Telephone disconnect apparatus

Abstract

An electronic telephone disconnect apparatus uses a phase locked loop circuit containing a voltage control oscillator (VCO) to detect a signal at a frequency above the audible range, and the output of the VCO is coupled, through the normal connection wires, to the earpiece of the telephone handset. In close proximity to the location of the earpiece when the handset is in the off-hook condition, there is provided a pickup coil, which is coupled to the input of the phase locked loop. The phase comparator output of the phase locked loop then provides an indication of the hook state of the telephone. In the on-hook state, the earpiece is inductively coupled to the pickup coil, the VCO signal is thereby coupled back as an input signal to the phase locked loop, and it rapidly achieves lock, at which time the phase comparator indicates zero error. This corresponds to an on-hook indication. When the handset goes off-hook, the earpiece and pickup coil are separated, the inductive coupling is lost, and the phase locked loop loses lock. At this time, the phase comparator indicates a non-zero error signal representative of the off-hook condition. Inasmuch as the VCO provides both the signal to the earpiece and the reference signal to the phase comparator, the frequency produced at the earpiece and the frequency to which the phase locked loop responds are automatically matched, regardless of environmental conditions or aging.