All objects with a temperature above absolute zero emit heat energy in the form of radiation. Usually this radiation isn’t visible to the human eye because it radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose.
The term passive in this instance refers to the fact that PIR devices do not generate or radiate energy for detection purposes. They work entirely by detecting infrared radiation emitted by or reflected from objects. They do not detect or measure “heat”.
The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to IR. The lens used here is not really doing much and so we see that the two slots can ‘see’ out past some distance (basically the sensitivity of the sensor). When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected.
The PIR sensor sometimes operates differently at different temperatures, not triggering when it gets hotter but in the colder months triggering for every small, distant, movement. It can also be insensitive when walking directly towards the sensor.
A Microwave sensor applies the Doppler effect to detect moving objects using microwave technology to analyze its environment by sending out a microwave signal and measuring the length of time it takes to return to the motion detector (referred to as “echo time”), similar to a RADAR system. It uses this echo time to calculate its distance from stationary objects. The motion detector conducts a baseline room analysis to detect its present distance from stationary objects, then reverts to its “normal” state. Upon a person entering the motion detector’s field of view, this causes an interruption in the emitted microwave beam, which changes the received echo time, which the motion detector perceives as a change in distance from an object. This action causes the motion detector to trigger the alarm.
Although these sensors detect the motion through wood and most building materials , microwaves do not penetrate metals. Metal objects act as a shield that creates shadows or dead zones behind them.
On the other hand, because the beam can penetrate the walls, the sensor will detect the motion behind the walls, even if such motion is normal as the sensors are extremely sensitive to the motion.
They are also prone to other false alarms. Everyday movements such as the objects blowing in wind may trigger false alarms. Even fluorescent lighting, that emits detectable light particles, can activate the sensor erroneously and they may trigger a false alarm.