Device for obtaining switching signal e.g. for telephone appts.

摘要

The device has an incoming telephone line joined to its input terminals (31,32), which are connected to an A.C. detector (33) via a first condenser (34) and a first resistance (34'). The detector has an associated second condenser (35) which causes the generation of a pulse output corresponding to the length of the call sign. This pulse enables the optocoupler (36) to charge a third condenser (37) via a diode (38). A second resistance (40) and a third resistance (41) determine the respective charge and discharge time constants of the third condenser (37), so that the output of a Schmidt trigger (39) stays at a predetermined voltage level during reception of several call signs. A second call may create a second pulse which changes the Schmidt trigger output voltage level, and hence the count in the counter (42). This produces a signal from the output terminal (49), for switching some equipment off.