Abstract: A device for indicating a certain proximity between movable units emitting to the units a synchronizing signal with a very high transmission speed. A transmitter device is situated at least on certain of the units. The transmitter device is activated by the synchronizing pulse to emit an information signal in the form of a wave propagation movement in the medium in which the units are located. The wave propagation movement has in comparison with the synchronizing signal a considerably lower transmission speed. In at least certain others of the units the information signal is received by and related to the difference in time between the synchronizing signal and the received information signal to a predetermined time value corresponding to a certain proximity between the units. Thereafter the difference in time is evaluated in relation to the predetermined time value.