The MA40S4 sensors cannot be used outdoors. RS Pro Compact Ultrasonic Sensors. Compact ultrasonic sensors are highly sensitive and compact sensors. The RS Pro range of open structure, compact ultrasonic sensors are used to receive ultrasonic waves. Features ; Benefits. Compact and light weight. High sensitivity and sound pressure. 40kHz operating frequency. Directivity 80°. Body is elastically fixed to the base. Housed in a case. Typical Application. Burglar alarms. Range finders. Automatic doors. remote controls