Interface unit for pushbutton selector

Abstract

An interface unit inserted between a subscriber line of a telecommunication system and a generator of dial pulses, including a signal converter associated with a pushbutton selector and a memory for the temporary storage of selected digits, comprises a rectifier bridge in series with an impedance network which includes a shunt branch with two complementary transistors interconnected in a positive-feedback circuit and a low-threshold Zener diode in series with one of these transistors across which a reduced supply voltage for the pulse generator remains available when the shunt branch becomes conductive with saturation of the transistors upon a closure of the subscriber's hook switch. An enabling signal for the pulse generator is emitted from the shunt branch upon such conduction. An ancillary transistor, with an input connection across a small series resistor of the network, and a further Zener diode in parallel with the shunt path protect the components thereof against overcurrents and overvoltages.