Passive Infrared Motion Detector
They detect or receive the infrared heat signature off of anything that is above absolute zero. If the sensor detects movement in an area where there was nothing visible, the sensor has picked up something invisible to the naked eye that is giving off an IR heat signature. When monitor or chime mode, the alarm will chimes to announce visitors and then stop. When it is switched to alarm mode, it will sound a siren when it detects motion up to 30ft away and continue to sound the alarm until it is turned off. We always use the alarms in chime mode. The models may vary and which model you receive may vary depending upon which is available to us at the time of your order. We have used all of these alarms many times and have gotten excellent results with each of them.