Panic proof public emergency call box - detects presence of caller by ultrasonic echo sounder, effective over defined area in front of box

摘要

The emergency telephone call box is connected to the public telephone system. It has an alarm button, fixed microphone and loudspeaker, control circuits and an ultrasonic echo sounder to detect the presence of a caller. Pressing the alarm button automatically dials the emergency code and sends call box identification, provided that the echo sounder detects that a caller is present i n a defined space in front of the call box. If he presses the alarm button again, the control circuits hang up and, provided that he is still there, repeat the dialling procedure. If he leaves the detection space, the echo sounder causes the control circuits simply to hang up. The monitor region for the echo sounder forms a petal shaped lob (UEB) extending in front of the call box unit (UWE).