Speed detecting device for a direct current motor, and electric fan with a motor speed detecting device

An electric fan comprising:

• a direct current motor having a power input line adapted to be connected electrically to a power source;
• a fan impeller mounted on said motor and driven rotatably thereby;
• and
• a motor speed detecting device including
• a coupling circuit connected electrically to said power input line of said motor, said coupling circuit generating a current-coupled pulse output that corresponds to current flowing through said power input line,
• a one-shot circuit connected electrically to said coupling circuit so as to receive the current-coupled pulse output therefrom, said one-shot circuit being triggered by the current-coupled pulse output to generate a pulse train signal in a form of a series of constant-width pulses, where spacing between adjacent ones of the constant-width pulses varies according to instantaneous operating speed of said motor,
• a converter circuit connected electrically to said one-shot circuit so as to receive the pulse train signal therefrom, said converter circuit generating a test signal having a magnitude that varies in accordance with the spacing between the adjacent ones of the constant-width pulses to provide an indication of a detected average operating speed of said motor,
• a comparator connected electrically to said converter circuit so as to receive the test signal therefrom, said comparator comparing the test signal with a predetermined reference signal that corresponds to a preset minimum operating speed of said motor, and generating an alarm activating signal when the test signal is less than the reference signal, and
• an alarm device connected electrically to said comparator for generating an alarm output upon receiving the alarm activating signal from said comparator.