Bial Alarm Chime Doorbell Wireless Infrared Monitor Sensor Detector Entry Door Bell,BL-YBQ021 (1 Receiver 2 Detector) Infrared sensor detects motion and sends a signal to the receiver Wireless installation, just place the sensor at the entrance Receiver requires 2x AA batteries, transmitter requires 1x 9V battery (batteries not included) Send audible chime sound, alerting you that somebody is approaching Material: Plastic. Melody: for Alice