| 摘要 |
The first arm (La) of the telephone line can be optionally connected via a key switch (S) either directly or via the relay winding of a relay (T) to the telephone device. Via the key switch (S), a first parallel arm is connected between the two arms (La, Lb) of the telephone line, which contains a constant current source (K) and, in parallel therewith, series-tuned circuits tuned to dialling frequency signals, and in series a field-effect transistor (FET1), the gate voltage of which is picked up across a voltage divider (R2, R3, R4) which can be connected to the first arm (La) of the telephone line via the switching contact (t) of the relay (T). The dialling block is switched off by a call detection device (RE) which with an incoming ringing signal outputs a control signal which is supplied to the gate electrode of a thyristor (Th) which is arranged in a control arm which is connected in parallel with a part-resistor (R3, R4) of the voltage divider, the resistors in the voltage divider and control arm being sized in such a manner that when the thyristor (Th) turns on, a signal blocking the field effect transistor (FET1) is generated at the tap (P) of the voltage dividers. <IMAGE>
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