Abstract: A hook state detector for a two-wire telephone line includes two comparators each for comparing a voltage on a respective wire of the line with a respective reference voltage, and a logic circuit or software responsive to outputs of the comparators for determining off-hook and on-hook states when one of the compared voltages is more negative and the other is more positive than the respective reference voltages, and maintaining a previous hook state determination when both of the compared voltages are more positive, and when both of the compared voltages are more negative, than the respective reference voltages. The arrangement is insensitive to common mode induced a.c. on the line. The comparators can comprise differential amplifiers or opto-couplers.

Abstract: An electronic switch is disclosed for switching relatively high voltages, such as telecommunications voltages of the order of -48 volts, in response to logic levels of typically 0 to 5 volts. The switch comprises a MOSFET having a source-drain switching path and a gate; a control transistor controlled from a logic level control terminal; and a potential divider MOSFET source and the control transistor and having a tapping point coupled to the MOSFET gate. The potential divider includes a zener dime and/or a resistor connected between the MOSFET source and gate. The sinitch can be used with a current sensing resistor and a monostable circuit to fonn an automatically-reset circuit interrupter or electronic fuse.