Passive infrared (PIR)
A passive infrared sensor detects body heat (infrared energy) by looking for changes in temperatures. This is the most-widely-used motion sensor in home security systems. When your arm your system, this activates the motion sensors to report possible threats.
Once the PIR motion sensor warms up, it can detect heat and movement in the surrounding areas, creating a protective “grid.” If a moving object blocks too many grid zones and the infrared energy levels change rapidly, the infrared sensor triggers an alarm.
Microwave (MW)
This type of sensor sends out microwave pulses and measures the reflections off of moving objects.1 They cover a larger area than infrared sensors but are more expensive and vulnerable to electrical interference.
Dual technology motion sensors
Some motion sensors can combine multiple detection methods in an attempt to reduce false alarms. For example, it's not uncommon for a dual technology sensor to combine a passive infrared (PIR) sensor with a microwave sensor.
Each sensor type operates in different areas of the spectrum (ranging from passive to active). Dual technology motion sensors are not as likely as other types to cause a false alarm, because both sensors need to trip in order to sound an alarm. However, this does not mean that they never cause false alarms.