Infra-red sensor for a burglar alarm. This device detects movement by sensing the change in infra- red radiation in the environment when a person walks into the room. It is a passive pyroelectric sensor. It is calibrated to infra-red radiation emitted at human body temperature. This radiation enters the device through the lens (red) that gives it a wide field of vision. The energy of the radiation will knock electrons out of the sensing material and trigger a cascade of electrons. This electronic pulse sets the alarm off. It will not detect someone standing still, as it will be set to respond only to changes in infra-red energy.