Passive Infrared (PIR) Motion Sensors protect space by ‘looking’ for changes in infrared (heat) energy levels caused by movement of an intruder.
The passive infrared detectors are passive devices. i.e., they do not transmit any form of energy. When a person enters and moves within the detecting zone, his body which is at its normal temperature of 98.6 degree F., radiates more (or less) thermal energy than the surroundings and such minute difference in thermal energy is detected by the PIR.
In simple words, PIR keeps a picture of the heat levels in its view and keeps on comparing the new view with the stored one. In case there is any change then they activate the alarm.