A movable door incorporates a proximity detection system for sensing obstructions in the path of the door. The system comprises an array of transmitter/receiver pairs (21, 22 and 23), each pair comprising a transmitter (1) for transmitting an energy pulse, a receiver (2) located adjacent the transmitter and directed to receive any reflected transmitter pulses, and a shielding (3) for shielding the receiver (2) from receiving energy pulses directly from the transmitter (1). At least two transmitter/receiver pairs (21, 23) are mounted on a leading edge or face of the door (12) to transmit energy pulses in a direction which is not at right angles to the leading surface or edge of the door, to monitor for obstructions in the path of the door (12). Another two transmitter/receiver pairs (26 and 28) are mounted on the leading edge or surface of the door (12) to transmit energy pulses at right angles to the surface or edge of the door (12). One of the latter two receiver pairs (26) has a different response to the other pair (27) so that when the door (12) approaches a stationary object the one pair (26) detects the object ahead of the other pair (27). Means are provided to respond to the output of the one pair (26) indicating the detection of an object to inhibit the output of the other pair (27). Different transmitter/receiver pairs may transmit on different frequencies, and means for suppressing unwanted reflected signals is provided. Means for transmitting data signals on a two conductor power supply cable for the system is also disclosed.