a room are not very capable of detecting snoopers or peeping toms trying to peek in through a window. That is because a motion detector sensitive to infrared energy is unable to detect it through glass windows.
If you have a four-legged friend in your home, you have to get sensors that are pet immune, to make sure the friend is not mistaken for an intruder.