SELECTIVE RINGING CIRCUIT FOR A TELEPHONE

摘要

SELECTIVE RINGING CIRCUIT FOR A TELEPHONE A selective ringing circuit for a telephone line includes a rectifier which is normally connected to the telephone line for rectifying the AC ringing and other signals. The rectified ringing signals will not activate the ringing circuit of the telephone equipment, because a capacitor in the ringing circuit is charged and will not pass current therethrough in response to the DC signal. The selective ringing circuit includes a switch-selective signal-detecting arrangement by which a predetermined one of the ringing signals is detected and is used as an activation signal, while the other ringing signals are rejected. The activation signal activates an electrical connecting device which, when activated, electrically bypasses the rectifier and directly electrically connects the predetermined selected ringing signal to the telephone equipment. An automatic number identification circuit includes a transistor switch which connects a predetermined ground resistance to the telephone line. A shunt resistance automatically shunts a tip or ring dropping resistor to compensate for the unbalanced resistive effects of a biasing circuit for the transistor switch. The rectifier supplies consistent polarity DC signals to the telephone equipment.