The Wireless Alert Sensor is a Passive Infra-Red (PIR) Motion Sensor that detects movement of people and vehicles, while allowing small animals to pass through undetected. Each sensor has its own unique ID, for a secure radio link. The base station (not provided) can learn the signals of up to 8 sensors. The base station gives a single beep for the first sensor that is programmed, a double beep for the second, and so on. Additionally, multiple base stations can work with the same sensor to sound in different places.