Interchangeable multifrequency telephone keyboard - has diode bridge across protection device, with supply and switching circuit connected to negative bridge terminal

摘要

The keyboard is connected to a telephone line across the subscriber loop and includes a multifrequency signal generator. A diode bridge is connected to the phone line across a protection device, and a supply circuit connected in series with a 600 ohm resistor and a first switching circuit which is further connected across the bridge output. An injection and filtering circuit is connected to the generator output, is parallel with the resistance and supply circuit. A keyboard circuit is connected both to the keyboard and to the bridge across the resistance with an audio switching circuit connected to the keyboard circuit and an output of the generator. The audio circuit has a switch connecting one telephone wire to an audio circuit and a generator attenuation circuit and connects with a control input of the first switching circuit.