| 摘要 |
A dialling restriction for a telephone device having a talk-listen device (HS) which is connected to the telephone line (a, b) and which is preceded by a cradle switch contact (GU) and the contact (nsi) for generating the dialling pulses. The dialling restriction can be switched on and off by a key switch (S). The first branch (a) of the telephone line is connected to the second branch (b) of the telephone line via a parallel branch to the telephone device (2), containing a field-effect transistor (Tr1) and a Zener diode (D1). The gate of the field-effect transistor (Tr1) is connected via a second field-effect transistor (Tr2), located in a second parallel branch, to the first branch (a) of the telephone line. A third parallel branch to the telephone device (2) contains a voltage divider (R3, R4), the tap (P) of which is connected to the gate of the second field-effect transistor (Tr2) and in which a second Zener diode (D2) is inserted, the anode of which is connected to the first branch (a) of the telephone line via a capacitor. The breakdown voltages of the two Zener diodes (D1, D2) are selected in such a manner that they are between the voltage values prevailing on the telephone line (a, b) with the loop closed and with the loop opened. <IMAGE>
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