Clearing signal generator for emergency phone service - has clock driven switches to send two different signals alternately

摘要

Free emergency phone calls are cleared by the called unit, in case the caller forgets to hang up. The caller may be on an electronic or an electromechanical exchange, which require different clearing signals. These are sent alternately, each for 150 ms with 150 ms pause between them. A clock (1) generates a square wave, 150 ms ON, 150 ms OFF, which drives two switches (i1,i2) and two bistable changeovers (f11,f12), i.e. the switches operate and the bistables change over every pulse. One bistable (f11) switches between -ve supply, -UB, and E and is connected to the a-wire by one switch (i1). The other bistable (f12) switches between (E) and (O/C)and is connected to the b-wire by the other switch (i2).