MALFUNCTION PREVENTING CIRCUIT FOR SIGNAL DETECTING PART

PURPOSE:To prevent a signal detecting part from generating malfunction due to noises, etc. by providing two pieces of constant-current sources in a closed loop of a connecting line of a DC feed path and a switchboard, detecting a variation of the potential between the connecting lines, and controlling a radio wave of the constant- current source. CONSTITUTION:Constant-current sources 25-1-25-2 are connected in series to a DC power source 21 through a connecting line 3. Also, an intermediate connecting point 27-3 of resistances 27-1-27-2 of a connecting line 3 side of each constant-current source 25-1-25-2 is set as a voltage monitor point, and the potential of an intermediate connecting point 28-3 of resistances 28-1-28-2 of the DC power source side is set as the reference voltage. In this state, a difference of a potential variation which has been fetched from the voltage monitor point 27-3, and the reference potential is brought to differential amplification by operational amplifiers 26-1-26-2, and when its output has been generated, output currents of the constant-current sources 25-1-25-2 are controlled in the direction for equalizing a current value. Therefore, when a high voltage large current is induced by a start of a ringer device, etc., it does not occur that a signal detecting part 22 becomes an abnormality detecting state by its influence.