Interface unit for pushbutton selector

Claim 1

In a telecommunication system wherein a hook switch in parallel with a high-ohmic shunt resistor is inserted in a line loop in series with a generator of dial pulses including a pushbutton selector and a signal coverter provided with digit-storing means, the combination therewith of an interface unit interposed between said hook switch and said generator, said interface unit comprising: a rectifier bridge having an input diagonal and an output diagonal, said input diagonal being inserted in a conductor of said line loop in series with the parallel combination of said shunt resistor and said hook switch; an impedance network connected across said output diagonal, said network being provided with two supply leads for delivering operating voltage to said generator irrespectively of the position of said hook switch, said network further having a resistive shunt branch including switchover means responsive to a voltage rise across said input diagonal due to closure of said hook switch for significantly lowering the resistance of said shunt branch with resulting reduction in said operating voltage; an outgoing conductor extending from said shunt branch to said generator for signaling the closure of said hook switch; and an incoming conductor extending from said generator to said network for carrying dial pulses generated upon manipulation of said pushbutton selector, said network further including pulse-receiving means connected to said incoming conductor for reversing said switchover means in the closed position of said hook switch to raise the resistance of said shunt branch to a value high enough to approach an open-circuit condition of said line loop.