BUSY TONE DETECTOR

PURPOSE:To attain the detection of busy tone by constituting the titled detector with a filter circuit outputting a frequency component forming a busy tone from the signal of a telephone line, a conversion circuit converting the signal from the filter circuit and a control circuit outputting a signal representing the busy tone. CONSTITUTION:When a telephone set makes a call, a sent signal is inputted to the filter circuit 1. When a frequency component around 400-500Hz is inputted from a telephone line to the filter circuit 1, it is outputted to cut off the other frequency component. A conversion circuit 2 acts like a circuit outputting a high level signal when the frequency component around 400-500Hz is inputted from the filter circuit 1 for the time and outputting a low level signal when non input given thereto. A microprocessor 3 being a control circuit receives the signal converted in this way from the telephone line at its port so as to discriminate a busy tone from signals other than the busy tone and a noise from a call tone and to output a signal representing the busy tone as well as a signal representing the call tone. Thus, the busy tone is detected.