Ground start and tip ground detection without the use of -48V or tip bait circuit

摘要

A loop detector circuit for detecting the loop current of a telephone circuit in a ground start mode and a normal mode which obviates the need for a separate TIP ground detector circuit. A pair of light emitting diodes (LED) 52, 54 are associated with a photoelectric device, such as a phototransistor 56, 58, connected across TIP and RING with a series resistor. Each resistor 60, 62, is selected to provide a predetermined voltage drop during a ground start mode and a normal mode, thus enabling the loop detector to detect loop current during both modes. The loop detector circuit is thus able to detect the off-hook status of a subscriber handset during a ground start mode without the use of a separate TIP ground circuit.