Panic proof public emergency call box - uses ultrasonic echo sounder to hang up when caller leaves and monitored time switch

摘要

An emergency call box, connected to the public telephone system, 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 the call box identification. If it is pressed again, while the echo sounder still detects the presence of a caller, this is taken to mean that the first call has failed. The control circuit therefore first 'hangs up' and then repeats the automatic connection and dialling. If the echo sounder detects that the caller has left the box for more than a specified time, the control circuit automatically hangs up. The system includes a monitor unit (UEW) with two signal output lines (SAE1, 2) in parallel, each in series with a time-switch (ZSE1, 2) and a common driver circuit (TZE).