Abstract: An automatic alarm is disclosed for detecting obstacles behind a vehicle as the vehicle travels backwards. The alarm includes a casing mounted at the rear of the vehicle and having a rearwardly facing opening. A revolving disc is mounted within the casing which is driven by a motor in the casing. A printed circuit plate is fixed within the casing under the revolving disc. A supersonic transmitting member is installed on the revolving disc. A supersonic receiving member is installed on the revolving disc. The transmitting and receiving members are simultaneously alignable with the opening and are electrically connected to a printed circuit on the printed circuit plate at least when the transmitting and receiving members are aligned with the opening, so that the transmitting member emits a signal which is deflected off adjacent obstacles and is detected by the receiving member.